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WORLD METEOROLOGICAL ORGANIZATION

WMO TROPICAL PROGRAMME (TMRP) COMMISSION FOR ATMOSPHERIC SCIENCES (CAS)

TOPIC CHAIRMAN AND RAPPORTEUR REPORTS OF THE FOURTH WMO INTERNATIONAL WORKSHOP ON TROPICAL (IWTC-IV)

HAIKOU,

21-30 APRIL 1998

WMO/TD - No. 875

Secretariat of the World Meteorological Organization Geneva, Switzerland 1998 NOTE

The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries.

This report has been produced without editorial supervision by the WMO Secretariat. lt is not an official WMO publication and its distribution in this form does not imply endorsement by the Organization of the ideas expressed. Foreword

This is the fourth in a series of very successful workshops organized through the World Meteorological Organization's Tropical Meteorology Programme. The concept of bringing together researchers and operational forecasters from all parts of the globe for discussion and interaction has proved to be enormously successful. The previous three meetings have produced significant tangible outcomes in the form of publications and forecaster guidance, and have recommended directions for organizations to pursue with respect to tropical cyclones.

There are also a of intangibles surrounding these workshops that may be equally significant in the longer term. The broadening of links between individuals and organizations, the cross-fertilisation of ideas and the raising of awareness about exciting advances that are taking place around the world are all contributing towards the amelioration of the impact of tropical cyclones.

My thanks go to the topic chairs, the rapporteurs and the working group members who compiled the following reports. The information represents the current state of knowledge on tropical cyclones and stands on the shoulders of previous IWTC-inspired publications such as A Global View of Tropical Cyclones, Global Guide to Tropical , and Global Perspectives on Tropical Cyclones.

Participants in IWTC-IV should familiarize themselves with the contents of this report. It is the starting point of the Workshop. May it serve to stimulate discussion, ideas and directions.

Gary Foley Chairman, ICTC

TABLE OF CONTENTS

1. LANDFALLING PROCESSES OF TROPICAL CYCLONES 1.0 Report of the Topic Chairman F. Marks 1.1 The USWRP Landfalling Research Program F. Marks 1.2 Transients, Vertical Structure and Gust Factors in Tropical Cyclones P. Black 1.3 Processes excited by Tropical Cyclones L. Shay 1.4 Modification of Track at E. Rappaport 1.5 Rainfall D. Renqing 1.6 Decay after Landfall F. Marks 1.6 Decay after Landfall L. Chen

2. TROPICAL CYCLONE INTENSITY AND STRUCTURE 2.0 Report of the Topic Chairman G. Holland 2.1 Air-Sea Interaction J. Lighthill 2.2 Environmental Interaction (Intensity) J. Molinari 2.3 Tropical Cyclone Maximum Intensity Theory K. Emanual 2.4 Tropical Cyclone Formation W. Frank 2.5 Small and Mesoscale Interactions (TC structure and intensity) L. Ritchie

3. TROPICAL CYCLONE MOTION 3.0 Report of the Topic Chairman R. Elsberry 3.1 Environmental Interaction J.C.L. Chan 3.2 Baroclinic Processes B. Wang 3.3 Barotropic Processes (TC Motion) R. Smith 3.4 Small Scale Interactions (motion) M. Lander

4. TROPICAL CYCLONE PREDICTION 4.0 Report of the Topic Chairman M. Andrews 4.1 Observational Issues S. Ready 4.2 Analysis F. Wells 4.3 Parametric Wind Fields B. Harper 4.4 Track Prediction Techniques L. Carr 4.5 Intensity Prediction Techniques C. Guard 4.6 Seasonal p-rediction Techniques W. M. Gray 4.7 Rainfall Prediction Techniques X. Xiangde 4.8 Upgrade to the Forecasters Guide to Tropical Cyclone Forecasting G. Holland

5. TROPICAL CYCLONE IMPACTS 5.0 Report of the Topic Chairman R. Pielke + K. McGuffie 5.1 Economic Impacts K. McGuffie 5.2 Societallmpacts R. Pielke 5.3 The Warning Processes L. Avilia 5.4 Information Issues C.Landsea

Table of Contents

TOPIC ONE

LANDFALLING PROCESSES OF TROPICAL CYCLONES

1.0 Report of the Topic Chairman F. Marks (USA)

1.1 The USWRP Landfalling Tropical Cyclone Research Program Rapporteur: F. Marks (USA)

1.2 Wind Transients. Vertical Structure and Gust Factors in Tropical Cyclones Rapporteur: P. Black (USA)

1.3 Ocean Processes excited by Tropical Cyclones Rapporteur: L. Shay (USA)

1.4 Modification of Track at Landfall Rapporteur: E. Rappaport (USA)

1.5 Rainfall Rapporteur: D. Renqing

1.6 Decay after Landfall Rapporteur: F. Marks (USA)

1.6 Decay after Landfall Rapporteur: L. Chen (China)

Topic 1. 0/1.1 Chairman's Report/ US WRP landfalling tropical cyclone research program Rapporteur: Frank D. Marks, Jr. NOANAOML Hurricane Research Division , FL 33149 email: [email protected] Fax (1) 305-361-4402 Working Group: R. Elsberry, G. Holland, R. Carbone, F. delSol, K. Abe 1.1.1 Introduction The U. S. Research Program (USWRP) (Emanuel et al1995) effort on tropical cyclone (TC) landfall (Marks and Shay 1998) is focused on what the meteorological research community can contribute to a reduction in the disastrous impacts on the nation. With the continuing shift of population and commerce to the U.S. coastal areas from to , the threat is continually rising. Although some ofthe losses from disruptions to industry, commerce, and transportation are not avoidable, better warnings for longer periods in advance of the landfall would allow for more prevention activities. Both science and technology opportunities are described in a five-year (2000-2004) research plan (Elsberry and Marks 1998) that has as its ultimate goal to produce better guidance for hurricane forecasters. A new facet of research is proposed whereby the meteorological community works with the public and private sector to learn what improvements will have the greatest socio-economic impact. One may categorize studies ofTCs into three periods: pre-landfall, landfall, and post­ landfall. These periods are artificial, but they demonstrate that different aspects of the TC forecast problem are paramount at different . Pre-landfall commences when a is within 120 h of a forecast landfall. One forecast challenge during pre-landfall is to specify the precise coastal location and time at which the 34-kt (17 m s -1) isotach will cross the coast, because the local emergency managers want all disaster preparedness activities to be completed by that time in the warned area. Thus, the outer wind structure that determines the radius of 34-kt must be forecast along with the track. In addition, an accurate forecast of at least 24 h of the inner core wind structure is also needed as the inundation areas and evacuation plans are linked to the intensity (maximum wind). Improved track forecasts are achievable with more accurate specification of the initial conditions of both the large-scale environment and the wind fields of the TC itself. Application of the new NOAA Gulfstream IV (G-IV), deployment of the Global Positioning System(GPS) (GPS sondes),identification of the most fruitful adaptive sampling strategies, validation of current -1.1.1- model forecasts, and the development of are all likely to improve track forecasts. Once the tropical cyclone is within 72 h of landfall the radius of the 34-kt wind speeds and rapid intensity change become vital issues. The landfall period is declared when wind speeds reach 34 kt; this is a practical consideration as the arrival of 34-kt winds causes most meaningful preparation to cease and it is when all those that should have been evacuated are out of harm's way. During landfall, the key forecasts are of the inner wind structure and . The inner wind structure, which is modified in complex ways while passing from open ocean to coastal water and then to land is the primary determinant of localized wind damage, tornadoes, storm surge, ocean surface wave run-up, and evenprecipitation during landfall (see section 1.2 below for more details). Our knowledge ofTC structure changes at landfall is in its infancy, as there are little hard data that survive the harsh conditions. The WSR- 88D , making standard wind measuring systems more robust, and the application of new sensors such as portable Doppler, wind profilers, and the deployment of expendable sondes all provide an opportunity to learn much about the evolution of a TC as it makes landfall. Theoretical work that addresses TC intensity is needed to speed progress. Precipitation forecasting requires special considerations and will be treated as a separate topic. Post-landfall begins once the TC has lost its warm core. Precipitation forecasting moves to the fore followed by genesis within the region influenced by TC wind shears and stability (see sections 1.5 and 1.6 below for more details). Development of non-hydrostatic resolving models and case studies that take full advantage of the WSR-88D radars are two fruitful avenues worthy of pursuit. For the purpose of prioritizing the research objectives and observational opportunities, the topics are organized into track and wind structure forecasts during pre-landfall, and the inner wind structure and precipitation forecasts during landfall. A fourth focus area is the socio-economic aspects ofTC landfall, which were considered in setting the priorities.

1.1.2 Pre-landfall objectives

Two goals for the pre-landfall track are to: (i) increase the preparation lead-time by at least 12 h via an improvement in forecasts of the location and timing of the TC landfall approach point, and by providing forecast guidance to 96 h with the same accuracy as the present 72-h guidance; and (ii) provide more accurate forecast guidance with confidence measures that will allow the U.S. Tropical Prediction Center/ National Hurricane Center (NHC) to reduce the present 3:1 overwarning of coastal areas to a ratio of 2:1 in 90% of the TC . A solid basis for prioritizing the research approaches in this topic is available from recent advances in understanding TC motion.

-1.1.2- a) Track The largest gain in track forecast accuracy may be achieved through improvements in the initial environment and specification. The variational assimilation technique has great promise to improve the initial analysis in the . Targeted observing strategies are being developed to make optimum use of the mobile platforms such as the new G-IV aircraft and future Unmanned Aerial Vehicles (UAV). Upgrades in the quality and quantity of environment and vortex specification are possible from other -based instruments, such as those to be tested in the combined NASA and Mesoscale (CAMEX) 3 and NOAA Hurricane Research Division (HRD) during 1998. A better understanding of the sources of forecast errors would lead to reductions in the outliers in the error distribution. Just as improvements in dynamical model guidance have contributed to recent impressive gains in track forecast accuracy, additional gains are waiting to be achieved from numerical model improvements. New approaches for statistical-dynamical techniques are also available, and post-processing technique to remove systematic track errors could be developed and applied. A reliable prediction of the accuracy of the track guidance may be achieved from similar ensemble prediction systems as have been applied to midlatitude forecasting. A number of theoretical aspects related to track forecasting could lead to further advances in assimilation strategies, scientific basis for TC ensemble prediction, and description of essential physical mechanisms via conceptual models.

b) Wind structure Because the basic physics of wind structure change are not well known, more basic research to understand the science, and also to validate hypotheses with observations, is required. The outer wind structure is a primary factor in specifying the timing of landfall. New satellite-based wind observations such as the and high resolution ct.imulus cloud-drift winds plus special aircraft-deployed GPS sonde data sets need to be collected. An immediate benefit would be validate the NHC 34-kt wind radii. The data sets are also needed to validate the prediction model guidance available to NHC and for diagnostic studies of the physical processes of outer wind structure changes The purpose of the inner-wind structure studies during the pre-landfall period is to improve the TC intensity prediction at the time oflandfall for use by the local emergency managers in setting the size of the evacuation area. Furthermore, it is a challenging scientific problem because of the nonlinearly interacting physical processes contributing to the intensity change, i.e., inner core dynamics; upper atmospheric influences; and upper ocean influences. Thus, one approach is to collect additional comprehensive data sets in which all physical processes are observed simultaneously. A prototype for such an experiment is the NASA CAMEX 3 and HRD coordinated

-1.1.3- field programs during 1998, which is expected .to yield a unique data set for diagnostic studies of inner-wind structure changes. Similarly, the data set can serve as initial and verifying conditions for real-data simulations. Furthermore, modeling is needed to understand the roles of convection and the atmospheric (ABL) structures and fluxes in TC intensity change. Coupled ocean-TC models need to be intercompared with comprehensive ocean structure and TC structure data sets to understand the extent to which such models will be required for real-time application in inner-wind structure prediction (see section 1.3 below for more details). Finally, theoretical studies are needed to clarify key physical mechanisms and provide conceptual models for understanding the elements.

1.1.3 Landfall objectives

a) Wind structure Another challenging scientific problem with exciting opportunities for new observations is the wind structure changes during landfall, which is the critical period in which most of the damage from localized winds, tornadoes, and storm surge plus ocean surface wave run-up actually occur. This understanding is critical to the real-time assessment of damage that is necessary for organizing efficient and effective rescue and damage/economy recovery activities, and for the revision of building codes and other mitigation functions that will minimize future damage. An additional goal is to understand the physics of the additional track deflections associated with the modifications of. the TC structure during landfall that change the precise center coastal crossing point (see section ·1.4 below for more details). A major thrust in this topic is to exploit new observing technology to obtain land-based observations coordinated with off- observations to understand the boundary layer structure and surface flux changes as the TC moves from open ocean to coastal ocean to land. Data assimilation, high-resolution modeling and theoretical studies are essential to complement these observations.

b) Rainfall forecasts Within the general context of the Quantitative Precipitation Forecasting (QPF) focus area of the USWRP, specific items more directly related to TC landfall precipitation were identified. The highest priority item is an improved data assimilation system for making optimum use of existing observations, which include both -based cloud, , and wind observations and land­ based reflectivity. New research non-hydrostatic models with high resolution, cloud­ resolving physics need to be tested with real data (e.g., the CAMEX 3 and HRD field experiment data sets). Case studies are needed to document the precipitation processes in observed and simulated TC landfall events, including both coastal and inland (perhaps topographically forced)

-1.1.4- flooding. Validation studies ofthe NOAA Geophysical Laboratory (GFDL) model precipitation predictions are needed to provide guidance to the NHC forecasters.

1. 1.4 Socio-economic objectives

A broad socio-economic impact research objective is to explore and document the potential savings that may be allowed from improved analyses and forecasts of the track, wind structure (including intensity), precipitation, and the associated effects such as storm surge, and local wind damage including tornadoes. A systematic collection of cost data from industry, commerce, and transportation sectors is required. In addition, the user community needs to be surveyed as to their needs for warnings and what they would do differently with more accurate forecasts oftrack, wind structure, precipitation, etc. Similarly, the emergency management community needs and benefits from improved forecasts need to be documented.

1.1.5 Hindrances to the research plan

Finally, some hindrances to progress in TC landfall research were defined. Some of these hindrances are related to establishing better linkages between the research community and the NOAA National Center for Environmental Prediction (NCEP) and original data sources. Ongoing problems such as inadequate computing resources and deficiencies in observations need to be addressed, along with assuring that all existing observations are communicated to the analysis and forecast centers.

1.1.6 Summary

The research and forecasting communities have arrived at a critical juncture. The U.S. has nearly completed its modernization. Emerging technologies have matured to a state where observations and models need to be integrated into a consistent framework to improve forecasts and analyses of surface winds, rainfall, and ensuing storm surge and during TC landfall. A recurrent theme in the deliberations has been a synergism between observations and models in isolating physical processes. High-quality, high-resolution observations of atmospheric and oceanic fields are crucial to evaluate and eventually validate models of the atmospheric, oceanic, and coupled processes. Integration of various components of the TC forecast problem (e.g., improved physical understanding of the processes that cause damage and track and intensity changes) has the best chance for success.

-1.1.5- To acquire the necessary understanding and provide the initial conditions for improved prediction, a focused, comprehensive observing system in a translating storm-coordinate system is required. Combining proven instrumentation with a number of new instruments and platforms that have become available in the last few years, three-dimensional wind and thermodynamic data sets can be obtained whenever a major TC threaten the . , aircraft, expendable probes released from aircraft, and coastal, moored, and drifting surface platforms are needed to estimate the upper-ocean and currents, air-sea interactions, the distribution of storm surge, ABL winds, rainfall, and potential damage. In the TC core, airborne , supplemented by a suite of other instruments, measures tropospheric winds including critical areas of the ABL and region. Microwave systems estimate surface winds, ocean waves, and storm surge. On the TC periphery, dropsondes released from turboprop and jet aircraft map the atmospheric , humidity, and wind structure. Satellites acquire both storm-scale and surrounding environmental data. To take full advantage of these new observations, techniques need to be developed to objectively analyze these observations, and initialize models aimed at improving prediction of TC track and intensity from global-scale and mesoscale dynamical models. Multi-nested models allow prediction of all scales from the global, which determine long-term TC motion, to the convective­ scale, which affect intensity. Development of an integrated analysis and model forecast system optimizing the use of observations and providing the necessary forecast skill on all relevant spatial scales is required. Detailed diagnostic analyses of these data sets will lead to improved understanding of the physical processes of TC motion, intensity change, the ABL, and air-sea coupling. Ultimately, the aim is on development of real-time analyses of surface winds, , and ensuing storm surge a11d severe weather before and during landfall, to improve warnings and provide local officials with the information needed to focus recovery efforts in the hardest hit areas as quickly as possible. The USWRP and World Weather Research Program (WWRP) have identified landfalling TCs as a major focus of their research programs. It is in the national interest to mitigate damage that occurs after a TC makes landfall. Over the next decade, these issues will have a significant impact on building codes, construction technology, preparedness lead times, and evacuation procedures, all aimed at saving and minimizing property damage. Even if the improvement in intensity predictions is only a few percent per year, the benefit to cost ratio is high, leading to an improvement over the present state of understanding and forecasting. Obviously, such an ambitious approach will require strong leadership from local, state, and federal government officials with demonstrated track record of accurate, timely forecasts. Thus, as a community including both scientists and forecasters, these improvements must be put in place to steadily improve required predictability by the public over the next decade.

-1.1.6- References

Elsberry, R. L., and F.D. Marks, 1998: U. S. Weather Research Program Hurricane Landfall Workshop Report, in preparation.

Emanuel, K., D. Raymond, A. Betts, L. Bosart, C. Bretherton, K. Droegemeir, B. Farrell, J.M. Fritsch, R. Houze, M. LeMone, D. Lilly, R. Rotunno, M. Shapiro, R. Smith, and A. Thorpe, 1995: Report of the first prospectus development team of the US Weather Research Program to NOAA and the NSF. Bull. Amer. Met. Soc., 76, 1194-1208.

Marks, F. and L.K. Shay, 1998: Landfalling tropical cyclones: Forecast problems and associated research opportunities: Report of the 5th prospectus development team to the U.S. Weather Research Program, Bull. A mer. . Soc., 79, 1-19.

-1.1.7-

Topic 1.2 Wind transients, vertical structure and gust factors in tropical cyclones

Rapporteur: P. G. Black NO AA!AOML Hurricane Research Division Miami, FL 33149) email: [email protected] Fax (1) 305-361-4402 Working Group: F. Marks, M. Powell, J. Kepert, S. Ready

1.2.1. Introduction

It is an important international priority to improve the forecasts of surface wind field, intensity, structure and storm surge in landfalling tropical cyclones (TC) in order to successfully mitigate the detrimental physical impacts associated with these . Coastal population growth in the U.S. of 4-5% per year, is outpacing the historic 1-2% per year rate of improvement in official hurricane track predictions. While specific track prediction models have indicated a 15-20% improvement over the past 2-3 years, very little skill has been shown in the prediction of intensity change or wind field distribution (Neumann et al 1997). For this reason, the average length of coastline warned per storm, about 570 km, has not changed much over the past decade, nor has the average overwarning percentage, about 75%. However, the average preparation costs have increased eight­ fold in the past 7 years from $50M per storm in 1989 (Sheets 1990) to an estimated $300M per storm in 1996, or about $1M per mile of coastline warned (Jarrell et al1992; Neumann et al 1997). The increasing potential for severe loss of as coastal populations soar, and potential monetary losses of tens of billions of dollars requires that greater effort be directed to understanding all physical processes which play an important role in modulating TC wind fields and storm surge at landfall. A major source of difficulty in past efforts to predict TC intensity, wind fields and storm surge at landfall has been the inability to measure the surface wind field directly and the inability to predict how it changes in response to external and internal forcing. The surface wind field must presently be estimated from a synthesis of scattered surface ship and/or buoy observations and aircraft measurements at 1.5 km to 3.0 km altitude (Powell 1980; Powell et al 1996; Powell and 1996). This task is complicated by variations with height ofthe storm's structure, such as the change with height of storm-relative flow due to environment~ll and to the variable outward tilt of the wind maximum with height. Numerous studies of TC structure have been made previously using various combinations of and in situ data. Numerous studies of TCs have been made concentrating on

-1.2.1- NOAA aircraft data, ground based radar and surface damage reports, e.g., Willoughby and Black (1996) and Black and Marks (1991). Based on NOAA aircraft observations. eyewall and attendant convection were associated with intense , vertical air motions in excess of 20 m s-1 at cloud base and momentary loss of aircraft control during low-level penetrations in rapidly developing Hurricanes Gladys (1975) and Hugo (1989). perturbations were deduced with magnitudes on the order of Midwest mesocylones. Land based radar observations and damage surveys after (1992) suggested that enhanced swirling motion associated with convectively initiated eyewall mesovortices may have been responsible for the intense inland wind damage associated with landfall in South (Wakimoto and Black 1994; Willoughby and Black 1996). Convective perturbations along the inner edge of the outer eyewall of Paka (1997) led to three distinct destructive wind surges over the northern half ofGuam. An in-depth examination of 3-5 s gust factors at 10-m level derived from a number of sources suggest that separate populations of gust factors are associated with mid-latitude storms, over­ water TC conditions and over-land TC conditions. The mean over-water gust factors for 1-min, 10-min and 30-min measurement duration in TC's were found to be 1.23, 1.40 and 1.48, respectively. The ratio of average maximum wind estimates in West Pacific TCs from the post-1970 data set over those from the pre-1970 data set, is found to be about the same as the ratio between flight­ level (cloud base) winds and surface winds, an average of 0.80 (Powell, 1980). Recent Global Positioning System (GPS) (GPS sonde) data. in the eyewalls of Hurricanes Guillermo (1997) and Erika (1997) in 1997 suggest that different ratios may apply in the TC outer region beyond the eyewall than in the eyewall itself. The difference in wind estimates between the two time periods in western Pacific may be due to the failure to take into account the decrease of the wind with height in the boundary layer of the TC for the pre-1970 era. Black (1993) suggested that the TC gust factor increased with height at the same rate that the mean wind decreases with height in the TC atmospheric boundary layer (ABL ), implying that convection conserves in the ABL, maintaining a nearly constant peak gust with height. The recent Guillermo and Erika GPS sonde data have shown that the vertical structure in the ABL is much more complex than previously thought containing multiple wind maxima. Multiple wind maxima in the ABL was first observed by Wilson (1979) with instrumented tower observations.

-1.2.2- 1.2.2. Eyewall mesovortex structure

In Hurricane Andrew (1992), an eyewall replacement cycle developed as the storm moved across the . The Miami WSR-57 radar was able to observe the dissipation of the old eyewall over simultaneously with its replacement by a larger eyewall as PC increased temporarily. A slow contraction ofthe new eyewall followed and two centers ofconvectiveactivity formed in the eyewall and rotated around the . As the storm approached Miami, two distinct cyclonic mesa-vortices could be seen adjacent to the convective on opposite sides of the eyewall and which rotated about a common center in the middle of the eye. This indicated a predominance of wave number-2 asymmetric structure which rotated about the center. However, as the eyewall approached the coast and came into view of the Tampa WSR-57 radar, as well as the Melbourne WSR-88D radar, a transition was observed from a mobile wave number-2 asymmetric flow to a fixed wave number-1 asymmetric flow relative to the translating center, even as the intense winds destroyed the Miami WSR-57 radar. The Tampa WSR-57 radar, which was being recorded digitally by a team from the Hurricane Research Division, scanned the eyewall at 30-sec intervals, covering an altitude band of 3-8 km. The WSR-57 revealed that a series of 7 intense raincells developed in the north portion of the eyewall as it made contact with the coast, moved across south Dade and dissipated in the south portion of the eyewall, south of Homestead, FL, during the 45 min required for the eyewall to move ashore. Development of the intense raincells, which began as the eyewall made contact with the coast, continued at intervals of 3- to 7-min, and ended once the eyewall was completely over land. Each had a life cycle of 8-15 min growing in intensity from 30 dBZ (~I 0 mm h-1) to 50 dBZ (~150 mm h-1). The rapid growth in cell intensity implied intense updrafts within the cells. These updrafts, imbedded within the background TC-scale vorticity, enhances the vorticity locally on.the scale of the updraft by means of vortex tube stretching and tilting ofhorizontal vorticity into the vertical. Thus, meso-vortices, similar to that associated with the tornado cyclone in rotating severe thunderstorm cells, were probably produced in Andrew's eyewall in association with each intense raincell. Through the process of geostrophic adjustment, the field adjusts rapidly to perturbations in the wind field on the mesoscale which are larger than the Rossby radius of deformation (on the order of 10 km for TC eyewalls). Hence, as the perturbation vortex grows with the raincell, an accompanying pressure perturbation develops near the inner edge of the eyewall, where the updraft is located. Anecdotal evidence for the existence of this pressure perturbation was obtained from two sources. First, by pure serendipity, the NOAA 11 polar­ orbiting spacecraft passed over at a time exactly midway through the raincell production process as the eyewall was moving onshore. The 1-km resolution infrared image seen

-1.2.3- on the cover resulted, which revealed a "hot " in the image, not in the center of the eye, as normally expected, but offset from the center immediately adjacent to the eyewall convection. The second serendipitous observation was made by a private citizen, who observed their minimum pressure reading for the storm from a home , later calibrated, which was situated exactly within the warmest pixel of the image and within 3 min of the satellite image time. Four additional private pressure observations in the nearby area confirmed the existence of a pressure perturbation along the edge of the eyewall that was 9 hPa lower than the minimum pressure in the center of the eye observed by Air Force reconnaissance aircraft at nearly the same time. Such a meso-vortex was actually penetrated by research aircraft at low altitude (300 m) in (1989), obtaining detailed flight-level measurements. The Hugo aircraft flight revealed a classic meso-vortex signature with a wind speed perturbation of 30 m s-1 and a pressure perturbation of 13 hPa and was tracked for 5 revolutions about the eye center during a 1.5 h period as 5 separate cells developed and decayed, an almost identical scenario to Andrew. An eyewall mesovortex was first photographed from above by a U-2 aircraft in (1958) (Fletcher et al1961). The eyewall actually folds into the eye as it attempts to partially wrap around the meso vortex in Ida. Both Ida and Hugo shared a common characteristic with Andrew in that they were both very intense and deepening rapidly. One might conclude thatmeso-vortices and their attendant enhanced and streaky surface winds are a characteristic of intense, deepening storms, and that, should this occur as landfall approaches, then intense inland wind damage is to be expected.

a) Hurricane Hugo On 15 September 1989, a NOAA WP-3D made the first aircraft penetration into Hurricane Hugo at an altitude of 450 m when it was ~500 km northeast of Barbados. The aircraft encountered intense turbulence and suffered the loss of an engine. Data from this aircraft showed that it penetrated a mesoscale vortex imbedded in the inner edge of the eyewall adjacent to intense convection. The mesovortex had a diameter of 1 km at 500 m altitude which is a factor of 10 smaller than the eye diameter. This is about the same ratio as suction vortex size to its parent tornado (Fujita, 1971). The 1 km is also 10 times smaller than a tornado's parent and about 5 times larger than a tornado. The wind and pressure perturbations about a linear pressure and wind speed profile are 30 m s-1 and 12 hPa, respectively. The former is about a factor of 3 smaller than the eyewall maximum winds. In contrast, suction vortex winds are about a factor of 2 greater than tornado rotational winds (Fujita 1971 ). The perturbation vorticity is of order 1o-I s-1, an order of magnitude greater than eyewall or mesocyclone vorticity and about the same as tornado vorticity.

-1.2.4- During 14 subsequent orbits of the eye the aircraft penetrated the vortex on nine different occasions with diameters of 1-2 km. The vortex penetrations, at gradually increasing altitude, showed that the vortex's maximum strength and smallest diameter appeared to be at cloud base and broadened with height. The vortex rotated around the center of circulation with a period of 19 min, implying a translation speed of ~30 m s-1. This was the first time that an aircraft had penetrated such a phenomena in a TC, although vortices of this type have been observed, and are probably more common than previously believed (e.g., Fletcher et al 1961; Marks and Houze 1984; Muramatsu 1986; Bluestein and Marks 1987).

b) During the daylight hours of 6 September 1995, 1-km resolution GOES-9 1-min interval images and GOES-8/9 15-min stereo images were obtained over Hurricane Luis. These data were concurrent with NOAA WP-3D one-min horizontal and vertical airborne radar scans. These observations revealed a series of 8 cycloidalloops. The low-level center is indicated by the primary low-cloud swirl, while the upper-level center is indicated by the center of the rim and the infrared maximum temperature. Secondary low-cloud swirls appeared to rotate around the primary swirl. The cycloidalloops had an average period of 90 min and an average diameter of 20 km. The upper eye diameter averaged 60 km, while the low-level diameter averaged 40 km. The upper..,level center was displaced to the east-southeast of the primary low-cloud swirl by an average of 25 km. Overshooting convective turret clusters were observed in the eyewall with similar 90- . min periods and duration averaging 20 min. One event, corresponding to the first WP-3D eye penetration, was analyzed in detail using the radar and one-min GOES-9 data. Several minutes after rapid increase in radar reflectivity occurred in the northeast eyewall sector, a cluster of overshooting turrets was observed emerging above the cirrus shield in the north eyewall sector. The cluster continued to bubble as the turret and reflectivity maximum rotated around to the west eyewall sector. Strong evidence of cyclonic rotation of the turret elements was observed during the peak overshoot period. Simultaneously, the GOES-8/9 stereo imagery revealed that thin cirrus elements at the upper edge of the eyewall were being entrained into the eye as they descended toward the primary low-level center adjacent to the overshooting turret cluster. During this period, the low-level center was migrating toward the eyewall sector where the turret cluster was observed.

c) Synopsis Features observed here are very similar to those encountered during a NOAA WP-3D flight into Hurricane Hugo (1989) (Black and Marks 1991). The storm structure was somewhat different in that case with a smaller eye than Luis, but the intense convection was clearly associated with a mesovortex which was separable from the intense background vortex circulation. The mesovortex

-1.2.5- had perturbation pressure on order of 15 hPa and wind perturbation on order 30 m s-1. Vertical reflectivity structure was similar to that in Luis with intense cores sloping outward and upstream. The mesovortex in Luis never penetrated to low levels, and did not appear to be as strong. However, the 6 revolutions about the eyewall in Hugo were very similar to the 8 loops observed for Luis. Only the revolution period was longer for Luis (90 min) compared to Hugo's (20 min) due to the larger eye diameter (60 km vs. 14 km respectively). A schematic composite of the flow fields, depicted in Fig. 1.2.1, illustrates how the me so vortex flow is superimposed upon the TC scale flow.

1.2.3. Recent ABL observations in Hurricanes Guillermo and Erika (1997)

The successful deployment of the new GPS sondes with reliable wind, temperature, humidity and pressure measurements every one-half second to within less than 10 m of the surface were made for the first time from the NOAA WP-3D and Gulfstream IV (G-IV) aircraft in the inner core of eastern Guillermo (1997). Additional deployments were made on three days in Atlantic (1997). Preliminary analysis of a few ofthe eyewall soundingshave revealed several new and unusual ABL structures. Fig 1.2.2 shows three normalized eyewall ABL soundings of potential temperature (8), specific humidity (q) and wind speed (WS) from Hurricanes Guillermo and Erika The vertical coordinate is height normalized by the depth of the ABL,. defined as the depth of the constant e layer (~300 m). These eyewall soundings indicate three consistent low-level wind maxima at approximately 0.7 normalized height units ( ~200 m), at 1.4 units (~400 m), and at 2.5 units (~ 750 m). The lowest wind maximum appears just above the top of a constant q layer, whose depth is shallower than the constant e layer. The other two wind maxima occur at and just above the constant e layer. These soundings are typical of other eyewall soundings that show multiple wind maxima in the ABL and exhibit a shallow constant q layer. The latter corresponds with a layer of nearly constant (not shown), and may be representative ofthe . These wind observations are consistent with Australian tower observations in the inner, high-wind core of tropical cyclones first reported by Wilson (1979) and recently discussed by Kepert and Holland (1997). These data showed a low level wind maximum consistently at the 60-m level, as well as a wind maximum near the to of the 400-m tower. The observation of a thin layer of elevated specific humidity in the high wind region beneath the eyewall is very suggestive of a spray layer which may enhance evaporation in the >90% relative humidity air. The existence of a wind maximum above this layer indicates that upward vapor fluxes in the ABL may be controlled more by shear-induced turbulence at the top of the high specific humidity layer than by direct flux from the sea. Sea spray processes such as discussed by

-1.2.6- Fairall et al {1994) may become important. Additional analysis of this revolutionary new data source should lead to profound new insights into the workings of the TC eyewall ABL.

1.2.4. Summary

A unique combination of one-min interval super-rapid-scan GOES-9 images and NOAA aircraft data provides unprecedented insights into the dynamics of eyewall convective outbreaks, eye motion, and low-level cloud motion in the eye. The mesovortices observed in concurrent GOES/Aircraft one-min animations appear to be similar to features of eyewall mesovortices observed by the flights into Gladys (1975) and Hugo (1989) and to observations in Andrew (1992). It is hypothesized that the mesoscale vortices are advected by the TC scale vortex causing significantly higher winds in regions where the wind of the two vortices are near their maxima and in the same direction. The insights gained from this work have potentially far reaching implications as to an improved understanding of the track and intensity fluctuations associated with these features and to strategies for mitigating attendant damages at landfall.

References

Black, P.G., 1993: Evolution of maximum wind estimates in . Tropical Cyclone Disasters, J. Lighthill and Z. Zhemin, eds., Peking University Press, Beijing, PRC, 104-115

Black, P. G. and F. D. Marks, Jr., 1991: The structure of an eyewall meso-vortex in Hurricane Hugo (1989). Proc. 19th Coriference on Hurricanes and Tropical Meteorology, AMS, Miami, FL, 579-582.

Black, P. G. and L.K. Shay, 1998: Observations of tropical cyclone intensity change due to air­ sea interactions. Proc. Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, 'AMS, 161-168.

Bluestein, H.B., and F.D. Marks, 1987; On the structure of the eyewall of (1984): Comparison of radar and visual characteristics. Mon. Wea. Rev., 115, 2542-2552.

Fairall, C. W., J. D. Kepert and G. J. Holland, 1994: The effect of sea spray on surface over the ocean. The Global and Ocean System, 2, 121-142.

-1.2.7- Fletcher, R.D., J.R. Smith, and R.C. Bundgaard, 1961: Superior photographic reconnaissance of tropical cyclones. Weatherwise, 14, 102-109.

Fujita, T. T., 1971: Proposed mechanism of suction spots accompanied by tornadoes. Proc. 7th Conference on Severe Local Storms, AMS, Kansas City, MO, 208-213.

Hasler, A.F., P.G. Black, V.M. Karyampudi, M. Jentoft-Nilsen, K. Palaniappan, D. Chesters, 1997: Synthesis of eyewall mesovortex and supercell convective structure in Hurricane Luis with GOES 8/9 stereo, concurrent 1-min GOES-9 and NOAA airborne radar observations. Proc. 22nd Conference on Hurricanes and Tropical Meteorology, AMS, Ft. Collins, CO, 201-202.

Jarrell, J. D., P. J. Hebert and M. Mayfield, 1992: Hurricane experience levels of coastal county populations from Texas to Maine. NOAA Tech. Memo. NWS NHC-46, 152 pp.

Kepert, J. D. and G. J. Holland, 1997: The Northwest Cape tropical cyclone boundary layer monitoring station. Proc. 22nd Conference on Hurricanes and Tropical Meteorology, AMS, Ft. Collins, CO, 82-83.

Marks, F .D., and R.A. Houze, 1984: Airborne Doppler radar observations in . Bull. Amer. Meteor. Soc., 65, 569-582.

Muramatsu, T., 1986: The structure of polygonal eye of a typhoon. J. Met. Soc. of , 64, 913-921.

Neumann, C., H. Nicholson, C. Guard, 1997: National Plan for Tropical Cyclone Research and Reconnaissance (1997-2002), FCM-P25-1997, Office of the Federal Coordinator for Meteorological Services and Supporting Research.

Powell, M. D., 1980: Evaluations of diagnostic marine boundary layer models applied to hurricanes. Mon. Wea. Rev., 108,757-766.

Powell, M. D., S. H. Houston and T. A. Reinhold, 1996: Hurricane Andrew's landfall in South Florida. Part 1: Standardizing measurements for documentation of surface wind fields. Wea. and Forecasting, 11,304-327.

-1.2.8- Powell, M. D. and S. H. Houston, 1996: Hurricane Andrew's landfall in South Florida. Part II: Surface wind fields and potential real-time applications. Wea. and Forecasting, 11, 329-349

Sheets, R. H., 1990: The National Hurricane Center, past, present and future. Wea. and Forecasting, 5, 185-232.

Wakimoto, R., and P.G. Black, 1994: Damage Survey of Hurricane Andrew and Its Relationship to the Eyewall. Bull. Amer. Meteor. Soc., 75, 2, 189-200.

Willoughby, H.E., and P.G. Black, 1996: Hurricane Andrew in Florida: Dynamics of a Disaster. Bull. Amer. Meteor. Soc., 77, 543-549.

Wilson, K. J., 1979: Characteristics of the subcloud layer wind structure in tropical cyclones. International Conference on Tropical Cyclones, , .

-1.2.9- Fig. 1.2.1. Illustration of mesovortex flow (blue concentric circles) superimposed upon the TC scale flow (red concentric circles). with maximum wind area in green (from Hasler et al 1997). e (K) 273 283 293 303 313 323 333 343 10 1 ~-r~~~~~~-r~~~~~~~~~~--~-T~~~~~

t .c...... Cl) 0.. "' (!) "0 "'0 .a 1o· 1 :;:::; aE

Fig. 1.2.2. GPS dropsonde profiles of specific hlimidity (q, red lines: units= g kg-1), potential temperature (8, black lines: units=°C) and wind speed (WS, blue lines: units= m s-1) for three typical profiles in the eyewall of Hurricanes Guillermo (thick solid lines) and Erika (thin dashed lines). Vertical coordinate is height normalized by the depth of the ABL defined as the depth of the constant e layer (~300 m).

Topic 1.3 Ocean processes excited by tropical cyclones

Rapporteur: Lynn K. Shay, MPO, RSMAS, University ofMiami, Miami, FL 33149 email: [email protected] Fax (1) 305-361- Working Group: I. Ginis, Duan Yihong

1.3 .1 Introduction

Warm, pre-existing sea surface temperatures (SST) (exceeding 26°C) are a necessary, but insufficient condition for tropical (Palmen, 1948). Once the TC develops and translates over the tropical , statistical models suggest that warm SSTs describe a large fraction of the variance (40 to 70%) associated with the intensification phase of the storm (e.g., DeMaria and Kaplan, 1994). However, these predictive models do not account either for the oceanic mixed layers having temperatures of 0.5 to 1°C cooler than the relatively thin SST over the upper meter of the ocean (30°C) or horizontal advective tendencies by basic-state ocean currents such as the Gulf Stream and warm core eddies. Thin layers of warm SST are well-mixed with the underlying mixed layer water well in advance of the storms where winds are a few meters per second. Thus, strong baroclinic features advecting deep, warm oceanic mixed layers represent moving reservoirs of high-heat content water available for the continued development and intensification phases ofthe TC. Beyond a first-order description of the lower boundary providing the heat and moisture fluxes derived from low-level convergence, little is known about the complex boundary layer interactions between the two geophysical fluids. ·

1.3 .2 Recent results on upper ocean processes in tropical cyclones

The use of surface current radars such as Over-the- Radar (OTH) (Georges et al 1993; Harlan et al1997) and the Ocean Surface Current Radar (OSCR) (Shay et al 1998a) have provided high quality surface currents in the coastal regime and in the deep ocean. These radars are based upon Bragg backscattering from surface waves having a wavelength of one-half the incident radar wavelength. Backscattered signals contain a first-order return for the surface current estimation whereas the wind directions and waves are embedded in the second-order return. Two stations, transmitting alternatively (at the US Navy facility in Texas and ), monitored the surface currents and wind directions at 15 km intervals from several hundred kilometers during Hurricane

-1.3.1- Hortense (Harlan et al 1997). As shown in Fig. 1.3 .1, surface currents diverged from the as part of the first upwelling cycle of the forced near-inertial response. The surface current response and cooling were also much stronger on the right side of the storm consistent with the rightward bias. Such radar measurements provide spatial context of the surface current response to tropical cyclones. moved over a warm core during a rapid deepening phase from 965 hPa to 916 hPa that was detected by radar altimeter data from the NASA TOPEX mission. Shay et al (1998b) found a net heat loss of about 30 Kcal cm-2 day-1 between pre- and post-Opal surface height anomaly fields derived from TOPEX relative to the depth of the 26°C isotherm (Fig. 1.3.2). This was well above the threshold of 4 Kcal cm-2 day-1 required for a tropical cyclone to maintain its strength (Leipper and Volgenau 1972). Unfortunately, in situ measurements of the and density structure were not acquired, thereby precluding any estimates of shear-induced mixing events at the mixed layer base and advective tendencies. Temperature structure measurements from the warm core eddy in indicated weak stratification in the upper layer of the eddy structure compared to the temperature structure outside ofthe warm eddy (Jacob et al1998). Warm water extends to depths of200 m in warm core eddies compared to outside these warm features, thereby requiring more and turbulence to cool and deepen the warm layers. An indirect confirmation of this effect was the Opal case where the net upper ocean cooling was about 1 to 1.5°C compared to about 3°C cooling away from the warm eddy feature observed from post AVHRR imagery (Shay et al1998b). More recently, Monaldo et al (1997) found sea surface temperature (SST) cooling in the wake ofHurricane Edouard of2-3°C from AVHRR imagery. The combination of the OTH and TOPEX with AVHRR-derived satellite products provide considerable insights into the upper ocean response to tropical cyclones. In addition, the satellite data must also be assimilated into the models for improved description ofthe pre-storm states for the prediction systems now underdevelopment (Ginis et al 1997). Basic process studies continue to provide valuable insights into the oceanic response problem. The three-dimensional TC-induced ocean current response was determined from velocity and temperature profiles acquired in the western Gulf of between 14-19 September 1988 during the passage of Hurricane Gilbert (Shay et al 1998c). In a storm-based coordinate system, along-track residual velocity profiles from 0 to 4 Rmax were fit to a least-squares model to isolate the near-inertial profiles (IP:::::30 h). Observed frequency shifts in the mixed-layer currents ranged from 1.03-1.05 f, where fis the local Coriolis frequency. This frequency was increasingly blue­ shifted in the upper 100 m to 1.1 f, but decreased towards f within the thermocline. These observed variations have yet to be confirmed with the numerical ocean response simulations due to neglect of background geostrophic flows.

-1.3.2- In addition to geostrophically balanced current fields, the forced upper ocean is ageostrophic and diabatic with vertical and horizontal mixing processes occurring in both the well-stirred mixed layer and in the thermocline. The vertical mixing generally determines the degree ofocean response to a tropical cyclone (e.g., Price 1981, Ginis and Dikinov 1989) which requires an accurate representation of the three-dimensional circulation patterns and the mixed layer physics. One such model is the Princeton Ocean Model (POM) developed by Blumberg and Melior (1987) that also contains a second-order turbulence closure scheme (Melior and Yamada, 1982). Ginis et al (1996) has tested the POM for the ocean response during Hurricane Gilbert (1988) using airborne field observations, and found good agreement with the temperature response (Shay et al1992). Using a diagnostic approach with the Gilbert observations, Jacob (1997) found that the geostrophic terms contributed significantly to the thermal balance, suggesting the importance of pre-storm variability. Jacob et al (1998) subsequently compared three mixing schemes and found similar patterns of entrainment mixing events although the locations of the maxima differed relative to the storm center. Mixing parameterizations continue to be an important topical area in modeling the oceanic response. Shay and Chang (1997) investigated the role of the free surface on the near-inertial current response and found a difference of 6 to 8 cm s-1 in the depth-averaged current and 4-5 cm sea level oscillations between a one-layer and 17-level models. Surface undulations are driven by the depth­ averaged nonlinear terms in the density equation, which represents an interaction between both the baroclinic and barotropic modes. Moreover, vertical velocity eigenfunctions at the surface indicate that the barotropic mode is at least a factor of 50 times larger than the baroclinic mode. Surface displacements by the barotropic mode have amplitudes of ~4 cm, explaining 90 to 95% of the height variations. One of the first confirmations of this effect was reported by Faber et al (1997) based upon observations acquired during the passage of off Cape Hatteras, where near-surface winds were 48 m s-1. As shown in Fig. 1.3.3, band-pass filtered directional wave spectral records from buoy 44014 indicated a modulation over inertial periods, which subsequently modulated the surface winds and significant wave heights (Fig. 1.3.3b,c). Using the Hurricane Gilbert profiles, the WKB-scaled clockwise (CW)-rotating vertical wave number spectra exceeded the counterclockwise(CCW)-rotating components by an average factor of 3.6 (Shay 1997). The downward vertical energy fluxes associated with these CW-rotating motions averaged 2 ergs cm-2 s-1. Globally, this near-inertial wave power is 0(1012 W), which is 30 times larger than the theoretical estimate of Nilsson (1995). Thus, the tropical cyclone wind field inputs considerable energy and power into the global internal wave field, and is as efficient as fronts and eddies in focusing the energy. Equating the observed flight-level winds reduced to 10 m to the near-inertial wave energy flux suggested a nonlinear relationship. Comparisons of this nonlinear function to the drag coefficient based on wave age (Donelan, 1990) were favorable between wind

-1.3.3- speeds of 12 to 20 m s-1. For wind speeds beyond 28 m s-1, this function leveled offto a nearly constant value in contrast to a monitonically increasing wind-speed dependent drag coefficient. Moreover, at the high winds of 40 m s-1, the drag coefficient equaled the monitonically increasing drag coefficient of the WAMDI group. Ocean response and storm surge models utilize a linear wind-speed dependent drag coefficient, which may not necessarily provide the most accurate surface wind stress at high winds. Recent field investigations of the air-sea fluxes (e.g., Hare et al 1997, Reider et al 1998) indicate that the surface fluxes may be significantly influenced by surface gravity waves (also suggested by Fig.l.3.3). Cione and Black (1998) documented the sea-air temperature differences during storms from NDBC buoys and found sizable differences of 3-4°C in the eye-wall region. This result suggests that the current values of a 1-2°C sea-air temperature differences used in numerical models may not reflect reality. Based on the bulk formula this is a doubling of the flux. Black and Shay (1998) also found a nonlinear sea-air temperature difference dependence on wind speed where the maximum difference of 4 °C occurred between wind speeds of25-30 m s-1. To address these questions, surface wave effects and their impact on air-sea fluxes require both observations such as the Scanning Radar Altimeter (Walsh et al 1985) and wave prediction models such as W AMDI coupled to oceanic and coupled models. Presently, one of the key questions is to what extent the explicit inclusion of surface wave models will influence simulations of the ocean response and air-sea interactions. Accurate simulation of the interaction between a tropical cyclone and ocean also requires the use of a very high-resolution atmosphere-ocean model with a series of nested grids with the finest resolution on the order of 10 km or even higher. This capability is especially important for coupled tropical cyclone-ocean simulations in the eastern and western . because it is not computationally practical to use a single very high-resolution mesh over the entire basin. Ginis et al (1998) and Rowley and Ginis (1998) have recently developed a movable nested mesh ocean model used in the GFDL/NOAA TC prediction model. As shown in Fig. 1.3.4, the ocean response to is simulated in the western North Pacific by initializing a large-scale general circulation model. For tropical cyclone mode ling a triply nested configuration was introduced (1 °, 1/3°, and 1/6°), in which the nested meshes of higher resolution in storm-based coordinates.

1.3.3 Roadblocks

One major obstacle in improving our understanding of the upper ocean response during high wind conditions has been the lack of simultaneous three-dimensional oceanic and atmospheric observations. These observations are crucial for the oceanic and coupled models. If careful attention is not given to acquiring detailed observations that the models could use, these models

-1.3.4- may be simulating the correct SST response, but have momentum budget that may be out of balance. Concurrent samples of the vertical structure of current, temperature, salinity and density in the ocean must be acquired with surface measurements of surface winds and stresses, directional surface waves and detailed vertical structure measurements of the atmospheric boundary layer (ABL). These in situ measurements will provide data to test modeling approaches and the inherent parameterizations that are required to couple the models. And, when combined with satellite and land-based measurements and well-constructed numerical models, the approach offers the best hope of understanding the upper ocean processes and the vigorous air-sea interactions that occur under high-wind conditions. From a modeling perspective, accurate simulations require realistic initialization of the subsurface ocean structure. However, there is currently no real-time subsurface ocean data in advance of TCs that are operationally available. Thus, the ocean model initialization remains one of the major foci of the ocean modeling research. The satellite data from ERS-1, TOPEX and AVHRR will have to assimilated into the models. To improve the forecast skill, systematic measurements of the three-dimensional ocean structure are required before, during and after the storm passage. Data validation will help to identify the weaknesses and strengths in our present understanding of the coupled TC-ocean system and improve empirical relationships at the very high winds. Thus, the community is poised to begin such an endeavor with the implication that this United States Weather Research Program could be used globally in understanding the upper ocean's role in the air-sea interactions and the intensity change.

1.3 .4 Opportunities

Over the past few years, recognition of this ocean effects problem has increased considerably. The interaction between the ocean and tropical cyclone is largely controlled not only by the SST, but also by other properties of the upper ocean, such as the mixed layer depth, heat content, the stratification in the thermocline, and upper ocean currents and their shears. Under the auspices of the US Weather Research Program (Marks and Shay 1998), research opportunities to examine upper ocean response and air-sea interactions continues to improve with respect to the ongoing programs at the Hurricane Research Division ofNOAA. Such experimental opportunities include observing the spatially evolving directional wave spectra when oceanic current and density profilers are deployed in the directly-forced regime. The new GPS sondes deployed from the aircraft would provide the three-dimensional synoptic flow structure which could be correlated to the upper ocean structure. Finally, remotely sensing the surface winds and stresses at the same time would greatly enhance our ability to examine the coupled interactions. All of these measurements would be embedded in the satellite-based oceanic and atmospheric fields thereby

-1.3.5- providing the spatial context. Modeling efforts continue to be developed for initialization schemes and movable mesh grids in storm coordinates. These efforts will have to ingest these types of data to fully exploit model deficiencies and improve model parameterizations, both of which are ultimately aimed at forecasting intensity change at the operational centers.

References

Black, P. G., and L. K. Shay. 1998: Observations of tropical cyclone intensity change due to air­ sea interactions. Proc. Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, AMS, 161-168.

Blumberg A.F. and G.L. Melior, 1987: A description of a three-dimensional coastal ocean circulation model. Three-Dimensional Coastal Ocean Models, Coastal Estuarine Sci., 4, edited by N.S. Heaps, AGU, 1-16.

Cione, J., and P. G. Black, 1998: Surface thermodynamic observations within the tropical cyclone inner core. Proc. Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, AMS, 141-145.

DeMaria, M., and J. Kaplan, 1994: Sea surface temperature and the maximum intensity of Atlantic tropical cyclones. J ofClimate, 7, 1324-1334.

Donelan, M., 1990: Air-sea interaction. The Sea: Ocean Engineering Series, 9, John Wiley and Sons Publishing Company, , 239-292.

Faber, T., L. K. Shay, S. D. Jacob, S. H. Houston, and P. G. Black, 1997: Observed air-sea interactions during Hurricane Emily. Proc. 22nd Conference of Hurricanes and Tropical Meteorology, Fort Collins, CO, AMS, 433-434.

Georges, T. M., J. A. Harlan, L. R. Meyer, andR. G. Peer, 1993: Tracking Hurricane Claudette with the US Air Force over-the-horizon radar, J Atmos. and Ocean Tech., 10, 441-451.

Ginis, I., Bender, M.A., and Kurihara, Y., 1997: Development of a coupled hurricane-ocean forecast system in the North Atlantic. Proc. 22nd Conference on Hurricane and Tropical Meteorology, Fort Collins, AMS, 443-444.

-1.3.6- Ginis, 1., and K.Z. Dikinov, 1989: Modeling of the typhoon Virginia (1978) forcing on the ocean. Meteorology and , 7, 53-60.

Ginis, I., R.A. Richardson, and L.M. Rothstein, 1998: Design of a multiply - nested primitive equation ocean model. Mon. Wea. Rev., 126, 226-251.

Ginis, I., L.K. Shay, L.M. Rothstein, and S.A. Frolov, 1996: Numerical simulations ofthe ocean response to Hurricane Gilbert using airborne field observations, Proc. 8th Conference on Air­ Sea Interaction and Symposium on GOALS, , GA, AMS, 33-37.

Hare, J. E., T. Hara, J. B. Edson and J. Wilczak, 1997. A similarity analysis of the structure of air flow over surface waves. J Phys. Oceanogr., 2 7, 1018-1 03 7.

Harlan, J., P. G. Black, and T. M. Georges, 1997: Observations ofHurricane Hortense with over the horizon radars. Geophys. Res. Letters, (in press)

Jacob, S. D., 1997: Effects of oceanic mesoscale variability on SST response: implications for intensity change. Proc. 22nd Conference of Hurricanes and Tropical Meteorology, Fort Collins, CO, AMS, 435-436.

Jacob, D. S., L. K. Shay, A. J. Mariano, and P. G. Black, 1998: The mixed layer heat balance during Hurricane Gilbert. J Phys. Oceanogr (submitted)

Leipper, D., and D. Volgenau, 1972: Hurricane heat potential of the . J Phys. Oceanogr., 2, 218-224.

Marks, F. and L.K. Shay, 1998: Landfalling tropical cyclones: Forecast problems and associated ·research opportunities: Report of the 5th prospectus development team to the U.S. Weather Research Program, Bull. Amer. Meteor. Soc., 79, 1-19.

Melior, G. and Yamada, T., 1982: Development of a turbulence closure model for geophysical fluid problems, Rev. Geophys. Space Phys., 20, 851-875.

Monaldo, F., T. D. Sikora, S. M. Babin, and R. E. Sterner, 1997: of sea surface temperature cooling in the wake of , Mon. Wea. Rev., 125, 2716-2721.

-1.3.7- Nilsson, J., 1995: Energy flux from traveling hurricanes to the oceanic internal wave field. J. Phys. Oceanogr., 25, 558-573.

Palmen, E., 1948: On the formation and structure oftropical cyclones. , 3, 26-38.

Price, J.F., 1981: Upper ocean response to a hurricane. J. Phys. Oceanogr., 11, 153-175.

Rieder K.F., and J. A. Smith, 1998. Removing wave effects from the wind stress vector. J. Geophys. Res., 103(Cl), 1363-1374.

Rowley C., and I. Ginis, 1998: Implementing a mesh movement scheme into a multiply-nested ocean model and its application to air-sea interaction studies. Submitted to Mon. Wea. Rev.

Shay, L. K., P. G. Black, A. J. Mariano, J. D. Hawkins, and R. L. Elsberry, 1992: Upper ocean response to Hurricane Gilbert. J. Geophys. Res., 97, 20,227-20,248.

Shay, L. K., 1997: Near-inertial wave energy fluxes forced by tropical cyclones. Proc. 22nd Conference ofHurricanes and Tropical Meteorology, Fort Collins, CO, AMS, 437-438.

Shay, L. K., and S. W. Chang, 1997: Free surface effects on the near-inertial ocean current . response excited by a hurricane. A revisit. J. Phys. Oceanogr., 27, 23-39.

Shay, L. K., S. J. Lentz, H. C. Graber, and B. K. Haus, 1998a: Current structure variations detected by HF radar and vector measuring current meters. J. Atmos. and Ocean. Tech., 15, 237-256.

Shay, L. K., G. J. Goni, F. D. Marks, J. J. Cione, and P. G. Black, 1998b: Role ofwarrn ocean features on intensity change: Hurricane Opal. Proc. Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, AMS, 131-13 8.

Shay, L. K., A. J. Mariano, S. D. Jacob, and E. H. Ryan, 1998c: Mean and near-inertial current response to Hurricane Gilbert. J. Phys. Oceanogr., 28, (in press)

Walsh, E. J., D. W. Hancock Ill, D. E. Hines, R. N. Swift, and J. F. Scott. 1985: Directional wave spectra measured with the surface contour radar. J. Phys. Oceanogr., 15, 566-592.

-1.3.8- - Surface Currents

Surface Current Magnitude {m/9) 0.{)I

' ' ...... -~- ,' ...... ,...... Door' .. .. ~~:;,-.; .... ~ ...... :...... -...... ------.. ; ...... -- 0 I ' ' 1 : : :

' ' .0 'I I () ...... ·--Y· ...... ---:-.... ·-· ...... : ...... ·-· ---l--- ·-· .... ·-· ·-· -i- ·-· ·-· .. 2:4 . ' .

: ..:::::-- : ---- ...... --- ...... ----- .. , __ --- ...... ' --- ...... ·- 1

>) ~ - ' ' ,_ . 0 I ...... --- ...... ------~-- ...... -:- ...... -...... --- ~-~- --· ·-· ·-· .t. ·-· ·-· ..22. . ' :' :' ' I 9,> P : I ·~;i_- '-=~ ·!~' _: ~ li- lf"t ' ' ' ---· ...... :-·-! · ·...... ·-r: --- ·__ .. --· ·-· --- :-·: ·--- ...... -- ...... ,'...... --- ...... I I ' ' ' . ' ' I I I' I 7:S Wl 71 w; 69 W: I' : : I

Fig 1.3.1. OTH-derived surface current patterns (arrows) relative to the track of hurricane Hortense using six hour positions (boxes) and the position of Hortense at the time of the radar image at 1600 UTC on 12 September 1996 (from Harlan et al 1997). SHA(cm) 30 30 N

25 N 0

20 N -30

30 N

25 N 0

20 N -50

Q(Kcal cm-2 ) 40 30 N 20

25 N 0

-20

20 N -40 100 w 95 w 90 w 85 w 80 w

Fig. 1.3.2. The change in a) surface height anomalies (cm), 2 b) Upper layer thickness (m), and c) heat content (Kcal cm- ) found by differencing the pre-storm and post-storm field relative to the track of Opal. -1 0 1 2 3 4 5 6 7 ....,.--.... 40 I m 30 g 20

(/) 10 ~ 0 10 2 1 8 I I 1.5 I -.1/\ I 6 6 I ,I 1\ ,,, I "--" \ 'J \"· \ 1 - - I ,, (/) 4 ~ ' ,,, I --,' \. . '," ' J I .,, 0.5 ::c 2 .., \\. r I 0 o· 0.4 'N' 0.3 "--"~ 0.2 3 0.1 0.0 =~§>a -1 0 1 2 3 4 5 6 7 Time (IP)

Fig. 1.3.3. of a) wind speed b) significant wave height (dashed: represents exaggerated scale after one inertial period), and c) contoured wave spectral energy density (0.05 to 2 m2 Hz -1) from an NDBC buoy. Notice that the time series is scaled by inertial period (20 h). Typhoon Roy (1988) 00 UTC 14 Jan 25

20

5

140 150 160 170 180 Longitude 0 E

24 25 26 27 28 29 30 oc

Fig. 1.3.4. Numerical simulation of the ocean response to Typhoon Roy (1988) using a triply nested movable mesh ocean model: velocity vector plotted over the SST at 0000 UTC 14 January. Only the area near the storm track is shown. The meshes have the following resolution: 1°, 1/3°, and 1/6°. Topic 1.4 Modification of track at landfall

Rapporteur: Edward N. Rappaport National Hurricane Center 11691 SW 17th St. Miami, FL 33165-2149 email: [email protected] Fax (1) 305-553-1901 Working Group: L. A vila, A. Soulain, Xu Xiangde

1. 4.1 Introduction

The pernicious variations in tropical cyclone track induced by land pose problems that compound the challenges of tropical cyclone track forecasting (cf., Brand and Blelloch 1973, Zehnder 1993b, Elsberry 1995). Alterations as small as the width of the eyewall have enormous local implications that are exacerbated near elevated terrain (e.g., Tseng and Yang 1997). Track excursions from a warned area result in reduced forecast credibility and diminished public response in subsequent events. Disastrous consequences can occur when a tropical cyclone veers to a landfall location having little or no advance warning. These variations sometimes begin hundreds ofkilometers from land (Zehnder 1993b) and can account for much of the track forecast error (e.g., Bender et al1987). Observational studies have established the magnitude of track changes and identified some acutely affected areas (e.g., Brand and Blelloch, 1973, 1974). Discontinuous tracks have also been described (e.g., Chang 1982; Y eh and Elsberry, 1993b). Laboratory experiments (Pao and Hwang 1977) and theoretical treatments (e.g., Chang 1982; Bender et al 1985, 1987; Yeh and Elsberry 1993a,b; Zehnder 1993b; Zehnder and Reeder 1997; Kuo and Wang 1997) have added to our understanding, but much remains unresolved (e.g., Lin et al 1996; Wu 1997) and competing hypotheses persist about the governing processes (Hirschberg et al 1997). Nevertheless, some observations and modeling results appear broadly applicable. Significant track anomalies are not generally attributed to relatively flat land (e.g., Tuleya and Kurihara 1978; Tuleya et al1984; Bender et al 1987). Indeed, both empirical and model results are limited to a few areas with extensive(> 100 km X 300 km) barriers containing peak elevations ;:::1 km not far from the coastline.

-1.4.1- 1.4.2 Regional idiosyncrasies

1.4.2.1 Island environment Research has emphasized westbound tropical cyclones approaching , The and . These islands have comparable physical characteristics, and lateral dimensions similar to the size of the inner circulation of cyclones (Zehnder 1993b).

a) Taiwan--continuous tracks Taiwan's Central Mountain Range provides the setting for one of the few published rudimentary forecast guides (Brand and Blelloch 1973) on alteration of track near land. For storms approaching the northern (southern) part of Taiwan from the east, a poleward (equatorward) deflection of the center can begin > 200 km from the island, with maximum deflections exceeding 100 km (Brand and Blelloch 1974) occurring about 24 h before landfall (Yehand Elsberry 1993a). Some systems approaching northern Taiwan then acquire a southward component, wrapping cyclonically around the northern part ofthe island (e.g., Brand andBlelloch 1974; Chu et al1977; Wang 1980; Chang 1982; Yeh and Elsberry 1993a) (Fig. 1.4.1). Systematic changes in forward speed often accompany the deflections. Following deceleration well east of the island, accelerations up to a few meters per second can occur in the hours . preceding landfall (Brand and Blelloch 1973), followed by a deceleration west of the mountain . The along track acceleration has been attributed to the cyclone's northward excursion into stronger, less disturbed low-level easterly currents. This steering is also consistent within an island-wide enhanced cyclonic flow maintained by the system's latent heating (Chang 1982). Alternately, it might be explained by general flow characteristics of the basin (Yeh and Elsberry 1993a). The northward deflection and acceleration are attributed to the influence of the mountain range on the basic flow and to an additional interaction between the mountain range and the storm circulation (Bender et al 1987). The latter interaction produces diabatic and effects that cause asymmetric circulations inducing a northward "ventilation" and storm propagation relative to the mean flow (Yeh and Elsberry 1993a). The B-effect might also be important (Kong and Soong 1997). The increase of vorticity on the lee side has also been attributed to an internal hydraulic jump (Lin et al1996). The relative importance of barrier-environment and barrier-vortex induced steering components varies with the basic flow strength (e.g., Yeh and Elsberry 1993a). Slow-moving storms deflect the most because a relatively strong southerly wind component develops in their environment.

-1.4.2- Lateral deviations are also relatively large in weak systems (Chang 1982; Yeh and Elsberry 1993a) because those systems respond readily to the development of asymmetric circulations. The observed (Chu et al 1977) and modeled (Yeh 1995) cases of storms approaching Taiwan from the south to southeast show some of the characteristics noted of westbound systems, and imply that impinging angle, landfall location and Reynolds number are factors in the development of secondary lows described below (Lin et al 1996).

b) Taiwan--discontinuous tracks Weak (or shallow) and slow-moving tropical cyclones, especially those approaching the south and central part of the island from the east, can follow a discontinuous track (Fig. 1.4.2). The mountain barrier blocks the westward progress of the low-level center while the upper portion of the vortex continues westward relatively unimpeded, leaving the system tilted or decoupled in the vertical. One or inore initially shallow secondary lows and (sometimes displaced) cyclonic circulation centers can form within a leeside (e.g., Yeh and Elsberry 1993b; Lee 1997). These centers can coexist or coalesce. Upward development can occur when a low-level center comes into phase with the advancing upper-level vortex (e.g., Chang 1982). Development can also occur as a downward extension of the original vortex aloft (Yeh and Elsberry 1993b; Lin et al 1996). Some secondary lows appear to be locked to the terrain, while others continue generally westward. Occasionally, centers moved westward in tandem. The formation and maintenance of secondary lows is associated generally with warming (as observed by Wu and Cho 1997; Kuo and Wang 1996 have another view), sometimes in combination with horizontal advection associated with the original vortex aloft. Secondary vortices shed westward when the barrier blocks the low-level upstream vortex, or form during flow blocking on the southwest slope. Alternately, wave overturning associated with turbulence and diffusion can increase vorticity (Lin et al 1997).

c) The Philippines Brand and Blelloch (1973) found track modifications near the Philippines that resemble those near Taiwan. Similarly, model simulations show a noticeable northward turn and acceleration induced by developing southerlies ahead of cyclones (Fig. 1.4.3). Some discontinuous model tracks occurred near the island. Restrengthening and vertical recoupling took place after the storm crossed the west coast, in general agreement with observations.

-1.4.3- d) Although the Greater Antilles are aligned parallel rather than normal to the climatological flow, modeled and observed tracks (e.g., David [1979] near Hispaniola [Hebert 1980] in the northern and western North Pacific Ocean have similarities (Bender et al 1987). Model runs of westward-moving tropical cyclones approaching Hispaniola show west-southwestward deflections of 70 km prior to a sharp northwestward turn and acceleration near landfall. The net result is a 150 km displacement in the track (Fig. 1.4.4). The evolving steering currents affected most the inner core area, eventually shifting the center northward within the storm's larger-scale envelope.

1.4.2.2 Extended topographic impediments The northwest -southeast oriented Sierra Madre has much larger dimensions and affects motion over longer periods and at greater distances than observed in the island environment (Zehnder 1993b).

a) East coast of Mexico and Tracks of westbound tropical cyclones in the Caribbean Sea and Gulf of Mexico depend on changes to beta gyres (asymmetric relative vorticity) 300-1000 km from the cyclone center (Zehnder 1993b). Up slope flow to the north of the circulation center is horizontally divergent and generates anticyclonic relative vorticity to the northwest of the center by compression of vortex tubes and advection (Zehnder 1993b). The opposite condition occurs to the southeast. The modified vorticity pattern is consistent with simulated southward track perturbations beginning 12 to 24 h prior to landfall (Zehnder and Reeder 1997). Comparable (1 00-200 km) deflections occur in (Zehnder 1993a) and are implied by climatological tracks in the area (Neumann 1996). Qualitatively similar model results have been obtained for a large idealized mountain range (Bender et al1987). The governing mechanisms and deflections are different for the Sierra Madre and the island environment. In the former, redistributed asymmetric vorticity induces a southward component. In the latter, flow blocking forms an upstream ridge leading to a northward component. The Froude number (relating inertial forces to gravity) helps determine which regime dominates (e.g., Zehnder and Reeder 1997, and Kong and Soong 1997). Efforts to understand the relevant processes continue (e.g., Hirschberg et al1995; Hirschberg et al1997). As in the islands, Zehnder and Reeder ( 1997) found that a decreasing zonal flow leads to increasing track deflection. Increasing vertical stratification does as well. Curvature of track does not appear to be a function of cyclone intensity, but along track accelerations are expected for stronger storms. The tracks of large simulated cyclones are relatively more affected (e.g., DeMaria

-1.4.4- 1985), with deflections beginning earlier and with a corresponding increase in forward speed (Zehnder and Reeder 1997). Mountain orientation and steepness are also important. The Sierra Madre ridge becomes more east-west and the range becomes sharper with decreasing latitude. Both characteristics are consistent with model observed tracks showing southward deviations maximized at low latitudes (Fig. 1.4.5).

b) West coast of Mexico and Central America The beta gyres west of the Sierra Madre produce a north to northeastward perturbation (Zehnder 1993b). Imposing a weak easterly basic flow yields rather realistic model tropical cyclone tracks (Fig. 1.4.6). There is also observational evidence (e.g., tropical cyclones Olaf [1997] and Pauline [1997]) that the inland penetration of some tropical cyclone centers is blocked by the mountain barrier near the southwest coast of Mexico. In these instances, the central portion of the cyclone, sometimes reduced to a low-level circulation, appears to change heading and move along the coast (roughly parallel to elevation contours) or back offshore.

1.4.3 Summary Modifications to tropical cyclone track have been observed and modeled near land. These changes are functions of the dimensions of land, environmental flow and stratification, and tropical cyclone structure and intensity. The impact on many areas, especially within the , has not been documented. Additional research is necessary before a unified theory and global forecast guide can be developed describing the impact of land on tropical cyclone track.

Ac.knowledgments Prof. Luo Zhexian contributed to material used to develop this report.

References

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Bender, M. A., R. E. Tuleya, and Y. Kurihara, 1987: A numerical study of the effect of island terrain on tropical cyclones. Mon. Wea. Rev., 115, 130-155.

Brand, S., and J. W. Blelloch, 1973: Changes in the characteristics of typhoons crossing the Philippines. J. Appl. Met., 12, 104-109.

-1.4.5- Brand, S., and J. W. Blelloch, 1974: Changes in characteristics of typhoons crossing the island of Taiwan. Mon. Wea. Rev., 102,708-713.

Brand, S., J.C. Herb, J. C. Woo, J. J. Lou and M. Danard, 1982: Mesoscale effects of topography on tropical cyclone-associated surfacewinds. Papers in Meteorological research, (Taiwan), 5, 37-49.

Chang, C.-P., T.-C. Yeh, and J.-M. Chen, 1993: Effects of terrains on the surface structure of typhoonsoverTaiwan.Mon. Wea. Rev., 121,734-752.

Chang, S. W.-J., 1982: The orographic effects induced by an island mountain range on propagating tropical cyclones. Mon. Wea. Rev., 110, 1255-1270.

Chen, D.-S., M. S. Peng, T.-C. Yeh, S. Chang, and K. N. Huang, 1997: Performance of the Typhoon-track Forecast System in the in Taiwan. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 248-249.

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Chu, K. K., S. T. Wang, and H. P. Pao, 1977: Surface wind fields and moving tracks of typhoons when encountering the island of Taiwan. Preprints, 11th Technical Conference on Hurricanes and Tropical Meteorology, Miami Beach, FL, AMS, 84-87.

DeMaria, M., 1985: Tropical cyclone motion in a nondivergent barotropic model. Mon. Wea. Rev., 113, 1199-1210.

Elsberry, R. L., 1995: Tropical cyclone motion. Global perspectives on tropical cyclones, WMO/TD-No. 693, World Meteorological Organization, Geneva, Chapter 4, 104-197.

Hebert, P. J., 1980: of 1979. Mon. Wea. Rev., 108, 973-990.

Hirschberg, P. A., R. L. Elsberry, and C.-S. Liou, 1995: Sensitivity of tropical cyclone topography interaction simulations to vertical resolution and orography cross-track scale. Proc., 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, AMS, 182- 183.

-1.4.6- Hirschberg,, P. A., C.-S. Liou, and R. E. Elsberry, 1997: Sensitivity of typhoon-terrain interaction simulations to vertical resolution and terrain scale. Proc., Weather Analysis and Forecasting & Marine Meteorology, Taipei, Republic of China, 225.

Hsu, Y. C., and S.-T. Wang, 1960: The problems of typhoon forecasting over Taiwan and its vicinity. Quart. J. Meteor., 2, 7-12.

Kintanar, R. L., and L. A. Amadore, 1974: Typhoon in relation to activities. WMO-No. 408. World Meteorological Organization: CP 5, CH-1211, Geneva.

Kong, J., and S.-T. Soong, 1997: A numerical study of orographic effects on typhoons. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 246-247.

Kuo, Y.-H., and W. Wang, 1996: Numerical simulation ofTyphoon Dot 1990 and its interaction with the Central Mountain Range. Proc., Weather Analysis and Forecasting & Marine Meteorology, Taipei, The Republic of China, 9-12.

Kuo, Y.-H., and W. Wang, 1996: Numerical simulation of 1990 and its interaction with the Central Mountain Range. Proc., Weather Analysis and Forecasting & Marine Meteorology, Taipei, Republic of China, 9-12.

Kuo, Y.-H., and W. Wang, 1997: Interaction of Typhoon Dot 1990 with the orography of Taiwan: A numerical study. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 238-239.

Lee, C.-s., 1997: An analysis of the secondary centers associated with a landfall typhoon in Taiwan. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology. Ft. Collins, CO, AMS, 236-237.

Lin, Y.-L., D. W. Hamilton, and C.-Y. Huang, .1996: Influence of orography on propagating cyclones. Proc., Conference on Weather Analysis and Forecasting & Marine Meteorology, Taipei, The Republic of China, 281-293.

Lin, Y.-L., D. W. Hamilton, and C.-Y. Huang, 1997: Orographic influence on propagating tropical cyclones. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 244-245.

-1.4.7- Meng, Z., X. Xiangde, and L. Chen, 1996: Influences of secondary circulation systems induced by Taiwan island terrain on the anomalous motion of typhoons, Studies on Typhoon Science, Professional Experiment and Synoptic and Dynamic Theories, Second Volume, Meteorological Press, Beijing, 185-197.

Neumann, C. J., 1996: Comments on "Determining cyclone frequencies using equal-area circle". Mon. Wea. Rev., 124, 1044-1045.

Pao, H. P., and R. R. Hwang, 1977: Effects on mountains on a typhoon vortex: A laboratory study. Preprints, 11th Technical Conference on Hurricanes and Tropical Meteorology, Miami Beach, FL, AMS, 88-91.

Suda, T., 1972: Orographic effects of the on the formation and movement of typhoons and extratropical cyclones. J Meteor. Res., 24, 495-509. (In Japanese.)

Tsay, C. Y., and S. T. Wang, 1991: Effects ofTaiwim's terrains on the intensity, structure and movement of typhoons, Part 1 and 2 (1992). Res. Rep., NSC. (In Chinese.)

Tsay, C. Y., 1993: Terrain effects on the intensity and movement of non-splitting typhoons in the Taiwan area (I). Res. Rep., NSC. (In Chinese.)

Tseng, H. Y., and C. C. Yang, 1997: (1996) and rainfall in Taiwan area. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 234-235.

Tuleya, R. E., and Y. Kurihara, 1978: A numerical simulation of the landfall of tropical cyclones. J Atmos. Sci., 35, 242-257.

Tuleya, R. E., M. A. Bender, and Y. Kurihara, 1984: A simulation of the landfall of tropical cyclones using a movable nested-mesh model. Mon. Wea. Rev., 112, 124-136.

Wang, S. T., 1980: Prediction ofthe behavior and strength oftyphoons in Taiwan and its vicinity. Research Report 108, Chinese National Science Council, Taipei, Taiwan, 100 pp. (In Chinese.) [Available from the author at Central Weather Bureau, 64 -Yuan Road, Taipei, Taiwan.]

Wu, C.-C., 1997: The effect of Taiwan terrain on typhoons. Proceedings, Workshop on typhoon research in the Taiwan area, National Center for , Boulder, CO, 27- 33.

-1.4.8- Wu, C.-C., and Y.-T. Cho, 1997: A numerical study of the effect of Taiwan terrain on Typhoon Gladys (1994). Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 240-241.

Yang, P., 1996: A study on the influence of topography on the motion of tropical cyclones, Studies on Typhoon Science, Professional Experiment and Synoptic and Dynamic Theories, Second Volume, Meteorological Press, Beijing, 395-399.

Yeh, T.-C., 1995: Effects of Taiwan orography on the motion and structure of northward approaching typhoons. Proc., 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, AMS, 179-180.

Yeh, T.-C., and R. L. Elsberry, 1993a: Interaction of typhoons with the Taiwan orography. Part!: Upstream Track Deflections. Mon. Wea. Rev., 121,3193-3212.

Yeh, T.-C., and R. L. Elsberry, 1993b: Interaction of typhoons with the Taiwan orography. Part II: Continuous and discontinuous tracks across the island. Mon. Wea. Rev., 121, 3213-3233.

Yeh, T.-C., K.-N. Huang, and D.-S. Chen, 1997: A numerical study ofTaiwan orography effect on Typhoon Herb. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, AMS, 242-243.

Zehnder, J. A., 1993a: The effect of topography on tropical cyclone landfall in Mexico. Proc., 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, AMS, 17-110.

Zehnder, J. A., 1993b: The influence oflarge-scale topography on barotropic vortex motion. J. Atmos. Sci., 50,2519-2532.

Zehnder, J. A., and M. J. Reeder, 1997: A numerical study of barotropic vortex motion near a large-scale mountain range with application to the motion of tropical cyclones approaching the Sierra Madre.Meteorol. Atmos. Phys., 64, 1-19.

-1.4.9- 220 -.

0 2f L-----~------L-----~------~----~~----~----~ 120° Fig. 1.4.1: Taiwan terrain and tracks of several typhoons crossing northern part of the island. Elevation contours plotted every 1000 m. (From Wang 1980, as shown in Chang 1982.)

-1.4.10- AGNES I (ISIJOJ

BETTY LOUJSE (IS6ti (19591

Fig. 1.4.2: Similar to Fig. 1.4.1, but for some discontinuous tracks.

-1.4.11- --- !XP. L5 Ocean Control ·

I!'

Fig. 1.4.3: Simulated storm tracks across northern Philippines, with topography included (solid line) and for ocean-only control (dashed line)in 5 m s· 1 easterly flow. Storm positions and minimum plotted at 6 h intervals (From Bender et al 1987.)

-1.4.12- ---DP~C5 - ... _ ~-Oeeo• (Oftife:t

HISPANIOLA

tr

Fig. 1.4.4: As in Fig. 1.4.3, but for Hispaniola. (From Bender et al 1987.)

-1.4.13- 10 -'------~­

~-·~----~--~~------~~~· l

lI i J ~

100 90 80 Longitude (degrees west) Fig. 1.4.5: Representative tropical cyclone tracks near eastern Mexico. (a) Tracks gaining a southward component: 1. Inez 1966; 2. Dora 1956; 3. 1954; 4. Item 1955; 5. Hermine 1980. (b) Tracks crossing shorelinenorthof23°N: 1. Alien 1980; 2. Alice 1954; 3. Ella 1970; 4. Caroline 1975; 5. Gilbert 1988. (From Zehnder and Reeder 1997.)

-1.4.14- 3.0 -- ' l ' l .. t J ' l t .. • J , ~ ' ' .. t " ' .-...... ,. E .. 6 11 ' - ' ' c 2.0 I i! ' ' > ,

f I t ' t t .. t l t t f tO ts 2.5 3.5. 4.5 X(1000km) Fig. 1.4.6: Simulated storm tracks in 2.5 m s- 1 easterly flow near barrier representing Sierra Madre. Vortex positions shown at 12 h intervals with "X" indicating an ocean-only control. Arrows indicate direction of potential vorticity gradient (From Zehnder 1993b.)

-1.4.15-

1.5 Rainfall

1. 5 .1 Recent advances in research on the mechanisms and causes of exceptional rainfall associated with tropical cyclones in China.

Rapporteur: Dang Renqing Department of Atmospheric Sciences Nanjing University, 210008, China email: [email protected] Fax Working Group: Yukio Takemura, Hyo-Sang Chung, Katsaros

1.5.1.1 Introduction

The author summarized recent advancements of in China on the mechanisms and causes of exceptionally heavy rainfall or abrupt enhancement of rainfall accompanying landed tropical cyclones (TC). Most results are obtained by numerical simulations. Both object and method of the studies have been changed from the past ones in the recent four years. The object was changed from ordinary TC rainfall to exceptionally heavy rainfall or abrupt enhancement of TC rainfall. The studying method was changed mainly to using numerical simulations.

1.5.1.2 Interactions between TC and mid-:-latitude weather systems

a) The influence of westerly troughs on the enhancement ofTC rainfall. . In studying TC 9012 (Yancy), the mid-latitude westerlytrough was artificially deepened in the initial field, then numerical simulation was performed. Results showed that amount of 24-h rainfall was increased in 52 mm than the control case.

b) The channel. By comparing cases of TCs causing exceptionally heavy rainfall and TCs without heavy rainfall, it was found in the former cases that there existed a strong and long conveyer belt of water vapor stretching from the sea to a landed TC, and a merging process took place between cloud systems of a TC and those of a westerly trough. It was not, however, in the latter cases.

-1.5.1- c) Asymmetric TC structure. TC 9216 carried a very large area of heavy rainfall from southern China to northern China. The TC has obviously asymmetric structures as in surface vorticity and divergence field, 850 hPa flow field and temperature field. The eastern half area of TC was warm whereas the western half was cold. Corresponding was strong at upper-level and thus enhanced divergence field, causing large-scale upward motion which is necessary for producing heavy rainfall.

d) Mechanism of the coupling of a with TC circulation. The coupling mechanism among a upper-level jet stream, a low-level jet stream and accompanying vertical circulation proposed by Uccellini and Johnson (1979) was extended to their interaction with TC circulation. When TC circulation approaches a jet region, a upward motion belt of a TC merges with the jet accompanied by vertical circulation. Meanwhile, convective release promotes the development of inertia- gravitational waves. Thus follows the strengthening of upward motion and water vapor flux at the period of the enhancement ofTC rainfall.

1.5.1.3 Mesoscale structure of flow field in a TC relating to heavy rainfall.

a) Upper-level mesoscale flow field above a TC heavy rainfall area. Four TCs, including TCs with heavy rainfall, a TC with light rainfall and a TC with abrupt enhancement of rainfall, were studied by numerical simulation and bandpass filtering for obtaining mesoscale flow fields. Results showed that obvious upper-level divergence structure appeared when a large area of heavy rain formed in a TC. On the other hand, no such divergence structure appeared for a TC with light rainfall.

b) Application of satellite data. Effects of moisture were numerically simulated by introducing moisture field derived from GMS satellite data into the ECMWF analysis. Comparison between the results obtained with satellite data and those without satellite data showed that locations or amounts of TC heavy rainfall areas were better simulated when using satellite data. The amount of heaviest rainfall accompanying a TC was increased in 80 mm/24 h by introducing satellite-derived moisture data.

c) The formation of mesoscale systems relating to heavy rainfall. Numerical simulations ofTCs showed that three factors mentioned below played an important role in the formation of mesoscale systems: (1) excitation by large scale fields (2) orography (3) latent heat release. The first effect was studied by slightly modifying Anthes'(1982) research

-1.5.2- approach, i.e., mesoscale waves in the initial field observed by were filtered out and only large-scale waves were retained. After several hours integration, mesoscale systems relating to heavy rainfall nevertheless appeared.

1.5.1.4 Orographic effect

a) A numerical model. Based on MM4 model, a triple-nested numerical TC model incorporating multiple scales of orographic effects was developed. The model was applied to operational prediction experiments in 1994 and produced good results in TC tracking and rainfall distribution for TCs 9413 and 9414.

b) Influence of orography on TC rainfall. By using the model mentioned above for TC 9216 similar simulations were performed with and without orography. Results showed that a rainfall amount simulated with orography was 60% more than that simulated without orography, the difference of the simulated rainfall amounts was 70mm/24h.

c) TC rainfall in the . The relation between upper-level mesoscale flow field and orography was studied on TC 7914 by numerical simulations. Results showed that when the orography of Island was . incorporated, an upper-level divergence area emerged above an exceptionally heavy rainfall area, and a distinct jet stream and a water vapor band appeared in low-levels. On the other . hand, these features did not appear when the orography was not incorporated.

1.5.1.5 Other aspects

a) Studies on nonlinear dynamics. Under the assumption of semi-geostrophic motion, using vorticity equation, divergence equation, continuity equation and adiabatic equation in stratified atmosphere, applying traveling wave method, an equation of one variable for vertical velocity W can be obtained as follows: W"=-a2 W-(vt/n) (W/(W+v/n)) One of the solutions of this equation is Q(t)=Q2+(Ql-Q2) Cn2 ~((Ql-Q3)/6)*t, which is a new kind of nonlinear wave called mixed Rossby gravity cnoidal wave. Under certain condition, it degrades to be a solitary wave. Very strong upward motion can be formed by both waves.

-1.5.3- b) Meteorological parameters used for forecasting abrupt enhancement of TC rainfall Meteorological conditions or parameters suitable for forecasting abrupt enhancement of TC rainfall were searched for in various candidates of them excluding products of numerical prediction. Several choices were combined with output of numerical prediction, then applied to operational prediction experiments in 1995, in which good results were obtained, among them, a maximum of 200 rnm/24 h was predicted in the east coast of Guangdong province and agreed well with observation.

1.5.2 Forecasting Systems for rainfall accompanying tropical cyclones used in operational forecasts at JMA

Rapporteur: Yukio Takemura RSMC -Typhoon Center Forecast Division, Forecast Department, Japan Meteorological Agency 1-3-4, Ote-machi, Chiyoda-ku Tokyo 100, JAPAN e-mail: [email protected] Fax Working Group: Dang Renqing, Hyo-Sang Chung, Katsaros

1.5.2.1 Introduction

.In the Japan Meteorological Agency (JMA), numerical weather prediction models play a central role in tropical cyclone forecasting. The role of numerical models in forecasting track and intensity of tropical cyclones at JMA is reported in Topic 4.4 by Dr. Masashi Nagata, Numerical Prediction Division, JMA. Forecast guidance (data provided by data processing system for supporting forecast operations) for heavy rainfall is derived from grid point values of numerical prediction models with Neural Network method. Very-short-range Forecasting System (Nowcasting system) using radar and surface rain-gauge data is also used for precipitation forecasting up to 3 h.

1.5.2.2. Forecast Guidance for Heavy Precipitation

The JMA introduced guidance for heavy precipitation to support issuing warning/advisory of heavy precipitation in June 1996. When the Computer System for Meteorological Services

-1.5.4- (COSMETS) of JMA was upgraded, new numerical prediction models (Global Spectral Model: GSM, Regional Spectral Model: RSM, and Typhoon Model: TYM)) were introduced on 1 March 1996. Forecast guidance derived from grid point values ofRSM was first developed to replace the former forecast guidance in JMA. Based on Kalman filter method, the new Guidance (hereinafter called RSM Guidance) are provided for several weather elements, i. e. probability of precipitation, amount of precipitation, and daily maximum and minimum temperatures to be used in routine weather forecast operations. RSM guidance is inevitably dependent on the character of RSM because it is derived from linear combination of grid point values of the model. The JMA then planned to develop a different type of guidance for heavy precipitation by using Neural Network method.

a) Development of Forecast Guidance for Heavy Precipitation As orographic effects play an important role in the occurrence of heavy rainfall events, Orographic Rain Index was also included in a set of predictors in addition to direct outputs of numerical prediction models. It is important to decide which type of disturbances should be the target of forecast guidance of heavy precipitation, small-scale disturbances of 3-6-h lifetime or large-scale ones of 12-24-h lifetime. Evaluation was made for this guidance for cases of precipitation using dependent sets of predicted and observed precipitation data in two warm . As basic data, amounts of regional-maximum 3-h precipitation were produced for every 40 km square area in Japan using Neural Network method. The amounts of 3-h precipitation were summed up in 6-h, 12-h and 24-h periods. The results obtained from those data showed that predicted amounts of rainfall by this guidance was in most cases much smaller then the observed ones for heavy rainfall events of 3-6-h lifetime because those rainfall events were caused by small scale disturbances that are hardly simulated by the present numerical models. As for events of heavy rainfall of 12- 24-h lifetime whose maximum amounts of rainfall were predicted to exceed 100 mm, they accounted for 12.2% of all the cases of rainfall predicted in warin-seasonal 82 days. Very few events of heavy rainfall of 3-6-h lifetime were found whose maximum amounts of rainfall were predicted to exceed 50 mm. Based on the results above, JMA decided that the target of forecast guidance of heavy precipitation (hereinafter called Guidance for Heavy Precipitation) be area-maximum amounts of rainfall in 24 h.

b) Some preliminary results of evaluation on Guidance for Heavy Precipitation Some preliminary results of evaluation were obtained on Guidance for Heavy Precipitation The results indicated Guidance for Heavy Precipitation would make a reasonable performance as well

-1.5.5- for predicting heavy rainfall events accompanying tropical cyclones, and it was introduced into the present forecast operations in JMA. The results were summarized below. There were 50 cases in which an amount of maximum 24-h precipitation was predicted to exceed 100 mm. Specifically, ( 1) In 28 cases, an observed amount of rainfall also exceeded 100 mm. (2) In 20 cases in above 28 cases, a predicted amount of rainfall exceeded 150 mm. An observed amount of rainfall also exceeded 150 mm in 14 cases in the 20 cases. In 3 cases there existed a big discrepancy between predicted and observed amounts of rainfall. (3) In 22 cases, observed amounts of rainfall were less than 100 mm. In 14 cases (about 30% of all the 50 cases) out of the 22 cases, observed amounts of rainfall did not exceed 70 mm (false alarm cases). ( 4) More than 200 mm of rainfall was predicted in 7 that were related to a tropical cyclone. An observed amount of rainfall also exceeded 200 mm in 4 cases out of the 7 cases and more than 150 mm of rainfall was observed in the other 3 cases. (5) Regional dependence in the character of the guidance was hardly found.

1.5.2.3. Very-short-range Forecasting System ofPrecipitation

A fully automated system for hourly analysis and very -short-range forecasting of precipitation are in operational use in JMA since 1 April 1988. The principal purpose of the system is to provide . guidance for real-time monitoring and nowcasting localized heavy rainfall. The basic method of very-short-range forecasting at JMA. is a linear extrapolation of movements of precipitation · systems. Development and decay of precipitation systems induced by orographic effects are also taken into account. Specifications of the Very-short-range Forecasting Model in JMA are summarized in Table 1.5.1.

-1.5.6- Table 1.5.1 Specifications ofVery Short-range Forecasting Model in JMA Forecast process Linear extrapolation Physical process Orographic enhancement and dissipation Motion vector Motion of precipitation system is estimated by the cross correlation method Iteration time of forecast step 10min Resolution 5 km square

Number of Grids 250 X 660 Initial field composite radar echo intensity. observed by 19 radars and calibrated by 1300 rain-gauges Forecast time Up to 3 h

a) Products The basic products used for issuing warnings of heavy precipitation of the Very short-range Forecasting system in JMA are (a) composite intensity and top height of radar echo (b) calibrated 1-h precipitation (analysis of precipitation) and (c) 1-h precipitation forecasts up to 3 h ahead (forecast of precipitation)

b) Verification of forecasts The Very-short-range Forecast Model was verified for data obtained in three years from 1993 to 1995 using Critical Success Index (CSI). Forecast and analysis data of precipitation were

averaged in 20 km X 20 km square grid, and were compared. Performance of the Very-short-range Forecast Model were compared with that of simple persistence method. Obtained results of CS Is averaged for the three years (from 1993-1995) are shown in Table 1.5.2.

Table 1.5.2 CSI (%)of the Very-short-range Forecast Model averaged for three years from 1993 to 1995 (a) 1-h forecast (b) 3 h forecast 1 mm h-1 4 mm h-1 10 mm h-1 : 1 mm h-1 4 mm h-1 10 mm h-1 Very-short-range F~~ecast [ 57.8 45.4 33.0 31.8 17.9 8.6 I Persistence i 45.2 29.5 16.8 24.9 11.1 4.1

Results show that the Very-short-range Forecast Model keep its superiority to persistence method even in 3-h forecast although the Indices decrease rapidly with longer forecast time.

-1.5.7- Threshold values of CSI were set at 1 mm h-1, 4 mm h-1, and 10 mm h-1, respectively. Values of CSI are smaller for larger thresholds of precipitation. This implies that heavy precipitation events are more difficult to predict.

References

Anthes, R. A. and T. N. Carlson, 1982: Conceptual and numerical models ofthe evolution ofthe environment of severe local storms. Joint US-China workshop on mountain meteorology, Beijing, China.

Dang, R. Q., and J. K. Chen, 1992: A survey of research on torrential rain induced by tropical cyclones. JCSU/WMO International Symposium on Tropical Cyclone Disasters, 1992, Beijing, China. Peking Univ. Press, 469-473.

Dang, R. Q.,1996: Recent advances of researches on the mechanisms of exceptional rainfall caused by tropical cyclones in China. Extended abstract collection of tenth session of national workshop on tropical cyclones, 10-12. Chinese Academy of Meteorological Sciences.

Uccellini, L. W., and D .R Johnson, 1979: The coupling of upper and lower tropospheric jet streams and implications for the development of severe convective storms, Mon. Wea. Rev., 107, 682-703

-1.5.8- 1. 6 Decay after landfall

Rapporteur: Frank D. Marks, Jr. NOAA/ AOML Hurricane Research Division Miami, FL 33149) email: [email protected] Fax (1) 305-361-4402 Working Group: Lianshou ehen, Luo Zhexhian, McCaul, Kaplan

1. 6.1 Introduction

Documentation of the kinematic and thermodynamic changes in storm structure after a Te crosses the coastline is very rare. Kaplan and DeMaria (1995) have shown that the intensities ofthe U.S.landfalling storms for the period beginning in 1967 (the most reliable portion of the record) can be modeled using a simple exponential decay equation. Such a relation can be used to show that even a strong category 5 storm (maximum winds ~75 m s-1) will decay to less than hurricane strength in~ 12 h. Thus, the potential for damage due to the sustained winds of a storm does not last very long after landfall (if a storm were moving rapidly ( ~ 10 m s-1), it could move about 430 km in 12 h). However, there is a need to determine how the overall changes in the Te vortex affect the temporal and spatial variations in the surface winds and rainfall and the generation of tornadoes as the storm moves inland. The importance of studying these inland effects was underscored by the recent U.S. landfalls of Hurricanes Hugo (1989), Andrew (1992), Opal (1995), and Fran (1996) which produced significant damage and fatalities hundreds ofkilometers inland. · Although observational and modeling studies have sho"':l that the primary mechanism responsible for the decay ofTC winds after landfall is the greatly reduced latent and sensible heat fluxes over land (e.g., Tuleya 1994), very few detailed data sets have been collected in landfalling TCs against which these results can be verified. Consequently, many key questions remain concerning the precise role that latent and sensible heat fluxes and various other physical processes play in the decay of landfalling res. Moreover, while modeling studies have investigated the effect of variations in surface temperature, moisture and roughness in some detail, relatively little time has been spent investigating the potential effect of such factors as stability, storm size, topography, the intensification rate of the cyclone prior to landfall, and the role of the large-scale environment on the spin-down of the re wind field. The collection of detailed wind fields in landfalling res is essential to answering these and other questions.

-1.6.1- 1.6.2 Wind field decay

Case studies of the wind fields of landfalling TCs (e.g., Powell et al 1991, 1996) reveal considerable small-scale variability. In some cases localized regions of high winds are correlated with local convective activity (Parrish et al1982; Wakimoto and Black 1994). Sustained wind and gustiness at a particular site is a strong function of topography and the small-scale roughness of the surrounding surface (Padya 197 5). Rapidly translating TCs, such as Hurricanes Hugo of 1989 or Andrew of 1992, present a greater inland wind hazard because they have less time to weaken after they cross the coast. Surface wind field representation and characteristics before and after landfall needs to be understood with respect to the nature of gustiness of the wind within the eye-wall as it makes landfall. In particular, there is a need to understand the relative roles played by mini-swirls of the eye-wall vortex compared to tornadoes spawned by localized eye convective towers. The potential for strong, negative as deep, upward motions in balance with the vortex circulation are suddenly cut off from the rich Se supplied from the ocean surface, which may result in locally strong dynamically forced downdraft or bow-echo type events. New data sets of the characteristics of the surface wind field at sea and at landfall must be acquired. Apart from atmospheric boundary layer (ABL) spindownand the collapse of the eyewall wind maximum, the surviving residual circulation is often not too different from the original TC. The TC circulation above the ABL is in large measure isolated from surface friction and may persist for days as it continues to exhibit a characteristic TC appearance on radar and to produce substantial rainfall (Bluestein and Hazen 1989). If the midtropospheric remnants return to a warm ocean, a TC may redevelop. The interaction of the storm circulation with its environment also probably plays an important role in the evolution of the storm wind field after landfall. For example, Bosart and Lackmann (1995) have shown that (1979) decayed at a very slow rate after landfall, and subsequently intensified well inland. In this case, the storm remained in a weakly baroclinic and moist environment (conditions normally associated with intensification of storms over the ocean). In contrast to Hurricane David which appeared to maintain tropical characteristics after landfall, many storms merge with or transform into extra-tropical cyclones. The evolution of the surface winds during this transition needs to be documented. In addition, the interaction of the TC wind field with low-level baroclinic zones may be important in some cases. For example, if a storm moves into a region with cold surface temperatures, the stability of the ABL might increase, and the high winds above the surface layer might be less likely to mix down to the surface. At the present time, the only guidance models available for operational forecasting of inland TC winds are the empirical model described by Kaplan and DeMaria (1995) and the operational version of the GFDL model (Kurihara et al 1995). However, it is not clear how well these models

-1.6.2- can predict the evolution of the surface winds. The empirical model performs well when measured against the best track data, but is far too simple to predict the complicated horizontal wind structure that is normally observed. The GFDL model has not shown skill in the prediction of intensity for Atlantic or East Pacific storms (e.g., Gross and Lawrence 1996). However, most of the forecast cases are for storms over the ocean. A separate verification should be performed for a sample that only includes landfall cases.

1.6.3 Severe weather

Tornado outbreaks are a common occurrence with landfalling TCs, and represent a significant cause of damage and casualties in some cases. While the U.S. NWS often issues tornado watches in conjunction with the landfalls of major TCs in the U.S., accurate forecasts of the scope and severity of the tornado activity remain elusive. For forecasters, this is an important issue, because some TCs may spawn only a handful of weak tornadoes, while others may spawn dozens, and in one case, more than 100, some ofwhich may be strong (Novlan and Gray 1974). McCaul (1991) described the statistical properties ofthe TC-tornado environment, and found significant correlations between the overall severity of tornado activity and certain atmospheric properties routinely derived from rawinsonde data. The best correlations were with certain measures of vertical shear, such as 0-3 km storm-cell relative helicity. Correlations with well­ known thermodynamic parameters such as total buoyant energy were, however, were negligible. On balance the correlations with severe weather were found to be too small to be useful as a basis for issuing definitive forecasts, at least on a case-by-case basis. Taken as a composite, however, the patterns of low-level helicity within TCs mirrored closely those for tornado occurrence, with both quantities reaching distinct maxima in the TC's right-front (RF) quadrant, relative to TC motion. McCaul (1991) attributed the helicity maximum in the RF quadrant to the additive effects of the TC's own self-generated vertical shear profile and that of a weakly sheared baroclinic steering current. Other investigators had already identified surface drag effects as being potentially important as a shear enhancement mechanism in the portions of the TC where the ABL winds are onshore (Gentry 1983).

a) Severe weather in the outer The characteristics of the convective cells that produce tornadoes in landfalling TCs are still not fully known. Through examination of non-Doppler radar reflectivity images, McCaul (1987) discovered evidence that many of the tornadoes produced in the outer rainbands of in 1985 were spawned by miniature supercell storms, some of which produced tornado families for several hours, as in classic springtime midwestern tornado outbreaks. The presence of

-1.6.3- reduced surface temperatures and the observed low storm top heights suggested that these developed in an environment of relatively small total buoyant energy. Recently, McCaul and Weisman (1996) used a cloud model to show that a form of shallow supercell storm could exist in such environments, when both the parcel buoyancy and the vertical shear are strongest in the lower , as typically found in landfalling TCs. Further observational confirmation of the existence of these small rotating storms came in in 1994, when Tropical Storm Beryl produced a major tornado outbreak that was observed by the WSR-88D in Columbia, SC (Cammarata et al 1996). Small but intense and persistent supercells accounted for most of the reported tornadoes, with some of the cells identifiable on radar for as long as an astonishing 9 h. The occurrence of these small supercells is apparently not rare in landfalling TCs. As far back as Hurricane Alien in 1980, long-track TC­ tomadoes were noted by some investigators (Stiegler and Fujita 1982). Their actual frequency of occurrence, however, and the characteristics ofthe other tomadic storms that can form in TCs, remain unanswered questions. These small supercell storms pose special obstacles for detection and warning, even for the WSR-88D radars. Because their rotational signatures are compact, the storms must be within about 50-60 km of the radar to be easily detectable. Because the WSR-88D network is more widely spaced, no multi-Doppler analyses of these small TC-spawned supercells have yet been accomplished, despite the landfall of many TCs in the U. S. since deployment of the WSR-88D network. Mobile Doppler radar systems appear to offer the most hope of obtaining more detailed observations about the structure and dynamics of these unusual storms. The lack of any significant correlation between buoyant energy and tornado incidence within landfalling TCs (McCaul 1991) went unexplained until August 1992, when Hurricane Andrew passed near a ABL profiler located in northwest . As severe rainbands passed over the instrument site, the profiler and its accompanying acoustic sounding system (RASS) obtained unprecedented kinematic and thermodynamic profiles (McCaul et al 1993). -scale fluctuations of ABL and wind were noted, with peak winds and temperatures both occurring in the central portions of the rainbands, in the midst of precipitation. The were most unstable precisely in the zones where it would be most difficult to launch rawinsondes. The principal period with which the rainband variations occurred was roughly 3 h, much smaller than the standard U.S. rawinsonde launch interval of 12 h. It is thus possible that many of the soundings used in previous studies were deliberately taken in those portions ofthe TC where buoyant energy (and shear) were small relative to that actually supporting the severe convection. Unfortunately, no rawinsonde data or surface temperature or moisture measurements were available, even in the more benign environment between rainbands, to validate the highly-

-1.6.4- patterned profiler data from the Andrew case. Furthermore, it is not yet known how representative of other TCs the Andrew results are. The Andrew profiler/RASS results lead one to suspect that higher correlations between tornado occurrences and kinematic and especially thermodynamic parameters might be obtained if finer­ scale data capable of resolving the complex fields in landfalling TCs could be obtained. A combined effort using mobile Doppler radars, profilers with RASS, rawinsondes, and possibly, coordinated aircraft over flights, appears to be the best hope for obtaining such comprehensive data near TC tornadic rainbands. Improvements in our dynamical and statistical understanding of the severe weather spawned by TCs should, of course, lead to better forecasts.

b) Severe weather in the inner core There remains, however, even more mystery about the exact nature of the convective events that spawn tornadoes in the core regions of landfalling TCs, as opposed to the outer rainbands. McCaul (1991) clearly showed that there are enough reports oftornadoes close to the TC centers to suggest that the core tornado phenomenon is real. Photographic documentation or other ground truth for these events may be very difficult to obtain, but a sample of cases of rotation documented by Doppler radars and correlated with tornado damage reports would offer a starting point for further research. The size, intensity, duration and dynamics of these core tornadic cells are as yet not understood. Their relationship, if any, to the eyewall miniswirls described by Fujita (1993) is also unknown. However, it is possible that they may be related to the large-amplitude ABL wave structures recently found in (Wurman and Winslow, 1998), featuring large transient vertical vorticity outbursts and narrow flow-parallel bands of strongly enhanced wind descending toward the surface (Fig. 1.6.1 ). A number of additional case studies of the core-region ABL flows in TCs, using both fixed and mobile Doppler radars, along with carefully mapped damage reports, is needed to determine which phenomena are the predominant damage producers there.

1. 6 .4. Future directions

From a surface wind perspective, the important scientific issues are: i) Surface wind field representation and characteristics before and after landfall needs to be understood with respect to the nature of gustiness of the wind within the eye-wall as it makes landfall. New data sets of the characteristics of the surface wind field at sea and at landfall must be acquired. ii) Even less is known about the vertical structure of the TC wind field after landfall. Recent results :from the operational version of the GFDL TC model suggest that the winds at the top of

-1.6.5- the ABL decay at a much slower rate than the surface winds. This result should be evaluated using sounding and radar observations, and may have important implications for the effect of topography and convection on the variability of the inland winds iii) Different coastal effects at the surface occur depending on whether a storm is filling or intensifying. The former has weak surface winds and strong shear in the atmospheric planetary ABL. By contrast, the latter effect mixes momentum downward producing stronger surface winds than a filling storm with the same minimum pressure. Research is required to determine what physical processes are responsible for this variability. iv) There is a need to understand the relative roles played by mini-swirls of the eye-wall vortex compared to tornadoes spawned by localized eyewall convection. At landfall there is the potential for strong, negative buoyancy as upward motions in balance with the vortex circulation are suddenly cut off from the rich Se supplied from the ocean surface, which may result in locally strong dynamically forced downdraft or bow-echo type events. v) Verification of surface wind fields from operational models using enhanced surface wind observations. The models available for verification are the Kaplan and DeMaria (1995) empirical model and the GFDL model. Forecasters need to know how well these models can predict the evolution of the surface winds. vi) There are many questions that must be answered when trying to forecast inland winds using numerical models. For example, are the ABL parameterizations adequate to accurately model the low-level wind structure? What horizontal and vertical resolutions are required? How can the model be initialized, given the sparse data that is routinely available? The model results must be compared with all available observations to answer these questions. The important scientific issues related to improved forecasting of severe weather in landfalling TCare: (i) Which TCs will produce many tornadoes versus few? To answer this question, we will need to understand more completely the rainband-scale structure within landfalling TCs. More field studies with mobile Doppler radars, profilers with RASS, rawinsondes, and research aircraft over flights are urgently needed. Special attention should be paid to mapping any dry intrusions that may enter the TC circulation at mid-levels. (ii) We must identify the characteristics of tornadic storms in the core regions of TCs. This will require study of WSR-88D data collected in landfalling TCs, and comparison with validated tornado damage reports from the core regions. (iii) There is a need for continued work in improving the numerical models used in studying TC structures. Of particular importance is the issue of accurate representation of the TC rainbands. Proper handling of the complex flows that occur in the TC ABL, where shears often exceed

-1.6.6- Richardson number criteria for turbulence, may also be crucial. The intriguing recent measurements made in the TC core regions using high-resolution mobile Doppler radars should be continued and expanded. The benefits of improved knowledge of the wind structure at landfall go beyond improved meteorological understanding. Wind engineers, designers, and interests in the construction and insurance industry require timely wind field information to establish the recurrence interval for the peak wind and whether design winds (the winds used to compute wind loads on structures) were exceeded. The national wind load standard references design winds as peak 3-s gusts (ASCE 1994). Powell et al (1996) found that over 1 h the peak 3-s gust over land is approximately 30% higher than 1-min sustained wind. Over water, peak 3-s gusts are not as large because of reduced turbulence there. The production of a gust swath map could determine whether design winds were exceeded for coastal, inland, and offshore structures and minimize confusion among various disciplines when referring to the peak winds caused by the storm. The ultimate cost of a disaster is related to the amount of time taken for a community to recover; a faster, more organized recovery helps to mitigate the losses associated with a storm. Real-time information on the actual areas impacted by the eyewall and the strongest winds would help minimize confusion and assist recovery management at the earliest stages of the disaster. Powell et al (1996) showed an example ofthe ability of a Geographical Information System (GIS)to link the Hurricane Andrew 1-min sustained wind swath to a map showing Florida Power and Light Company (FP&L) electrical utility substations in the Miami area. They proposed using . georeferenced statistics of the damaged facilities at each substation to correlate losses with meteorological fields believed to be associated with damage. Unfortunately, damage data bases of this nature are not readily available, and are for geographic areas too large to correlate with high­ resolution meteorological fields.

References:

Bluestein, H.B., and D. Hazan, 1989: Doppler radar analysis of a tropical cyclone over land: (1983) in , Mon. Wea. Rev., 117, 2594-2611.

Bosart, L.F., and G.M. Lackmann, 1995: Postlandfall tropical cyclone reintensification in a weakly baroclinic environment: A case study ofHurricane David (September 1979),Mon. Wea. Rev., 123, 3268-3291.

Cammarata, M., E. W. McCaul, Jr., and D. Buechler, 1996: Observations of shallow supercells during a major tornado outbreak spawned by Tropical Storm Beryl. Reprints, 18th Conference Severe Local Storms, San Francisco, CA, AMS, 340-343.

-1.6. 7- Fujita, T. T., 1993: Wind fields of Andrew, Omar, and Iniki, 1992. Reprints, 20th Conference Hurricanes and Tropical Meteorology, San Antonio, TX, AMS, 46-49.

Gentry, R. C., 1983: Genesis of tornadoes associated with hurricanes. Mon. We a. Rev., 111, 1793-1805.

Gross, J.M., and M.B. Lawrence, 1996: 1995 National Hurricane Center . Minutes of the 50th Interdepartmental Hurricane Conference, Office ofthe Federal Coordinator, Washington, DC.

Kaplan, J., and M. DeMaria, 1995: A simple empirical model for predicting the decay oftropical cyclone winds after landfall. J. Appl. Meteor., 3 4, 2499-2512.

Kurihara, Y., M.A. Bender, R.E. Tuleya, and R.J. Ross, 1995: Improvements in the GFDL hurricane prediction system. Mon. Wea. Rev., 123, 2791-2801.

McCaul, E. W., Jr., K. R. Knupp, and W. L. Snell, 1993: Observations of tornadic storms and rainbands within Hurricane Andrew's remnants. Reprints, 17th Conference Severe Local Storms, St. Louis, MO, AMS, 272-276.

McCaul, E. W., Jr., and M. L. Weisman, 1996: Simulations of shallow supercell storms in landfalling hurricane environments. Mon. Wea. Rev., 124, 408-429.

McCaul, E. W., Jr., 1991: Buoyancy and shear characteristics of hurricane-tornado environments. Mon. Wea. Rev., 119, 1954-1978.

McCaul, E. W., Jr., 1987: Observations ofthe Hurricane "Danny" tornado outbreak of 16 August · 1985. Mon. Wea. Rev., 115, 1206-1223.

Novlan, D. J., and W. M. Gray, 1974: Hurricane-spawned tornadoes. Mon. Wea. Rev., 102, 476-488.

Padya, M, 1975: Spatial variability and gustiness of cyclone winds Gervaise, Mauritius, February 1975. Aus. Meteor. Mag., 23, 61-69

Parrish, J.R., R.W. Burpee, and F.D. Marks Jr., 1982: Rainfall patterns observed by digital radar duringthelandfallofHurricaneFrederic(1979).Mon. Wea. Rev., 110,1933-1944.

Powell, M. D., 1980: Evaluations of diagnostic boundary layer models to hurricanes. Mon. We a. Rev., 115,2479-2506.

-1.6.8- Powell, M.D., P.P. Dodge, and M.L. Black, 1991: The landfall ofHurricane Hugo in the Carolinas: Surface wind distribution. Wea. Forecasting, 6, 379-399.

Powell, M.D., and S.H. Houston, 1996: Hurricane Andrew's landfall in south Florida. Part 11: Surface wind fields and potential real-time applications. Wea. Forecasting, 11, 329-349.

Stiegler, D. J., and T. T. Fujita, 1982: A detailed analysis of the San Marcos, Texas, tornado induced by Hurricane "Alien" on 10 August 1980. Reprints, 12th Conference Severe Local Storms, San Antonio, TX, AMS, 371-374.

Tuleya, R.E., 1994: Tropical storm development and decay: Sensitivity to surface boundary conditions. Mon. Wea. Rev., 122,291-304.

Wakimoto, R., and P.G. Black, 1994: Damage Survey of Hurricane Andrew and Its Relationship to the Eyewall. Bull. Amer. Meteor. Soc., 75, 2, 189-200.

Wurman, J., and J. Winslow, 1998: Intense sub-kilometer boundary layer rolls in Hurricane Fran, Science, submitted.

-1.6.9- 56

so

44

38

..d 32

26

Doppler On Wheels (DOW) radar data in hurricane Fran just before landfall in Wilmington, NC on 96 September 05/06.

The plotted region is approximately 15 km inland from the North Carolina coast, near the point of landfall of the hurricane. Blue lines are a 1 km grid. Blue arrow denotes the general easterly windfl.ow from the storm.

Doppler velocity in sub-}illometer scale boundary layer rolls is shown. All velocities are away from the radar, ranging from about 28 to 54 m!s in the waves. The peak to peak amplitude of the waves is as high as 25 m!s 1 over scales of300-1000 m, resulting in azimuthal shear values of up to 0.1 s- .

Fig. 1.6.1. (from Wurman and Winslow, 1998)

-1.6.1 0- WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic:1.6: Decay after landfall

Rapporteur: Lianshou Chen Chinese Academy of Meteorological Sciences 46 Baishiqiao Rd., Beijing, 100081, China

Email: [email protected] Fax: 86-10-62175931

Working Group: M. DeMaria

1.6.1 Introduction Tropical cyclone landfall is a very special problem, which is significantly different from those on deep ocean. What would happen after the landfall of a tropical cyclone is related to not only its track, but also the distribution of accompanying torrential rain, TC structure and intensity, topography and continental shelf effect, etc.

Generally speaking, the period that a TC sustain on land is different for different tropical cyclones. Some TCs may dissipate less than six hours after their landfall, while others may sustain for three to five days on land before disappearance. Sometimes they may be transformed into an extra-tropical cyclone or move back to the ocean and develop under certain conditions. Whether a tropical cyclone will sustain or dissipate is decided by the following several factors.

1.6.2 Water vapor supply Tropical cyclone is a kind of circulation supported by heat. Latent heat released by water vapor is its main energy source, which forms the warm core of the tropical cyclone. After a tropical cyclone makes landfall, water vapor transportation from the sea is cut off, which will reduce the cumulus convection inside the tropical cyclone. So the warm core will be destroyed, the horizontal will decrease, which is unfavorable to kinetic energy production. So the tropical cyclone will dissipate soon.

On the other hand, if a tropical cyclone can get other water vapor supply continuously after landfall, the water vapor will make the TC sustain for a certain period. Usually, a TC may obtain water vapor by the following three ways after landfall. Firstly, the TC is still in ITCZ, especially when the ITCZ shifts to the north while the TC is moving to northwest or north. Consequently, the TC can get enough water vapor from the ITCZ (Fig.1a). Secondly, there are southeast or southwest jet from ocean spiraling into TC circulation at lower troposphere, which will transport wet air from western Pacific and South China Sea even Bay of Bangle into TC (Fig.1b). The third way is the merging of

1 GJH February 23, 1998 TC and another tropical cloud mass. Then the TC will develop due to supply of both water vapor and positive vorticity (Fig. 1c). In addition, large inland lake is also a water vapor source if the TC happens to move on them.

Fig. 1 Water vapor supply to a tropical cyclone after landfall

1.6.3 Baroclinic energy or transformation As we know, latent heat released by moisture condensation is the main energy source of TC development. Once a TC makes landfall, the latent heat will be cut off and the TC will dissipate soon without other energy supply. The interaction between a tropical cyclone and a is a way to make tropical cyclone obtain baroclinic energy i'nstead of barotropic energy. lt is also a way maintaining the cyclone over land.

Fig.2 TC structure change caused by moderate cold front

When the tropical cyclone meets moderate cold air, a north-south front will appear in the middle of the tropical cyclone at lower troposphere. The temperature field of the TC circulation will become a structure with half-cold-half-warm (Fig. 2). This front provides the baroclinic potential energy, which will be transformed into kinetic energy with warm

2 GJH February 23, 1998 air ascending and the cold air descending. Then the cyclonic circulation at lower level will intensify and the central pressure of the TC will decrease. Typhoon 7416 is a good example of development due to the release of baroclinic potential energy and transform into kinetic energy.

But if the cold air is very strong, the warm core of the tropical cyclone will be destroyed completely, the middle and upper cloud will disappear, the maximum wind velocity will decrease rapidly, the symmetric distribution of wind, rainfall and temperature will be destroyed also. Then the tropical cyclone will dissipate soon.

1.6.4 Impact of large scale circulation Usually, when a tropical cyclone makes landfall and moves to higher latitude, it will be affected by large scale circulation there.

Fig.3 Stream line field of typhoon 7220 at 200hPa on Nov. 7, 1972

Upper level (around the divergence level of tropical cyclone) westerly trough can apparently influence the intensity of an approaching tropical cyclone. When the tropical cyclone is in front of a westerly trough or upper level jet, the positive vorticity advection will intensify the upper divergence field of the tropical cyclone (Fig.3). Besides, the southwest wind can take the heat above the tropical cyclone away and maintain its vertical circulation. When the tropical cyclone is at the trough area or under the upper level jet, the strong vertical wind shear will weaken the storm. When the tropical cyclone locates behind a westerly trough or upper level jet, the negative vorticity advection will produce upper convergence and then increase surface pressure of the tropical cyclone. As a result, the tropical cyclone will disappear. Sometimes, there exits an at about 200mb on land, when a tropical cyclone moves under it, the tropical cyclone will be intensified. When the tropical cyclone departs the upper high, it will decay gradually because of weaker divergence field at upper level.

Subtropical high also has influence on tropical cyclone circulation. When a tropical cyclone moves toward the subtropical high from the south, the subtropical high may

3 GJH February 23, 1998 the transportation of baroclinic energy from the higher latitudes. Then the tropical cyclone will disappear soon.

1.6.5 Vertical wind shear Vertical wind shear is another important factor to affect TC intensity after landfall. The smaller the vertical wind shear is, the longer the TC will sustain. For example, the vertical wind shear of (7503) after landfall was less than 5m/s and made the typhoon sustain for five days. Then the wind shear increased suddenly to 1Om/s on Aug.7, which indeed made the typhoon disappear the next day.

1.6.6 Topography impact Once a TC has passed onshore, suddenly increased friction will lead to immediate reduction of surface winds. Topography can affect not only tropical cyclone track, storm · surge, rainfall, but also TC intensity. In the topography impact on typhoon intensity change after landfall, island topography impact is very complicated, which will be mainly discussed here. Influence of island topography on TC intensity are mainly due to two . reasons: one is friction increase, which is related to the surface characteristics and area of the island. The other is decrease of heat and water vapor supply. According to the study of Brand in 1974, Taiwan island can affect TC intensity significantly. Based on the statistics on 22 TCs crossing Taiwan and landing on China from east during 1960-1972, it is shown that TC intensity may decrease 41% from 6h before landfall to 6h after departing the island (Fig.4). Furthermore, no how TC intensity changes before landfall, the TC will always weaken by similar speed and amount.

Fig.4 Variation of averaged TC intensity during crossing Taiwan island (Brand, 1974)

In addition, the impact of Taiwan island is different to different intensity of tropical cyclone. The stronger the TC is, the more significant the impact will be. Furthermore, due to the north-south trend mountain on the west of the island, those TCs with sustained maximum wind below 25m/s usually disappear while crossing the island. Those between 25-50m/s usually cause induced low or topographical trough and show "jumping" phenomena, which has been simulated clearly in limited region numerical

4 GJH February 23, 1998 Fig.5 Simulated 'Jumping" process of typhoon Dot across Taiwan island in 1990 (Meng, · 1996) model (Fig.5) (Meng, 1996), and those beyond 50m/s generally cross the island continuously.

1.6.7 Summary Based on above discussion, after a TC makes landfall, the general tendency is getting weakened. The sustaining period and decaying speed of landfalling tropical cyclone are mainly determined by water vapor supply, baroclinic energy or transformation, impact of large scale circulation, vertical wind shear and topographical effect, etc.

Reference Brand, S., Blelloch J. W.,1974: Changes in the Characteristics of Typhoon Crossing the Island of Taiwan, Mon. Wea. Rev., 102, 708-713pp

Meng Zhiyong, Nagata Masashi, Chen Lianshou, 1996: A Numerical Study on the Formation and Development of Island-Induced Cyclone and Its Impact on Typhoon Structure Change and Motion, ACTA Meteorologica Sinica, Vol.1 0, No.4, 430-443pp

5 GJH February 23, 1998

Table of Contents

TOPIC TWO

TROPICAL CYCLONE INTENSITY AND STRUCTURE

2.0 Report of the Topic Chairman G. Holland (Australia)

2.1 Air-Sea Interaction Rapporteur: J. Lighthill ()

2.2 Environmental Interaction (Intensity) Rapporteur: J. Molinari (USA)

2.3 Tropical Cyclone Maximum Intensity Theory Rapporteur: K. Emanual (USA)

2.4 Tropical Cyclone Formation Rapporteur: W. Frank (USA)

2.5 Small and Mesoscale Interactions QC structure and intensity) Rapporteur: L. Ritchie (USA)

WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic 2.0: TROPICAL CYCLONE INTENSITY AND STRUCTURE

Chairman: G.J. Holland Research Centre PO Box 1289K Melbourne, Vie 3001, Australia

Email [email protected] Fax (Australia) +613 9669-4660

Rapporteurs: James Lighthill, John Molinari, , Bill Frank, Liz Ritchie

2.1 Introduction

This report should be taken as an extension of previous work on tropical cyclone intensity and structure, including the extensive IWTC review volumes by Holland (1993) and Elsberry (1995), which have provided the basic concepts and governing relations. Thus, no attempt is made to provide a comprehensive coverage of all aspects of the processes involved in defining intensity, structure and the related changes. Rather, we c·oncentrate on current issues and questions, and on progress since IWTC-111.

The report draws strongly from the accompanying rapporteur reports on: Air/sea Interaction (Lighthill), Environmental Interaction (Molinari), Maximum Potential Intensity (Emanuel), Formation (Frank) and Small Scale Interactions (Ritchie). However, this ordering has not been explicitly followed. Rather, we start with a brief recapitulation of the life cycle of a tropical cyclone, together with the basic physical scaling of a hurricane

1 GJH 13 March, 1998 force system (Section 2.2}. This is followed by a discussion of intensity, including formation and intensity changes in Section 2.3, then structure in Section 2.4. The major issues and questions are summarised in Section 2.5. We note that, since there are essentially no techniques for forecasting structure changes, and very little work on the associated processes, the structure discussion concentrates on the core region only.

2.2 Fundamental Scaling Parameters

a) Intensity Stages The life cycle of a tropical cyclone consists of a number of stages that can be reasonably well defined, but which will vary in specific definition from study to study. For the purpose of this report, we break the life cycle into the stages indicated in Fig. 1 : preformation and initial development; intensification; mature stage (with potential for intensity cycles}; and decay. We choose not to explicitly define exact changeover points, as these are meaningless in what is an essentially continuous process. This report will focus on the preformation and initial development, through to intensification and mature stages.

b) Physical Scales in a Developed Cyclone As discussed in Elsberry (1995}, the physical processes that can occur, and the manner in which a tropical cyclone adjusts to influences varies considerably between the core and the outer region, and between the lower and upper atmosphere. This leads to a natural separation of the developed cyclone into the regions indicated in Fig. 2.

2 GJH 13 March, 1998 Preformation an Initial Development - ~ Intensification ...t: Q) 0

Mature Stage Time Figure 1. Tropical cyclone intensity stages used in this report, shown relative to a schematic time history of central pressure.

15 km Anticyclonic Outflow r----Anticyclonic Outflow

Outer Region Core Region Outer Region

Atmospheric Boundary La er' SFCr---~------~~~~m=r---~--~~~~----1 Oceanic Boundary Layer

--~="""·- a. LJi CS Perspective View Figure 2. Schematic of the major regions in a tropical cyclone, based on physical The scaling principles.

3 GJH 13 March, 1998 details have been provided in Holland and Willoughby. The major intensity change processes occur in the core region, where there is very high inertial stability and strong radial windshear. Convective processes dominate in such an environment and the strong radical windshear produces a tendency towards symmetric development. The air- sea interaction processes, which optimally provides the energy for the cyclone, are confined to this region. Questions raised in this report on the ocean atmosphere interaction, including the surface spray regime, are largely confined to the core region.

The outer region includes the major rain bands, or feeder bands. The cyclone size and outer wind strength are defined in this region, where the relatively low inertial stability requires that wind forcing mechanisms from passing weather systems dominate over convective processes. Processes in this region are known to have a major role in the cyclone motion (Eisberry 1995), but have previously been thought to have only a minor direct impact on cyclone intensity. More recent work, described in (Ritchie 1998) has shown that mesoscale convective systems developing in the outer region, with their associated mid-level vortices, could have a substantial direct impact on both intensity and motion. Further, there are indications that the thermodynamic structure of the atmosphere in this outer region has a direct effect on the maximum intensity that a cyclone can achieve.

The low inertial stability outflow layer has long been recognised as providing a potential channel for direct influence by external systems on the cyclone intensity. The outflow regime also often contains potential vorticity anomalies of various sizes that have been extruded from the . Defining the precise interaction mechanisms effecting intensity has proven to be a difficult task, however, as described by Molinari (1998)

2.3 Intensity

a) Analysing Intensity One of the most remarkable features of a tropical cyclone is that the intensity can be estimated simply from its shape and overall cloud features (Dvorak 1976). Without

4 GJH 13 March, 1998 such a potential, most regions would have only a poorly defined idea of the intensity of cyclones, as aircraft reconnaissance is only sustainable in the North Atlantic and occasionally in the East Pacific. Perhaps this characteristic feature of tropical cyclones holds important clues to our understanding, clues that have not been fully exploited by the research community.

Care needs to be taken with the use of archives, however, in that the lack of proper verification leads to a tendency to assume a higher degree of accuracy to the than is valid. This point has been made in previous workshops, but there remains a lack of a reasonable error assessment of the historical archive. Indeed in several regions, the empirical relationships to cloud features has not ever been properly validated.

All analysis techniques make use of some form of wind-pressure relationship to relate the maximum winds in a tropical cyclone to the central pressure. The techniques that are used vary considerably in their maximum wind for a given central pressure. There are undoubtably differences in individual storms and across ocean basins, For example, there has been substantial discussion in recent years on the potential for small storms to have different pressure-wind relationships. Whilst this work has been largely inconclusive, due largely to the lack of good data, it indicates the involved. But the variability also can arise from different analysis methodologies and even in the definitions of "wind". The range of wind averaging times identified in Holland (1993) indicates some of the , but there also is uncertainty with relating aircraft observed winds, which sample a spatial domain, to surface observed winds which sample the flow over a set time period. Further, as numerical models become more useful at indicating intensity, what is the relationship between a wind in a numerical model and that at a surface observing point?

Added to these difficulties is the mesoscale structure that is becoming obvious in tropical cyclone surface wind fields. This occurs particularly at landfall (see Marks 1998), but is present in the form of eye-wall vortices and the like over the ocean.

5 GJH 13 March, 1998 When a forecast office indicates a maximum wind speed for a cyclone, is this the worst wind that could be experienced anywhere in the system? If so, what is the circumstances (over ocean, at landfall, topographical effects)? What is the potential error in such an estimate?

b) Preformation and Initial Development Frank (1998) provides a summary of processes and issues related to cyclone formation. An earlier review by Holland (1995) also placed the overall initiation of a tropical cyclone in terms of a complex interaction of diverse processes arising from wave dynamics associated systems in the equatorial duct, depressions and pre-existing tropical cyclones. These interactions both preconditioned the atmosphere for mesoscale development and provided a modulation that enhanced or weakened the development potential.

As noted by Frank, the current status of knowledge is that there are a number of paths to cyclone development and that these comprise processes and interactions at both the synoptic and mesa scales. The synoptic-scale processes seem to be reasonably well definable from current observing and analysis systems, as is evidenced by recent research and numerical forecasts. For example global models are showing increasing skill at predicting "genesis" without a requirement for bogussing. Whilst the system that develops in these models has only a passing resemblance to actual tropical cyclones, the overall outer structure is reasonable and indicative of strong large-scale influences. Unfortunately, there has been little work undertaken to understand the processes that could be contributing to such development in the global models.

Frank (1998) lists a set of theoretical and observational issues to be considered by the meeting. These include: the relationship between deep convection and vorticity during the genesis period; theprocessess whereby the low -level vorticity maximum is produced; the environmental conditions required to form an MCS and MCV, and how these conditions may be specified or parameterised in numerical models; the processes of modulation of a developing vortex by the large - scale flow patterns, and the age-old bogey of defining genesis.

6 GJH 13 March, 1998 Of critical importance, however, is the continuing question of whether forecasters actually care about improved understanding or capacity to forecast genesis. The survey by McBride and Holland (1987) for the first IWTC found that forecasting offices considered genesis to be well forecast and of little importance. Perhaps it is sufficient to simply place a genesis alert out and wait for development? However, perhaps the move towards longer period forecasting will place a higher priority on forecasting development?

c) Intensification Tropical cyclone intensification is monitored by falling central pressure and increasing maximum winds, which arise from a combination of internal and external processes (Eisberry 1995, Molinari 1998). The processes that have become a focus at this workshop include the interaction that occurs at the air- sea interface in the core region (Lighthill1998), the intensity limits dictated by thermodynamic conditions in the cyclone environment (Emanuel1998), and the lateral interactions that occur both in the upper troposphere (Molinari 1998) and with nearby mesoscale systems (Ritchie 1998).

The fundamental dynamic associated with intensification is the development of an eye and eye wall and the subsequent contraction inwards as the cyclone intensifies. This dynamical process has its own inherent timescale, which may vary from a number of hours to several days. Importantly, once started the process has a tendency to continue unless it is disrupted in some way. The manner in which a troprcal cyclone will respond to lateral influences from the environment, and the like, therefore may depend on the relative timing of the influence within this dynamical timescale. For example it is conceivable that an interaction in the upper troposphere early in the cyclone lifetime could initiate a cycle, whereas the same forcing during an existing intensification could disrupt an existing intensification cycle.

The energy to sustain cyclone intensification comes directly from the underlying ocean. lt is relatively straightforward to show that the pre-existing energy in the atmosphere can at most lead to surface pressure falls of a few tens of hPa. The additional energy is provided by a feedback mechanism, whereby the lower pressure

7 GJH 13 March, 1998 over the warm ocean enables an additional upward flux of energy, which can provide further lowering of the pressure, and so on. The thermodynamic theories that have been developed to describe this process are summarised in Emanuel (1998). These theories provide explicit predictions of the maximum achievable intensity from the thermodynamic state of the environment. The climatological results provide a good upper bound on maximum intensity, but do not explicitly predict the intensity that can be reached by an individual cyclone. There are a number of reasons for this behaviour.

1. The cyclone may modify its own environment to be less favorable for intensification. A well-known consideration here is the changes to the ocean temperatures arising from the presence of the cyclone, which is particularly effective on slow moving systems. However, recent work by in Australia indicates that the thermodynamic capacity of both the oceanic and atmospheric environment may be substantially reduced by the changes that occur during both the formation and subsequent intensification of the cyclone. An area highlighted by Lighthill (1998) as one in which there is very little understanding is the detailed processes occurring in the high wind region, where the majority of the energy is transferred from the ocean. Of particular interest is the potential negative role of the spray layer that develops once the winds move above force.

2. Internal could inhibit development to the full thermodynamic potential. One example is the potential for eye wall breakdown associated with barotropic of ring of potential vorticity near the maximum wind radius.

3. A range of negative influences may be provided from the environment. Chief amongst these in the literature is movement over land and the role of vertical shear. However, it is likely that the focus on vertical shear is as much a result of our strong reliance on visual interpretation of patterns from satellite imagery as it is a real process. A developing cyclone with vigorous convection could resist a degree of vertical shear, whereas a cyclone that has lost its thermodynamic energy source could shear quite easy. Whilst an impinging upper-level trough has associated vertical shear, it also can substantially effect

8 GJH 13 March, 1998 the thermodynamic capacity for intensification (either positively or negatively). We need to re-evaluate our empirical emphasis on shearing as a primary mechanism in the decay of cyclones.

Lateral interactions with the surrounding environment have been associated with tropical cyclone intensification for several decades. Chief amongst these are those that occur with mid-latitude troughs and TUTTs in the upper levels. The status of this work has been summarised by Molinari (1998), who includes those interactions in the deep tropics and the process of extra tropical transition. lt is of interest that despite the significant amount of research and the improved understanding that has occurred in this area, very little objective forecasting information has been provided for operations. Molinari (1998) indicates that this may be partially due to the different effects of potential vorticity anomalies of various sizes and intensities. Our earlier comments on the relevant timing of the forcing and the internal dynamical timescale for intensification may also be important. Improved applied research in this area is recommended. This research can only be seriously undertaken if it can be supported by direct observations at the appropriate scales and times.

Research into the impacts of mesoscale system's both within and outside the core region has expanded substantially in recent years. Partially this has arisen from improved observations that are now capable of resolving the mesoscale features, and partly it has arisen from a stronger emphasis on the vorticity view of potential interactions. The current status is summarised by Ritchie (1998). This entire field of work is still somewhat in its infancy and there are a range of interesting areas with strong potential operational application that should be pursued. Of major importance, for example, is the manner in which mesoscale convective systems and their associated potential vorticity anomalies in the outer circulation of a tropical cyclone can directly affect its intensity. Our limited understanding at this stage indicate that the convective systems may initially weaken the intensification potential, but intensification may be enhanced as the smaller system merges with the tropical cyclone.

9 GJH 13 March, 1998 The potential influence of mesoscale processes in cyclone intensity change can perhaps be illustrated by the following thought experiment. There has been some emphasis in recent years on the major intensification potential that occurs when a cyclone moves over a localised region of warmer ocean. The canonical example has been hurricane Opal in the Gulf of Mexico. A simple explanation is that the enhanced thermodynamic potential over the warm oceanic pool provides conditions for rapid intensification. However, Opal has also been used extensively as an example of intensification following interactions with an upper level trough (Molinari 1998). Further, there is evidence from numerical model experiments that a cyclone approaching a warm ocean anomaly initially weakens as the convection is disrupted by mesoscale development over the anomaly. Is the subsequent rapid intensification a result of the enhanced thermal potential, or ofthe trough interaction, or of the dynamic interactions arising from the formation of a new eye wall and its contraction? Perhaps the clue to the difficulties that we are having in relating these diverse interactions lies in the associated mesoscale processes?

d) Mature Stage Variability lt is highly likely that tropical cyclones arely, if ever, reach a steady state in maturity. The occurrences of such steady states in cyclone archives is more likely due to analysis errors or uncertainties than a true steady state. This raises the question of why a steady state is not reached? Several potential answers present themselves:

1. The region just inside the RMW is barotropqically unstable to horizontal perturbations, and this instability increases as the eye wall cpntracts and the horizontal gradient of vorticity sharpens. Further, as the convective processes that provide the forcing for the cyclone eye wall contraction weaken, the potential for barotropic processes to become dominant may increase.

2. Eye wall replacements have been observed in several intense hurricanes. Whilst the exact forcing mechanisms have never been well elucidated, there is a definite ten dance for intense cyclone cores to be destroyed by a new eye wall developing at larger radii.

10 GJH 13 March, 1998 3. Once a cyclone reaches its thermodynamic potential, any small changes in the underlying oceanic thermal state can produce significant intensity changes. These oceanic changes could result from modification by the cyclone itself, or by movement to colder oceanic regimes.

2.4 Structure

The structure of the core region should not be neglected in any consideration of the intensity of a tropical cyclone. The obvious and well-known example is the asymmetry that arises from the movement of the cyclone. Whilst this can often be roughly neglected for intense cyclones, it can be the dominant process for a weak, rqpidly moving system. For example, Tropical in produced 90 kt winds, of which around 60 kts were directly associated with its rapid poleward translation ahead of a major mid-latitude trough. The normal method of including the effects of motion in operations is to simply add the motion to the cyclone intensity. This can be markedly incorrect in regions of reasonable vertical wind shear, or asymmetric convective processes.

Almost all other forms of core asymmetry are neglected in operations. Is this a pragmatic solution to a difficult analysis problem? Or are we making significant errors, especially with regard to transient wind maxima? For example, the presence of vortices inside the RMW have been observed on and off for over nearly 100 years. Access to hoigh resolution (time and space) satellite imagery and Doppler radars, has recently lead to a re-discovered of such vortices and an understanding that they are quite common. How should operational warnings accoun for the transient high winds that result from these vortices?

Further, there is evidence, not conlcusive but certainly indicative, of vortex breakdown at landfall producing extreme winds at the surface during and following the passage of the eye. Such processes are obviously of major potential impact on vulnerable coastal communities. Yet they also are not accounted for in warnings!

11 GJH 13 March, 1998 2.5 Summary I try to tie this discussion together with the following summary:

1. The maximum achievable intensity is determined by the thermodynamic environment of the cyclone, which may also determine the rate of intensification. This environment may change with time. lt may be negatively effected by the presence of the cyclone. There is evidence that the thermodynamic environment of a developing storm is always Jess conducive to development of an intense system than is indicated by the climatological mean. That is the process of cyclone development inherently reduces the thermodynamic energy potential;

Questions relate to the processes occurring at the ocean interface, especially the effects of sea spray; on the manner in which the cyclone may change its own environment (both oceanic and atmospheric) to reduce the maximum intensity potential.

2. Lateral dynamical interactions typically effect the rate of intensification. They may reduce the maximum intensity, but it is highly unlikely that they will directly effect the maxmum intensity that can be achieved;

Questions relate to the actual processes involved, both with regard to the imposition of a mid-latitude trough (or potential vorticity anomaly) and the effectes on mesoscale developments within or near the cyclone circulation.

3. The response to a particular external forcing depends on the current state of the environment and particularly the presence of other forcing processes. Further, the internal dynamics have their own time scale and life cycles, which dictate to some extent the capacity to adjust to transient external forcing;

There is relatively little thought given to this in operations, yet the intensity stage of the cyclone is something that is reasonably easy to determine. Is it possible to explicitly

12 GJH 13 March, 1998 include the intensity stage as a predeterminant in estimating external forcing responses (for example as is done with SHIFOR).

4. The three dimensional shape of the cyclone is an important intermediate analysis tool for understanding interactions and processes of importance to intensification;

5. Details of the core region structure may be very important in defining the actual intensity. Whilst such details are often transitory, they can produce very substantia/local winds.

How do we determine the relative importance of such core process, and how do we include them into operations?

Bibliography Only a few references are included here. Full lists may be found in the rapporteur reports and in the review papers listed below.

Dvorak, V.F., 1975: Tropical cyclone intensity analysis and forecasting from satellite imagery. Mon. Wea. Rev., 103, 420-430.

Elsberry, R.L., 1995 (ed): Global Perspectives on Tropical Cyclones, WMO/TD-No. 693, World Meteorological Organization, Geneva, 289pp. ·

Emanuel, K.A., 1998: Rapporteur report 2.3

Frank, W.M., 1998: Rapporteur report 2.4

Holland, G.J., ed, 1993: The Global Guide to Tropical Cyclone Forecasting WMO/TD- 560. World Meteorological Organization, Geneva, 342 pp.

13 GJH 13 March, 1998 Holland, G.J., 1995: Scale interaction in the western Pacific monsoon. Met. Atmos. Phys., 56, 57-79.

Lighthill, J., 1998: Rapporteur report 2.1

Molinari, J., 1998: Rapporteur report 2.2

McBride, J. L., and G. J. Holland, 1987: Tropical cyclone forecasting: A worldwide summary of techniques and verification statistics. Bull. Amer. Met. Soc., 68, 1230-1238. Ritchie, L., 1998: Rapporteur report 2.5

14 GJH 13 March, 1998 1.3-NOV-' 96 FRI 1o: 12 ID: UCL MAlo ~RX NU:071 383 ~519 U'::;.89 P02 r. 1/9

WMO/CAS/ICSU

. FOURTH IN'I':ERNA'I'IONAL WORKSHOP ON TROPICAL CYCLONES

'I'opic 2.1: Air-sea interaction Rapporteur: James Lighthill Department of University College London Gower St., London WClE 6BT 1 England.

Fax (+44)171 383 5519

working Group: P.G. Black, I. Ginis, K.B. Katsaros, J.D. Kepert, V.D. Pudov, L.I<. Shay

2.1.1 Introduc~ion In the report (Lighthill 1991) f~m its Vienna Workshop (held Aug.

1990) ~ the International Council of scientific Unions (ICSU) proposed an

interdisciplinary WMO/ICSU project on ~ropical Cyclone Disasters within which ICSU'e special contributions would be on Tropical Cyclone and, Air-Sea Interaction. It was above all the presumption, that track forecasting - for all its importance - would increasingly need to be supplemented with forecasts of intensity changes, which appeared to call for such an interdisciplinary development. After the proposals had been agreed (March 1991) by WHO, G.J. Holland represented WMO/CAS in preparing, jointly with ICSU, the Beijing meeting of Octobe:r· 19.92 on Tropical cyclone Disasters (Lighthill et al. 1993) . The success of this first TC meeting to include numerous specialists on 'I'C oceanography and air-sea interaction was recognised as making a powerful case for continued interdisciplinary collaboration. A year later, attendance at IWTC-III (, Dec. 1993) followed a similar pattern, which was reflected

strongly in 'Global Perspectives on Tropical pyclones• (Elsberry 19~5) with the 60-page 'Chapter 5. Ocean Response to Tropical Cyclones' as one of its six chapters. By contrast, the \Global Guide to Tropical Cyclone Forecasting'

(Holla~ 1993), compiled before IWTC-III, had hardly mentioned TC oceanography and air-sea interaction, while its brief account (2. 3. 3) of Forecasting Intensity Changes concentrated on •evaluating intensity tendency from cloud

pattern~:· - rather than by any use of ocean data. Yet instances of substantial intensity growth from TC motion over warm currents, or warm-core eddies, are now copiously documented. Ocean-data acquisition systems have expanded hugely, and lend themselves to use with coupled atmosphere-ocean models. Furthermore, the close coupling of an ___...,;;;1;;;;.3-...:.NQU-' ~P FR I 16: 13 ID: UCL MATS FAX N0:071 3~~ ~~l~

specialised for TC forecasting with a high-resolution ocean model is now developed far beyond the level described in section 5.5 of ElsDerry (1995) and is confirming, not only major increases of intensity from passage over warmer ocean areas, but also significant decreases in the presence of oceanic heat-source depletion. These chree remarks, connected to possible future advances in intensiey-change forecasting, are expanded in the next three sections.

Then a final ~ection reconsiders the role of air-sea interaction in possible effects of global change on TC frequency a.nd intensity. The report (Lighthill ee al. 1994) of the symposium held during IWTC-III on 'Global and Tropical cyclones' was later supplemented by a longer paper (Henderson-Sellers et al. 1998) which came nonetheless to similar conclusions. Both papers discussed thQ. possibility that ocean spray at extreme TC wind speeds may ultimately limit tendencies for further rises in sea-surface temperature to go on intensifying Tropical Cyclones, and the present text is concluded with a re-examination of this 'spray cooling' hypothesis.

2.1.2 Intensity growth over warm ocean areas :Even though TC intensities can be reduced by certain key air-sea . interaction mechanisms, this section aims to highlight the importance of

oceani~ data for forecasts and warnings by dramatic evidence that d~ngerous intensification has often accompanied passage of a TC over warmer waters. Briefly, over a pre-existing warm ocean structure, strong latent-heat fluxes are able to re-energize the storm. During 1995 in the Gulf of Mexico, just before striking the coast, hurricane opal moved (Marks and Shay 1997) over a warm core eady which was detected by radar altimeter data from the NASA TOPEX mission, and deepened from 965 hPa to ~16 hPa over 14 hours (Fig.2.1.1). Shay et al. (1998), using surface height anomaly and the underlying density field, inferred a net ·oceanic heat loss after TC passage more than sufficient to have produced the observed intensification. This very detailed evidence supplements 3 well documented •:.ases of hurricanes which similarly intensified, but this time on passing over the warm

Gulf Stream. Out of them, by far ehe most damaging was Andrewi wher~ a combination (Willoughby and Black 1996) of low wind shear with warmw,ter east of Florida deepened ie to 920 hPa just before landfall on 24 August 1992. In subsequent: years (Black. 1998), intensification from the same cause 111as

··) FAX NO:I2l71 3S3 .. ;!:;i19 *=1989 Pl2l4 13-NOU-'96 FRI 16:14 ID:UCL MATS ='------P. 3/ct

observed for hurricanes Jerry (1995) and Bertha (1996) .

2.~.3 Advanced ocean-data acquisition and its uses Any combined use of air and ocean data for future intensity forecasting will demand routine acquisition of relevant ocean data in real time. This

sectio~l lists promising acquisition systems,which have already shown their worth in experimental studies - both in cases of intensification (see above) and in other important ca.ses where depletion of the oceanic heat source reduced TC intensity. satellite systems, with their global applicability, appear specially promising. Admittedly, direct determination of SST from polar-orbiting meteorological satellites can be misleading (Schluessel et al. 1990, Wick et al. 1996). Fortunately, however, a signiiicant coefficient of expansion for warm seawat.er '(ex = o. 00032K"1 at 28°C) allows the presence of a substantial mass of water at elevated temperature to be inferred by use of existing NASA and ESA satellites which either register local distortion of the geoid or make direct radar-altimeter measurements. Yet another parameter affecting air-sea interaction is surface roughness, which can now be routinely determined with a satellite-borne C-band radar measuring Bragg back-scatter from the rough ocean surface. This usefully yields a directional spectrum for waves generated by strong local winds - and thus can be of additional value as suggesting an approximate distribution of surface wind vectors (Quilfen et al. 1994) . In more limited areas, mobile drifting buoy arrays deployed from aircraft have proved highly effective for demonstrating TC intensity changes

from air-sea in~eraction. Surface wind speed and direction are themselves measured by some of these buoys, while others determine SST and ocean mixed­ layer depth together with ocean current profiles. Recent analysis (Shay and Mariano 1996) of 1988 data using airborne expendable buoys showed complex interactions when Gilbert passed to the left of a warm eddy, with wind stress generating a near-inertial internal-wave field tending (Nilsson 1995) to entrain colder water into the mixed layer. In l995 again, a huge array of working buoy platforms demonstrated the oceanic heat-source depletion from successive hurricanes Felix, Luis and Marilyn, with the later hurricanes weakening as they entered the cold wakes of earlier ones (Black 1998).

2-~-4. coupling TC forecasting models to fine-mesh ocean models Among those TC forecasting models which predict track b•st is the 13-NOU-'96 FRI 16:14 ID:UCL ~ATS

mo'9"able triply-nested-mesh model developed at NOAA' e GFPL, which became operational for the U.S. National Heather Service in 1995 (KUrihara et al.

~997); yet it offers relatively poor intensity forecasts (over-predicting weak and under-predicting strong storms), probably because it assumes fixed SST. However, numerical experiments (Bender et al. 1993, Falkovich et al. 1995a,b) used an SST computed after the TC model's fluxes of momentum heat and moisture had been passed into a fine-mesh multi-level ocean model (generating currents in the mixed layer and upper thermocline layer) and then iterated to obtain a fully interactive model, satisfactorily tested against field measurements of ocean response to Norbert in 1984. In the coupled model, as might be expected from chapter 5 of Elsberry (1995), a slower-moving TC both cools the ocean more and itself suffers greater weakening as a result; yet

both effects remain significant for TC ~peeds up to 7.5 m/s (Fig.2.1.2). Moreover binary' TCs, common in the West Pacitic, experience still stronger Tc.:· ocean interaction as each TC crosses the other's wake, with computations from the coupled model confirmed by 1974 observations of the Ione/Kirsten binary system off . More recently (Ginis et al. 1997) the GFDL model was coupled to a high­ resolution version of the Princeton Ocean Model. Experiments were then run (Bender and Ginis 1997) including and excluding the coupling for hurricanes Gilbert {1986), Felix and opal (1995), Fran (1996) and.Erika (1997), with the coupling improving intensity prediction in every case. For still further

improvement, the authors look towards possible advances (see next se~tion) in understanding air-sea exchange under extreme wind conditions.

2.1.5 spray cooling at extreme wind speeds This report ends with a more speculative section suggesting the existence of an ultimate upper limit on increases of a TC's latent-heat uptake as SST rises, associated with air temperatures falling below SST more and more in extreme winds. Evidence of this inc:r:easing temperature difference l:!T between SST and wind temperature has expanded greatly since the IWTC- III Symposium (Lighthill et al. 1994), which relied on 1988 measurements in the Russian research ship Priliv as described by V.D. Pudov at the Beijing meeting (Lighthill 1993). Those, while agreeing (Fig.2.1.3) with conclusions from the HEXOS p:r:ogramme (De Cosmo et al. 1996) in finding no such effects below,lB

m/s, had obtained at higher wind speeds rising values of ~T, tentatively analysed as spray cooling by Fairall et al. (1994). Later Russian work (Petrichenko and Pudoy 1996) analysed data from 30 ships, confirming these ------13-NOV-'96 FRI 16:15 ID:UCL MRI5 FAX N~:071 383 5~~1~9------~-9-89 P06 P. S/'i

increases in 8T up to winds of 40 m/s (Pig.2.1.4). In the meantime, the U.s.

National Data ~uoy Center's moored buoys had found closely comparable data (Black et al. 1993), as would the 1997 recordings (Black 1998) ot SST and 10- metre wind temperature, obtained in Erika by concurrent airborne drops of

bathy~hermograph buoys and GPS dropsondes (Fig.2.l.S}. These data may be important because the ,.igorous energization of a Tropical Cyclone, which is provided by buoyancy forces acting on eyewall air as it rises along a moist-air adiabat, appears likely eo be. diminished by such a depression 8T in wind temperature below SST and by the associated reduction in latent-heat content. A new model by Lighthill (1998) tends to confirm

interpretation of the results as spray cooling; briefly, ~pray becomes eo dense at extreme wind speeds that almose all vapour transfer to air comes

directly from spray droplets, after which~oyancy-powered ascent of saturated air is able to ·begin before its cooling from evaporation can be compensated by sensible-heat transfer from the ocean surface.

As regards trends in ~T at still higher wind speeds, much uncertainty remainSi the only datum point above 45 m/s is a single buoy record at 60 m/s which shows no increase, but this is not statistically significant. The

•spray cooling' interpretation makes ~T likely to go on rising with wind speed

-which could constitute a self~limiting factor on increase in TC intenaities.

2.1.6 Summary Beyond such speculations, this report includes a principal message for forecasting authorities who need to achie,.e, not only good track torecasts, but also good intensity-change forecasts. That will demand real-time oceanic data, and we envisage developments in which their routine acquisition - for use as initial data in a TC forecasting model coupled to a fine-mesh ocean model -will significantly improve the prediction of intensity change.

Bibliography Bender, M.A., Ginis, I., and Xurihara, Y., 1993: Numerical simulations of the tropical cyclone-ocean interaction with a high-resolution coupled model. J. Geophys. Res., 98, 23245-232G3.

Bender, M.A., and Ginis, I., 1997: Real case simulations of hurricane-ocean interaction using a high :resolution coupled model: Effect on hurricane intensity. Mon. Wea. Rev. (to appear).

Black, p. G., 19.98: Air-sea interact:ion processes relevant to Tropical Cyclone intensity change, in Proceedings of the Special Session on Tropical Cyclones, 78th Annual AMS Meeting, 11-16 January 1998, American Meteorological society, Bo5ton.

. ~' --- 13-NOU-'96 FRI 16:16 ID:UCL MR-15

Black, P.G., Holland, G.J. and Pudov, V., 1993: Observations of sea-air temperature difference in tropical cyclones as a function of wind speed: Importance of spray evaporation. In BMRC Report No.46, Bureau of Meteorology Research Centre, Melbourne.

De Cosmo, J., Katsaros, K.B., Smith, s.o., Anderson, R.J., oost, W., Burnke, K., and Chadwick, H., 1996: Air-sea exchange of water vapor and sensible heat: The Humidity Exchange Over the Sea HEXOS results. J. Geophys. Res., 101, 12001-12016.

Elsberry, R.L., 1995 (ed) ~ Global Perspectives on Tropical Cyclones, WMO/TD· No.693, World Meteorological Organization, Geneva, 289 pp.

Emery, W.J., Yu, Y., Wick, G.A., Schluessel, P., and Reynolds, R.w .. , 1994: Correcting infrared satellite estimates of sea surface temperature for atmospheric water vapor attenuation. J. Geophys. Res., 99, 5219-5236.

Faber, T., Shay, L.K., Jacob, S.D., Houston, S.H., and Black, P.G., 1997: Observed air-sea interactions during hurrisane Emily. Proceedings of the 22nd Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, Boston, pp.433-434.

Fairall, c. W. , Kepert, J. D. , and Holland, G. J. , 1994: The effect of sea spray on surface energy transports over the ocean. The Global Atmosphere and Ocean System, 2, 121-142.

Palkovich, A.L., Khain1 A.P., and Ginis, I., l995a: The influence of air-sea interaction on the development and motion of a tropical cyclone: numerical experiments with a triply nested model. Meteorol. Atmos. Phys., 55, 167-184.

·Falkovich, A.L., Khain, A.P., and Ginis, I., 1995b: Motion and evolution of binary tropical cyclones in a coupled atmosphere-ocean model. Mon. Wea. Rev., 123, 1345-1363. Ginis, I., Sender, M.A., and Kurihara, Y., 1997: Development of a coupled hurricane~ocean forecast system in the North Atlantic. Proceedings of the 22nd Conference on Hurricanes and Tropical Meteorology, American Meteorological society, Boston, pp.443-444.

Henderson-Sellers, A., Zhang, H., Berz, G., Bmanuel, K., Gray, W., Landsea, C., Holland, G., Ligbthill, J., Shieh, S.L., Webster, P., and McGuffie, K., 199S: Tropical Cyclones and Global Climate Change: A Post-IPCC Assessment. Bull. Amer. Met. Soc. (to appear).

Holland, G. J., 1993 (ed) : Global Guide to Tropical Cyclone Forecasting, WMO/TD-No.560, World Meteorological Organisation, 300 pp.

Holland, G.J., 1997: Maximum potential intensity of tropical cyclones. J. Atmos. Sci. (to appear).

Kurihara, Y., Tuleya, R..E., and Bender, M.A., 1997: The GFDL hurricane pz;ediction system and its performance in the 19.95 hurricane season. Mon. w~a. Rev. (to appear).

Lighthill, J., 1991: Tropical Cyclone Disasters, Appendix 2 of First Report of ICSU Special Committee for IDNDR, International Council of Scientific Unions, Paris, 14pp. FAX NQ;, 07,1 31:13 !?:.;;;5.;::.19~--- ~9B9_P_0_B_____ - 13-NOv-'96 FRI 16:16 ID:UCL MATS 1 :r. 7/C{

Lighthill, J., Zheng Zhemin, Holland, G., and Emanuel, K., 1993; Tropical Cyclone Disasters, Peking University Press, Beijing, 588pp.

Lighthill, J. , Holland, G. J. , Gray, w. M. , t.andsea, C. , Craig I G. 1 Evans, J. 1 Kurihara, Y. 1 and Guard, C. 1994: Global climate change and tropical cyclones. Bull. Amer. Met. Soc., 75, 2l47-2l57.

Lighthill, J., 1998: Ocean spray and the thermodynamics· of Tropical Cyclones. J.Engrg. Maths. (to appear).

Marks, F., and Shay, L.K., 1997: Landfalling tropical cyclones: Forecast problems and associated research opportunities. Bull. Amer. Met. Soc. (to appear).

Nilsson, J., 1995: Energy flux from traveling hurricanes to the ;:>ceanic

internal wave field. J. Phys. Oceanogr., 25 1 55Sw573.

Petrichenko, S.A., i Pudov, V.D., 1996: 0 parametrizatsii teplo- i vlago­ obmena mezhdu okeanom i atmocferoi v t:w:>pikakh v shtormovykh usloviyakh. Meteorologiya i Gidrologiya 1996, No. 6, 92·100.

Quilfen, Y., Katsaros, K. et Chapron, B., 1g94: structures des champs de vent de surface dans les cyclones tropicaux observ~s par le diffusiometre d'ERS-l et relations avec les vagues et lea . M~moires de l'Institut oce&nographique no.lB, Monaco.

Schade, L.R., 1995: How the ocean affects hurricane intensity- a numerical investigation. Proceedings of the 21st Conference on Hurricanes and Tropical Meteorology. American Meteorological Society, Boston, pp.33S-338.

Schade, L.R., 1997: A physical interpretation of SST-feedback. Proca~dings of the :?.2nd Conference on Hurricanes and Tropical Meteorology. American Meteorological Society, Boston, pp.443-444.

Schluessel, P., Emery, W.J., Grassl, H., and Mammen, T., 1990: On the bulk­ skin temperature difference and its impact on satellite remote sensing o'f the sea surface temperature. J. Geoph. Res., 95, 13341-13356.

Shay, L.K. and Chang, s.w., 1997: Free surface effects on the near-inertial ocean current response to a hurricane: a revisit. J. Phye. oceanogr. 27, 23- 3.9.

Shay, L.K. and Mariano, A.J., 1996: ~nergy flux from hurricane Gilbert winds to near-inertial motions. Proceedings of 8th Conference on Air-Sea Interactions and Symposium on GOALS, American Meteorological Society, Boston.

Shay, L.K., Goni, G.J., Marks, F.D., Cione, J.J., anc.i Black, P.G., 1998: Role of warm ocean features on intensity change: hurricane Opal, in Proceedings of the Special Session on Tropical Cylones, 78th Annual AMS Meeting, 11-16 January 1.998, American Meteorological Society, Boston.

Wick, G.A., Emery, W.J., Kantha, L.H., and Schluessel, P., 1996: 'l'he belavior of the bulk-skin sea surface temperature difference under varying wind speed and heat flux. J. Phys. Oceanogr., 26, 1969-1988.

Willoughby, H.E., and Black, P.G., 1996: Hurricane Andrew in Florida: Dynamics of a disaster. Bull. Arner. Met. Soc., 77, 543-549. t" H.X. NU: l:!:f( J. .,::,c.,::, ;:J:?-:.:J.:..:';):------t-t-"J-O"J--:"'r_~_"J____ ,-. \ 13...:.NOU-'96 FRI 16:17 ID:UCL MRT5 f. ~/9 1 I

1 1 1 1 1 1 · Upp~r-le~el T~ugh ~ ..4 .~' ' ' ~ (PV Anomaly) 1" 1 ~· \ ; ; ', Landfall T , , , • 940 hPa ( .... ., , 1:!~,<·~014 22Z ~,~/ • v--."\.. ·~ r ,~ . 1 sa... ., .~·· ~ ,...- • '-----91&; hP ,I ~ 10/4·\~JZ 965 hPa • • 939 hPa-_., · r10/3 21Z~. 1014 09Z . -~\ 1 • WarmOce~~~-~ J. " I. • t1 J-.-• .-dddy ~ , ':\: \ --.,. . ... '\ ~·I o j;> • '·~ '\~ :. -~ Hurrlcane.Op'al ,~

p I f. 191~S:1o;o~~ ~

Fig. 2.1.1: Intensification of Opal over a warm ocean eddy. -.:

- XUIIIICA.Mt ..OCb.N C.OUf~O•ODil -'-V••IC'.Urii·OClAN COVPLID "0011. ---•IIIIOIICANI •OIW ••••ltiRIICANI.OIIfl I i ii I

~--u~--=u----~.---.-----~~~R a~o--~R~~~~~.~~q~~--~n·

&A/1 !:ASTER!. T IAJIC FlOW 7..!G/o !A$T£Rl Y I ASIC FI.OW I a~ I

Fi1.2.1.1.rTune series o£ minimum sea level pressure (bPa) tor the ao basic ftow e,.;perimet:lts and all experiments with easterly basic ftows run with hurrlcane-occa.n coupling (solid curve) and without coupling (dashed ~urvc}. ---1•3;;;.-..:.Nq_~-:. ~-9.,6. f.R.I 16: 20 ID: UCL MATS FRX N0:071 383 5519 ~990 P02

6

5 g 4 .J"" 3 2

1

00 4 8 12 16 20 24 28 Wind Speed (rnls)

Fig. 2.1.3: P~iliV measurements of bT, from Fairall et al. (l994) "' . ..

mj&

· Fig. 2.1.4: Observations of ~T from 30 ships (Petrichenko and Pudov 1996) 5.0 . 5.0. Q 0 0 4.0 ,.o 4.0 li. o. (I .oCl - e 0 oo . -.._.. l ...... ! ~ . .... E'r--(9> ~ 3.0 I 3.0 lcftt d:J Q .a. ... ."aca ~ 0 (f) ~g_o 2.0 -' ' C) SST·TI.I 2.0 Cf.) ·~ 0 ~·- -• o,._ 0 . 1.0 r----· 0 A. 1.0 ... I ... ESSTJ .a. 0.0 __..___.._ 0.0 0 10 20 30 40 50 60 1 0-m Wind Speed (mls) Fig.2.1.5: Data from airdropped buoys and sondes (triangle&) and from moored buoys (circles) ; / WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic: 2.2 Environmental Interaction (Intensity)

Rapporteur: John Molinari Department of and Atmospheric Sciences University at Albany State University of New York 1400 Washington Avenue Albany, NY 12222 USA

Email: molinari@ atmos.albany.. edu Fax (518) 442-4494

Working Group: J. Abraham, P. Harr, S. Jones,'R. Smith, C. Thorncroft

2.2.1 Introduction

Tropical cyclone intensity remains one of the most poorly predicted parameters by forecasters (Eisberry et al, 1992). lt will be argued that major potential gains·could arise from the study of relevant mechanisms.

In the broadest sense, three factors play the major role in tropical cyclone intensity change: sea surface temperature, internal dynamics (such as eye wall cycles), and external interactions with the atmosphere (including vertical shear). Air-sea interactions . have their own separate working group, so this document will deal only with the latter two factors. Internal dynamics does not strictly belong in this section on environmental interaction· and its effects upon intensity, but because intensity change depends so much on internal dynamics, the topic will be briefly examined.

2.2.2 Internal dynamics

The phrase "internal dynamics" encompasses everything that goes on in the core of the hurricane, including the role of thermodynamic processes. For the purposes of this report, one phenomenon will be isolated, that of eye wall cycles, in which a second eye wall develops outside the first and often, as it contracts, replaces the first eye wall. Eye wall cycles occur most commonly in very intense storms, especially those storms that are near their maximum potential intensity (Willoughby, personal communication). Their development often coincides with large changes in intensity in both directions. Their structure and balanced dynamics has been well described by Shapiro and Willoughby (1982) and Willoughby et al (1984), but the cause of their initial development is unclear. Willoughby (1990) has provided possible internal dynamical mechanisms ("internal" in the sense that no environmental interaction with an independent feature is needed). Molinari and Vollaro (1990) noted that such cycles

1 GJH 11 March, 1998 frequently appear during interactions with upper tropospheric troughs, although the mechanism by which this occurs is unknown. The chapter by Willoughby in the 1995 IWTC report covered eye wall cycles well, but some work has been done since that is described below.

Samsury and Zipser (1995) show that secondary (outer) eye walls have a similar structure to primary eye walls. Willoughby (personal communication, 1997) notes that outer eye walls can be found at large radii, exceeding 100 km, in both weak and strong storms, although these latter features appear to have little influence on intensity. Ritchie and Holland (1993) and Guinn and Schubert (1993) have shown that features resembling secondary eye walls can be produced by filamenting of positive potential vorticity anomalies outside the core by the primary circulation. These papers have contributed to our understanding of eye wall cycles, but it is clear that major gaps still exist in our understanding. Montgomery and Kallenbach (1995) have formally described the filamentation process in terms of vortex Rossby waves, and this provides a potentially rewarding path for understanding such events.

2.2.3 External atmospheric interactions

Two primary factors can be listed here: vertical ,wind shear and interactions with upper tropospheric troughs. Here "trough" is used as a generic term that encompasses any vorticity maximum (positive potential vorticity anomaly). lt includes small-scale cutoff lows ("TUTT" cells) as well as propagating baroclinic waves.

2.2.3.1 Vertical shear influences

In the simplest sense, vertical shear tilts the column and thus tilts some of the warm air out of the vertical, raising the surface pressure and weakening the storm. Raymond (1992) noted that vertical circulations occur in respond to this tilt that act to move the storm back toward a vertical orientation. Upward motion occurs downshear in this diagnostic framework. DeMaria (1996) noted that the vertical circulations induced by vertical wind shear also produce significant stability changes that may act to produce asymmetric convection. Simple models of hurricane-like vortices without convection were used by Jones (1995) to examine the time evolution of tropical cyclones experiencing vertical shear. She showed that once the vortex is tilted, extremely complex behavior can follow, suggesting that the ideas of Raymond do not describe the time-dependent behavior of a storm experiencing shear. lt is also true, however, that convection strongly couples upper and lower levels and may act to limit the behavior seen by Jones.

Vertical wind shear can effectively be specified independently of the tropical cyclone in idealized model simulations, but in nature the vertical wind shear evolves as a consequence of two-way interactions between the storm and its environment, with convection playing a large role in the interaction. As a result, the influences of vertical shear and the mechanisms by which it acts remain to be fully explained, a conclusion certainly recognized by the above investigators.

2 GJH 11 March, 1998 When upper tropospheric troughs are approaching, vertical shear influences become even more complex, because the vertical wind shear evolution is coupled to that of the trough. These effects will be discussed in the following section.

2.2.3.2 Trough interactions without extratropical transition

This topic was not covered to any degree in the previous IWTC report. Molinari et al. (1995; 1997} have argued on the basis of potential vorticity dynamics that a hurricane will intensify in response to an upper positive PV anomaly if the upper anomaly is reduced in scale to that of the tropical cyclone, and if the upper anomaly becomes superposed over the cyclone. This explanation covers only some of the possible interactions that occur in nature. A number of questions remain on the nature of tropical cyclone-trough interactions:

1. What is meant by trough interaction? When a synoptic scale trough is, for instance, 1500 km northwest of a hurricane, it almost always will influence hurricane motion, as has been shown by PV . Is the trough then interacting with the hurricane? A theoretician might say yes, a synoptician no. The key operational question is whether the trough interaction is having an impact on tropical cyclone intensity. lt is not trivial to determine this! What is needed is an objective way to define a trough interaction. Eddy momentum flux convergence has been used with some success, as have PV fluxes and nonlinear balanced vertical motions near the core. But no formal way yet exists of defining a trough interaction.

2. How does a synoptic scale trough interact with a mesoscale hurricane?

Upward motion ahead of a synoptic scale trough is on a scale much larger than that of a hurricane. If large-scale upward motion occurs over a hurricane, will it not induce heating over a large scale as well? If so, it will produce weakening, not strengthening of the storm. This has been established from theory, modelling, and, to a lesser extent, observation.

Molinari et al. (1995) argued that synoptic scale troughs do NOT interact favorably with tropical cyclones. Rather, some process must occur to reduce the scale of the upper trough before it approaches the tropical cyclone. The argument was made using PV principles. The scale reduction in one case (Eiena, 1985) was hypothesized to occur via the process of synoptic-scale wave breaking induced in part by the hurricane outflow anticyclone. In the absence of that, Molinari et al {1997) argue that a rather complex mix of vertical shear and diabatic heating influences together act to pinch off a small scale .. piece .. of the upper PV maximum and bring it over the storm. This scale matching of upper PV and tropical cyclone is essential in this view.

3. How does the tropical cyclone core respond during trough interactions? If this question is not answered, no understanding of intensity change during environmental interactions is possible. In general, we need much more data from the inner core at all levels during such interactions.

3 GJH 11 March, 1998 4. Does the nature of tropical cyclone-trough interaction differ for existing storms from pre-hurricane disturbances? An argument can be made in terms of PV dynamics that trough interactions should have similar impacts regardless of the stage of the tropical cyclone. But eye wall cycles do not occur in pre-genesis stages, and the tropical cyclone internal dynamics obviously differs greatly for a mature storm. This issue still must be resolved.

5. Do identifiable parameters exist for determining whether a tropical cyclone will intensify or weaken in response to an upper tropospheric trough? This is the fundamental forecast problem. Forecasters are well aware that "bad troughs" exist that produce weakening of tropical cyclones, although only one such case has appeared in the literature. Many researchers have speculated on this issue, but no method has proven successful. It is likely that basic understanding will have to improve before such an advance can be made.

6. Hurricane Opal Hurricane Opal intensified in the Gulf of Mexico when an upstream trough was quite distant from the storm, then rapidly filled when the trough got close. Was this a favorable or an unfavorable .trough interaction? Or both? By what mechanisms did the deepening and filling occur? And, of course, what about other factors such as sea surface temperature fluctuations? Opal seems to be an excellent case for testing any theories of trough interaction.

7. What sort of numerical model is needed to simulate tropical cyclone-trough interactions? Real cases are greatly complicated by other factors such as SST fluctuations, both pre-existing (as in warm eddies in the Gulf of Mexico) and tropical-cyclone-induced. As a result, there is a tremendous need to simulate tropical cyclone-trough interactions with idealized numerical models in which such variables can be controlled. This may prove to be a daunting task, because it is essential that details of the tropical cyclone core be resolved in order to simulate the response of the tropical cyclone to.the trough. This almost certainly requires the resolution of a clou9 ensemble model or something close. The simulation of tropical cyclones on such scales is still in its infancy. Early results (Tripoli, personal communication, 1993; Eastman, personal communication, 1997) indicate considerable sensitivity to the details of the microphysics when less than 5 km resolution is used. Such difficulties were anticipated long ago by Ooyama (1982). It may be that successful simulation of hurricane-trough interactions will have to await the resolution of these issues.

2.2.3.3 Trough interactions with extratropical transition

Extratropical transition provides a major forecast problem for middle latitude countries, including Australia, , China, Japan and the United States. This issue has not received a great deal of interest in previous IWTC publications. The circulations that result during this process can have high winds and heavy rainfall, and thus are an important forecast problem for many regions.

4 GJH 11 March, 1998 Matana and Sekioka (1971) defined two basic types of extratropical transition (ET) of a tropical cyclone (TC). A complex transition occurs when a pre-existing midlatitude front or trough interacts with a TC to produce a new on the front or trough. A compound transition results when a pre-existing midlatitude cyclone appears to merge with a TC such that the mid latitude low usurps the TC and intensifies. The original descriptions of Matana and Sekioka (1971 ), which were based mainly on surface pressure patterns did not explicitly acknowledge the roles of mid- and upper-tropospheric circulations. Dimego and Bosart (1982) studied .the complex transition of in 1972 and suggested that low- level heating and cyclonic vorticity associated with the remnants of Agnes combined with upper-level positive vorticity advection to permit the transformation of Agnes into an extratropical cyclone through typical Petterssen-Smebye Type-B development. Based on a study of Tropical Cyclone Patsy in the western South Pacific, Sinclair (1993) suggested that coupling of the weakening TC circulation with a 200-mb subtropical jet streak played a significant role in ET. These type of case studies have documented the complex three-dimensional aspect of the ET problem

The problem of ET has also been examined with respect to different types of circulation patterns over the tropics, subtropics and midlatitudes. The character of ET may be dependent on the tropical or subtropical circulation that helped steer the TC to higher latitudes or on the midlatitude circulation that the TC is moving into. Foley and Hanstrum (1994) identified two types of ET along the west coast of Australia. Those TCs caught in pre-cold frontal westerlies were said to be captured, which is similar to a complex transition. Those that moved poleward while remaining in a synoptic pattern with easterly flow on the poleward side were said to be cradled. This category has no counterpart in the original descriptions of Matana and Sekioka ( 1971). Over the western North Pacific, development of the systematic approach to tropical cyclone forecasting (Carr and Elsberry 1994) has lead to recognition that several circulation patterns may steer a TC into the midlatitudes. A study by Harr et al. (1996) indicated that ET is not sensitive to the circulation that steered the TC poleward but it is sensitive to the type of midlatitude circulation pattern that the TC is moving into. The impact of the TC on the midlatitude circulation and the ability of a global numerical model to forecast the impact varies with midlatitude circulation pattern.

2.2.4 Roadblocks to further advances

------The major roadblocks to further advances in the first two of the above topics is lack of data. The new Gulfstream IV aircraft, which flies at or above the level of trough interactions and is capable of realeasing a large number of dropsondes, will ultimately contribute greatly. Currently the aircraft does not fly across the storm circulation. To the extent that this can be done in coming years, a three-dimensional dataset that reaches from the storm core to the 400 or 500 km radius should be achievable during a trough interaction.

The inner core remains a greater challenge. Many Atlantic storms have specialized data sets collected by the Hurricane Research Division of NOAA, but these are typically at a single level. The development of SS M/I channels, especially the 85 GHz channel, hold promise for observing eye wall cycles through the dense cirrus overcast.

5 GJH 11 March, 1998 The extratropical transition problem suffers much less from lack of data. Even there, however, msesoscale developments along pre-existing fronts, for instance, are poorly observed even by the land-based network.

2.2.5 Research topics with the greatest potential return

The greatest potential return would be in study of trough interactions and extratropical transition, both before and after landfall. The dynamics and evolution of inner core regions are of great importance, but fewer data sources are available at this time that could give significant new insights.

2.2.6 References

Anthes, R.A. 1990: Advances in the understanding and prediciton of cyclone development with limited-area fine-mesh models. Extratropical Cyclones, Palmen memorial volume, C.W. Newton and E.O. Holopainen, Eds. Amer. Meteor. Soc., 221-253.

Bosart and Dean 1991: The Agnes rainstorm of June 1972: surface feature evolution culminating in inland storm redevelopment. Wea. Forecasting, 6, 515-537.

Bosart, L.F. and G.M. Lackmann 1995: Postlandfall tropical cyclone reintensification in a weakly baroclinic environment: a case study of hurricane David (September 1979). Mon. Wea. Rev. 123, 3268-3291.

Brand, S. and C.P. Guard, 1979: An observational study of extratropical storms evolved from tropical cyclones in the western north Pacific. J. Met. Soc. Japan, 57, 479-483.

Carr, L. E., Ill, and R. L. Elsberry, 1994: Systematic approach to tropical cyclone track forecasting. Part I. Approach overview and description of meteorological basis. Tech. Rep. NPS-MR-94-002, Naval Postgraduate School, Monterey, CA 93943, 96 pp.

Chien, H.H. and P.J. Smith, 1977: Synoptic and kinetic energy analysis of (1969) during transit across the southeastern United States. Mon. Wea. Rev. 105, 67-77.

DeMaria, M., J.-J. Baik, and J. Kaplan, 1993: Upper level eddy angular momentum fluxes and tropical cyclone intensity change. J. Atmos. Sci., 50, 1133-1147.

DeMaria, M., and J. Kaplan, 1994: A statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic Basin. Weather and Forecasting, 9, 209-220.

DeMaria, M., 1996: The effect of vertical shear on tropical cyclone intensity change. J. Atmos. Sci., 53, 2076-2087.

DiMego, G.J. and L.F. Bosart, 1982: The transformation of tropical storm Agnes into an extratropical cyclone. Part 1: The observerd fields and vertical motion computations. Mon. Wea. Rev., 110, 385-411.

6 GJH 11 March, 1998 DiMego, G.J. and L.F. Bosart, 1982: The transformation of tropical storm Agnes into an extratropical cyclone. Part 11: Moisture, vorticity and kinetic energy budgets. Mon. Wea. Rev., 110, 412-433.

Elsberry, R.L., G.J. Holland, H. Gerrish, M. DeMaria, C.P. Guard, and K. Emanuel, 1992: Is there any hope for tropical cyclone intensity prediction? Bull. Amer. Meteor. Soc., 12,264-275.

Emanuel, K.A., 1997: Some aspects of hurricane inner-core dynamics and energetics. J. Atmos. Sci., 54, 1014-1026.

Foley, G.R. and B. N. Hanstrum 1994: The capture of tropical cyclones by cold fronts off the west coast of Australia. Wea. Forecasting, 9, 577-592.

Frank, W.M. and E .. A. Ritchie, 1997: Environmental effects on the structure of tropical storms. Preprints, 2Z'd Conference on Hurricanes and Tropical Meteorology, Fort Collins, CO, Amer. Meteor. Soc., 352-353.

Guinn, T.A., and W.H. Schubert, 1993: Hurricane spiral bands. J. Atmos. Sci., 50, 3380-3403.

Harr, P.A., R. L. Elsberry, P. M. Klein, T. F. Hogan, and W. M. Clune, 1996: Impacts of the extratropical transition of tropical cyclones on midlatitude circulation systems. Preprints, 18h Cont. on Wea. Analysis and Forecasting, Norfolk, VA, Amer. Met. Soc., Boston, MA 02108, 509-512.

Jones, S.C., 1995: The evolution of vortices in vertical shear. 1: Initially barotropic vortices. Quart. J. Roy. Meteor. Soc., 121; 821-851.

Kornegay, F.C. and D.G. Vincent, 1976: Kinetic energy budget analysis during interaction of tropical storm Candy {1968) with an extratropical frontal system. Mon. Wea. Rev., 104, 849-859.

Lewis, B. M., and D.P. Jorgensen, 1978: Study of the dissipation of Hurricane Gertrude (1974). Mon. Wea. Rev., 106, 1288-1306.

Matano, J. 1958: On the synoptic structure of , 1954, over the eastern United States. J. Met. Soc. Japan, 36, 23-31.

Matano, H. and M. Sekioka, 1971: On the synoptic structure of typhoon Cora, 1969, as the compound system of tropical and extratropical cyclones. J. Met. Soc. Japan, 49, 282-295.

Mclntyre, M.E., 1993: lsentropic distributions of potential vorticity and their relevance to tropical cyclone dynamics. Tropical Cyclone Disasters, Lighthill, Holland, Zhemin, and Emanuel, Eds., Peking University Press, 143-156.

7 GJH 11 March, 1998 Moeller, J.D., and R.K. Smith, 1994: The development of potential vorticity in a hurricane-like vortex. Quart. J. Roy. Meteor. Soc., 120, 1255-1266.

Mohr, T., 1971: Beitrag zur Umwandlung tropischer Wirbelstuerme in intensive aussertropische Zyklonen. Berichte des Deutschen Wetterdienstes, 121, 1-32.

Molinari, J., S. Skubis, and D. Vollaro, 1995: External influences on hurricane intensity. Part Ill: Potential vorticity structure. J. Atmos. Sci., 52, 3593-3605.

Molinari, J., and D. Vollaro, 1990: External influences on hurricane intensity: Part 11. Vertical structure and response of the hurricane vortex. J. Atmos. Sci., 47, 1902-1918.

Molinari, J., S. Skubis, D. Vollaro, F. Alsheimer, and H. Willoughby, 1997: Potential vorticity analysis of hurricane intensification. J. Atmos. Sci., in press.

Montgomery, M.T., and R.J. Kallenbach, 1997: A theory for vortex Rossby-waves and its application to spiral bands and intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc., 123, 435-466.

Ooyama, K. V., 1982: Conceptual evolution of the theory and modelling of the tropical cyclone. J. Meteor. Soc. Japan, 60, 369-379.

Palmen, E. 1958: Vertical circulation and release of kinetic energy during the development of hurricane Hazel into an extratropical cyclone. Tel/us, 10, 1-23.

Petterssen, S. and S.J. Smebye, 1971: On the development of extratropical storms. Quart. J. Roy. Meteor. Soc., 97, 457-482.

Raymond, D.J., 1992: Non linear balance and potential vorticitythinking at large Rossby number. Quart. J. Royal Meteor. Soc., 118, 987-1016.

Ritchie, E.R., and G.J. Holland, 1993: On the interaction of tropical-cyclone-scale vortices. 1: Observations. Quart. J. Royal Meteor. Soc., 119, 1363-1380.

Samsury, C. E., and E.J. Zipser, 1995: Secondary wind maxima in hurricanes: and relationship to rainbands. Mon. Wea. Rev., 123, 3502-3517.

Sekioka, M., 1970: On the behavior of cloud patterns as seen on satellite photographs in the transformation of a typhoon into an extratropical cyclone. J. Met. Soc. Japan, 48, 224-233.

Sekioka, M., 1972: Note on the extratropical transformation of a typhoon in relation with cold outbreaks. Arch. Met. Geoph. Biokl., Ser. A, 21, 413-418.

Sekioka, M., 1972: A kinematical consideration on behavior of a front within a typhoon area. Arch. Met. Geoph. Biokl., Ser. A, 21, 1-12.

8 GJH 11 March, 1998 Shapiro, L.J., and J.L. Franklin, 1995: Potential vorticity in . Mon. Wea. Rev., 123, 1465-1475.

Shapiro, L.J., and H.E. Willoughby, 1982: The response of balanced hurricanes to local sources of heat and momentum. J. Atmos. Sci., 39, 378-394.

Sinclair, M. R., 1993: Synoptic-scale diagnosis of the extratropical transition of a southwest Pacific tropical cyclone. Mon. Wea. Rev., 121, 941-960.

Sinclair, M.R., 1993: A diagnostic study of the extratropical precipitation resulting from tropical cyclone Sola. Mon. Wea. Rev., 121, 2690-2707. ·

Sinclair, M. R., 1993: Synoptic-scale diagnosis of the extratropical transition of a southwest Pacific tropical cyclone. Mon. Wea. Rev., 121, 941-960.

Smith, R.K., 1997: On the theory of CISK. Quart. J. Roy. Meteor. Soc., 123, 407-418.

Thorncroft, C. D., B. J. Hoskins, and M. E. Mclntyre, 1993: Two paradigms of baroclinic-wave life-cycle behavior. Quart. J. Roy. Meteor. Soc., 119, 17-55.

Willoughby, H.E., 1990: Temporal changes of the primary circulation in tropical cyclones. J. Atmos. Sci., 47,242-264.

Willoughby, H.E., H.-L. Jin, S.J. Lord, and J.M. Piotrowicz, 1984: Hurricane structure · and evolution as simulated by an axisymmetric, nonhydrostatic numerical model. J. Atmos. Sci., 41, 1169-1186.

Wu, C.-C., and K.A. Emanuel, 1994: On hurricane outflow structure. J. Atmos. Sci., 51, 1995-2003.

9 GJH 11 March, 1998

WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic 2.3: Tropical Cyclone Maximum Intensity Theory

Rapporteur: Kerry Emanuel, Program in , Oceans, and Climate Institute of Technology Cambridge, MA 02139, USA

1. Introduction

Hurricanes are complex dynamical systems whose intensities at any given time are affected by a variety of physical processes, some of which are internal and others of which involve interactions between the storms and their environments. Many of these processes are poorly understood, and there is presently little if any skill in forecasts of the intensity change of individual storms (DeMaria and Kaplan, 1997).

Although it is at present very difficult to forecast individual intensity, it is generally agreed that there exist thermodynamic limits to intensity that apply in the absence of significant interaction between storms and their environment While there remains some uncertainty about how to calculate such limits, they do appear to provide reasonable upper bounds on the intensities of observed storms, and recent evidence suggests that they may even be useful for predicting the change in intensity of individual cyclones (DeMaria and Kaplan, 1994}. One particular advantage of limit calculations is they depend only on sea surface temperature and the vertical temperature structure of the atmosphere, so they are easily calculable from standard data sets.

Here we review progress in the development and application of upper bound theory since the last International Tropical Cyclone Workshop.

2. Theoretical Advances

There are two principal methods of estimating upper bounds on tropical cyclone intensity, but they are closely related. The first method was pioneered by Miller (1958) and involves first estimating the maximum temperature that can be achieved in the eyewall given the thermodynamic properties of air near the sea surface, and then estimating the maximum temperature that can be achieved in the eye by compressional warming of sinking air. Miller's technique has been expanded upon and improved by Holland (1997). The second technique uses the energy cycle of the storm to estimate the maximum possible surface wind speed and was developed by Emanuel (1986, 1995}. The central pressure can also be estimated using this technique by assuming a particular radial profile of azimuthal wind inside the eye, but this is not necessary if all that is desired is the maximum wind speed.

Holland's (1997} method consists of two steps: First, the surface pressure under the

2.3.1 eyewall is estimated by integrating the hydrostatic equation up the eyewall, assuming that the pressure perturbation vanishes at the altitude corresponding to the level of neutral buoyancy of eyewall air. The temperature perturbation used in this calculation is estimated by assuming that the eyewall is composed of air that has risen adiabatically from the subcloud layer, assuming that the surface temperature is closely related to the local sea surface temperature. The surface relative humidity is prescribed.

In the second step, the central surface pressure is estimated by integrating the hydrostatic equation up the central axis, and for this purpose, the eye temperature is estimated by means similar to those employed by Miller (1958). Here air is assumed to descend through the eye while mixing at some prescribed rate with eyewall air.

Emanuel's (1995) method is based on the energy cycle illustrated in Figure 1.

Leg 1

Figure 1

The same result was also derived previously (Emanuel 1986) based on a dynamical argument. lt differs from techniques aimed at estimating central pressure (Miller, 1958; Emanuel, 1988; Holland, 1997} in attempting instead to estimate maximum wind speeds. Basically, it works by equating the total rate of heat input (sensible and latent) from the sea surface, multiplied by a thermodynamic efficiency, to the total rate of dissipation, which is also assumed to take place mostly at the sea surface. The destruction of moist entropy by heat input at high temperature (the sea surface) and heat export at low temperature (the upper troposphere) is balanced by the creation of entropy by dissipation in the boundary layer. Sister and Emanuel (1998) showed, however, that it is important to include in the heat input term the dissipative heating itself. That is, some of what would otherwise be "waste heat" is recycled into the heat

2.3.2 engine, thereby increasing its effective thermodynamic efficiency. This results in an expression for the maximum wind speed, V max given by

(1)

where Ck is a surface transfer coefficient, CD is the surface drag coefficient, k ~·is the saturation enthalpy of the sea surface near the radius of maximum winds, ka is the boundary layer enthalpy in the same region, and e' is defined

_~ ::;;; T, - ·T"' t - ·T. • (2) 0

Here Ts is the sea surface temperature and To is an entropy-weighted mean outflow temperature at the top of the storm. The specific enthalpy, k , is defined (3) where CP is the at constant pressure, Lv is the latent heat of vaporization, T is the absolute temperature and q is the specific humidity. The effect of the dissipative heating is to make the denominator of (2) To ,rather than Ts which results when dissipative heating is neglected, as it was in earlier work of Emanuel (1986, 1988,

1995). Since Ts is typically 300 K while T0 is around 200 K, this is a substantial difference. Note that there is a positive feedback in (1) that arises from the surface pressure dependence of k ~ , which in turn arises from the pressure dependence of q .

lt is important to recognize that the derivation of (1) depends in no way on what happens inside the eye of the storm and does not directly yield an estimate of central pressure.

To actually evaluate potential intensity it is necessary to know several things. In the energy cycle method one needs to know, among other things, the ratio of heat to momentum exchange coefficients, Ck I CD . Unfortunately, no measurements of either of these coefficients have been made at tropical cyclone wind speeds. In the results presented to date, it has been assumed that this ratio is 1. The saturation enthalpy of the ocean surface under the eyewall, k~ , is also needed. This depends mostly on sea surface temperature, but also on sea level pressure at the radius of maximum winds. To get this, it is necessary to assume something about the distribution of wind outside the radius of maximum winds. The calculation of k; is not terribly sensitive to what one assumes about this distribution, as long as it is reasonable. For both the energy cycle and Holland methods, one also has to estimate the enthalpy of the air just above the

2.3.3 sea surface, ka , at the radius of maximum winds. This must be at least as large as the enthalpy of unperturbed boundary layer air, and it depends on temperature, pressure, and relative humidity. Both Holland (1997) and Emanuel (1995) assume that the air temperature is closely linked to the sea surface temperature. Holland assumes that the relative humidity is 85%, while Emanuel assumes that it is the same as that of the unperturbed environment. Finally, one has to estimate e', in the energy cycle method, or, equivalently, the level of neutral buoyancy in Holland's or Miller's method. This can be done rather easily using a representative environmental sounding and assuming some thermodynamic ascent path in the eyewall.

The results of using the two basic methods described above to calculate limiting storm intensities have been rather thoroughly compared by Tonkin (1997).

An example of the results of maximum wind speed calculations is shown in Figure 2.

Maximum Wind .Speed (m/s)

-lOO -95 -90 -85 -80 -75 -70 .... -65 -.> -60 -55 -0 .,.I -50 -45 -40 -35 -30 -25 -20 -15 -10 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 37.5 40.0 42.5 45.0 47.5

SST (C)

30 40 50 60 70 80 90 100 110 120 130 140

H =0.75 C_k/C_D =1

Figure 2

2.3.4 3. Advances in Applications of Maximum Intensity Theory

Comparison of calculated maximum intensity with observed tropical cyclone intensities shows that the maximum intensity is just that: an upper bound beyond which few if any storms intensify. A major exception occurs when strong storms move over a strong sea surface temperature gradient toward colder water; in this case, the storms may temporarily have intensities far beyond the local potential intensity. This has been thoroughly documented by Holland (1997).

Figu·re 3 shows a cumulative distribution function (CDF) for all Atlantic storms south of 30 N occurring between 1958 and 1994, classified by the ratio of their observed intensity to the local monthly mean value of the potential intensity. The latter has been calculated from NCEP daily mean reanalysis data and then averaged over all the days in a given month, using data from 1976 to 1994. Maps of the climatological potential intensity are available on the world wide web at http://www-paoc.mit.edu/-emanuel/pcmin/climo.html. The CDF shows the total number of occurrences in the record with relative intensities exceeding the value on the x axis. Clearly, the CDF falls off sharply with intensity, approaching zero at a relative intensity of one.

storms south of 30 N

~------, i • Data 4000 • Lower error bound " Upper error bound 3500 - Interpolation formula

;..3000

:I~ ~2500... 11. ~ ;2000 :i E :I 0 1500

1000

500

a~~~~~~~~~~~~~~~ 0 ~ ~ M U M M V M ~ Normalized Wind Speed

Figure 3

Real-time maps of potential intensity are also available on the world wide web at http://www-paoc.mit.edu/-emanuel/pcmin/hurdes.html.

2.3.5 Two examples of the evolution of tropical cyclone intensity, compared to official forecasts and to real-time calculations of potential intensity, are shown in Figure 4 and in Figure 5.

Hurricane Bertha

120

100

80 , ••. a---•• m••------' v_max (kts) 60 Forecast lntensities V· , ' \ 40

20

o~~~~~~~~~~~~~+-~~ 8 9 10 11 12 13 14 July 1996 Figure 4

Hurricane Lili

14 1\ ...... / \ 12 """ .... \ Potentiallntensi' Actual lntensi1y";? '------\ \ 8 '•\ ··- -~, ', ', '\ ..... ' '.....- ' , ' 4 ( Forecast lntensitie~, ._;:· ;... __·: :- - ~:

2

o~~~~~~~~~~~~~~~~ 17 18 19 20 21 22 23 24 25 26 27 0 ctober

Figure 5

2.3.6· The thin dashed lines show the official intensity forecast. In the case of , forecasters could have forecast the storm's decline and, with less confidence, its re-intensification just before landfall by taking into account the potential intensity. In the case of , forecasters continued to predict weakening in spite of maintained potential intensity, simply because the local sea surface temperature was marginally below the 26 C threshold.

4. Summary

There has been a surprising degree of progress in this topic since the last IWTC. A new theory by Holland (1997) has appeared, and the energy cycle method was improved to account for the effects of dissipative heating. Also, during this period, a climatology of potential intensity was undertaken, using NCEP reanalysis data, and real-time maps of potential intensity have been made available over the world wide web.

We are clearly at a stage when the theory of tropical cyclone potential intensity is reaching maturity. Comparison by Ton kin (1997) of the two main extant methods shows good agreement between them. Yet it is clear from Figure 3 that the theory is useful only as an upper bound, not as a predictor of actual storm strength. Given the findings of recent research on the effect of ocean interaction, it may be possible to take into account both the potential intensity and the probable reduction of intensity by ocean interaction effects in making tropical cyclone intensity forecasts. Such an undertaking should be encouraged.

References

Bister, M., and K. A. Emanuel, 1998: Dissipative heating and hurricane intensity. Meteor. Atm. Physics, Accepted. ·

DeMaria, M. and J. Kaplan, 1994: Sea surface temperature and the maximum intensity of Atlantic tropical cyclones. J. Climate, 7, 1324.

DeMaria, M. and J. Kaplan, 1997: An operational evaluation of a statistical hurricane intensity prediction scheme (SHIPS). Preprints of the 22nd Conf. Hurricanes and Tropical Meteor., Amer. Meteor. Soc., Boston, pp 280-281.

Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones. Part I. J. Atmos. Sci., 43, 1062-1071.

Emanuel, K. A., 1988: The maximum intensity of hurricanes. J. Atmos. Sci., 45 1143-1155.

Emanuel, K. A., 1995: Sensitivity of tropical cyclones to surface exchange coefficients and a revised steady-state model incorporating eye dynamics. J. Atmos. Sci.,52, 3969-3976.

Holland, G., 1997: The maximum potential intensity of tropical cyclones. J. Atmos. Sci., 54, 2519-2541

2.3.7 Miller, B. I., 1958: On the maximum intensity of hurricanes. J. Meteor., 15, 184-195.

Tonkin, H., 1997: Investigating Thermodynamic Models of Tropical Cyclone Intensity. MSc thesis, Climatic Impacts Centre, Macquarie University, Sydney, Australia.

2.3.8 Rapporteur's Report for Working Group 2.4- Tropical Cyclone Formation

Prepared by: William M. Frank (Rapporteur) with input from working group members Kerry A. Emanuel, William M. Gray, Patrick A. Harr, Christopher W. Landsea and Raymond M. Zehr.

2.4.1 Introduction

During the past five years or so there has been a significant increase in research on the formation of tropical cyclones. This burst of activity was spurred in part by a combination of · improvements in global analysis fields, mesoscale models and remotely sensed wind fields and data sets obtained during special tropical cyclone experiments during the early 1990s. These developments led to improved ability to observe and simulate the conditions associated with individual events of .

There has been general agreement for the past two decades concerning the large-scale climatology of tropical cyclogenesis (Gray, 1978; Frank, 1985; McBride, 1993). Tropical cyclones form over warm tropical waters with sea surface temperatures (SST) greater than about 26.5 C, though there has been some discussion of hurricane-like storms forming over colder waters (e.g. - certain polar lows, and even a few rare Mediterranean storms - Blier and Ma (1997)). They typically form in regions conducive to widespread occurrence of deep convection (at least some and significant mid-level moisture- the latter possibly a result of large-scale uplifting). The genesis regions also exhibits relatively weak vertical wind shear directly over the forming storm. Further, formation is favored in regions with significant large-scale absolute vorticity in the lower levels, and they always begin as some sort of convective disturbance.

It has long been recognized that the thermodynamic conditions associated with tropical cyclogenesis occur over broad areas and seasonal time scales. There is no clear evidence that tropical cyclogenesis is triggered by local temporal or spatial perturbations of the temperature, moisture or SST fields to above-climatological values (Gray, 1998). Hence, most recent research has been based on the premise that tropical cyclones form due to perturbations of the wind field on the mesoscale and/or synoptic scale.

The majority of the research described below tends to fall into three major areas. There have been several studies that have examined the range of synoptic-scale flow characteristics and satellite-observed convective patterns associated with individual cases of tropical cyclogenesis. Perhaps the greatest amount of research has focussed on mesoscale phenomena, particularly observational and numerical modeling studies of the processes by which a mesoscale convective system (MCS) can form a mesoscale convective vortex (MCV) and of the role of MCV s in cyclone formation. Third, there have been both modeling and observational studies of the relationship between cyclogenesis and fluctuations in the structure of the Intertropical (ITCZ). 2

2.4.2 · Research Results

a) The Synoptic Climatology of Tropical Cyclogenesis

As noted by Gray (1998) and Holland (1987), there seem to be several different paths to tropical cyclogenesis in nature, and there are definitely several different sets of conditions or physical mechanisms that can produce hurricane-like vortices in numerical models. Thus, the task of elucidating the essential characteristics and processes associated with tropical cyclogenesis remains an important research area. Zehr (1992) analyzed many individual cases of tropical cyclogenesis in the Pacific and determined that there were distinct statistical differences between the wind fields and convective patterns of developing and non-developing tropical disturbances. These differences generally supported the hypothesis of Gray (1978) that genesis tends to occur in local environments that exhibit favorable perturbations of the dynamic parameters that are climatologically associated with genesis (strong vorticity, weak shear, etc.). Zehr (1992) also presented a conceptual model of genesis that included a two-stage process (Fig.1 ). The first stage consists of a convective event (an individual MCS) that produces an MCV, usually with maximum vorticity in the middle troposphere. In most cases a second MCS event occurs within a few days, and it is during this latter stage that the system develops a warm core and hence maximum vorticity in the lower troposphere, which is necessary to set off the self-intensifying sea surface flux instability associated with development into a mature cyclone.

Ritchie (1995) and Briegel and Frank (1997) analyzed circulation patterns associated with cyclone formation within the NW Pacific ITCZ. They identified several large-scale circulation patterns that tended to perturb the monsoon trough and produce genesis (Fig. 2). These included the presence of upper-level troughs north-northwest of the genesis location, the existence of a mature tropical cyclone west of the genesis location, and propagation of tropical easterly waves into the ITCZ. Harr et al. (1996 a,b) and Lander (1994) analyzed the large-scale circulations associated with formation of a midget typhoon, and of genesis from a monsoon depression and a monsoon gyre, respectively. Molinari et al. (1997), Brister and Emanuel (1997) and Pytharoulis and Thomcroft (1997) examined storm formation from easterly waves, while Farfan and Zehnder (1997) analyzed orographic effects on E. Pacific genesis. Zhang and Bao (1996 a,b) studied the interactions between large-scale and mesoscale processes in a case of Atlantic genesis. These and several other studies were able to examine many detailed aspects of the synoptic-scale flow associated with cyclogenesis.

Although several of the above studies utilized global analysis fields, which do not resolve any of the core properties of hurricanes, others made use of aircraft data and were able to explore links between the synoptic-scale and mesoscale processes that occur during genesis. Understanding the physical links between the core structure and the immediately surrounding flow is the major focus area of current genesis research. It is somewhat artificial to separate studies by spatial scale, but it is done here for organizational convenience. 3

,,.______GENESIS j-INTENSIFICATJON-

STAGE ___ L STAGE I TROPICAL 1 ONE TWO 4..- STORM -

r I I ! I I 3 2 I 0 I I Named After I Days Be f ore I Storm . I j Deep Convective SURGE SURGE ~ ~ I

MSLP

mb

990

980

I . Maximum Surface Wind Speed f I I . I I 15 Vmox I 10 5 ms·1

Dvorok lntensi1y No. I I I I I I 1.0 1.5 2.0 2.5 3.0 3.5

Figure 1. Conceptual model summarizing the important changes of Cb, MSLP, and V max during tropical cyclogenesis with estimates of the approximate numerican values. The associated Dvorak intensity T - are also shown. (Adapted from Zehr, 1992.) 4

a b

ASIAN MONSOON

c d

~ ~ MONSOON ...' Jt PACIFIC .-=-...-... GYRE ''- EASTERLIES / A ·, (c)." I ·- ·' c --+ -::::::/ V' \ MONSOON .. \. DEPRESSION / . ( +- '~.

Figure 2: Schematic of four large-scale flow patterns for cyclogenesis: (a) monsoon trough, (b) easterly-wave, (c) energy accumulation, (d) energy dispersion. From Ritchie (1995).

b) Mesoscale Processes

During the it became apparent that the majority ofMCSs, both in the middle latitudes and in the tropics, tend to form MCV s with approximate diameters of 50-200 km within their stratiform rain regions (e.g. - Velasco and Fritsch, 1987). These MCV s are typically strongest in the middle troposphere, slightly above the . They are usually cold-core in the lower troposphere, and hence they usually have little or no circulation at the surface. Chen and Frank (1993) hypothesized that these MCVs could develop downwards with time producing enough vorticity in the boundary layer to initiate tropical cyclogenesis. In their modeling study this downward growth of the vortex was caused by downward extension of the stratiform cloud. There are several other hypotheses for how this downward extension of the vortex may occur (most ofwhich are not mutually exclusive). Emanuel (1995) and Bister and Emanuel (1997) argued that evaporation of rain within an MCS could cause such extension (Fig. 3). Ritchie (1995), Simpson et al. (1997) and Ritchie and Holland (1997) proposed that mergers ofMCVs, or an MCV with a larger vorticity center, could produce a stronger, deeper vortex (Fig. 4). Fritsch (1994) and Rogers (1997) argued that deepening of the vortex could be explained by favorable alignments of the vortex and th~ vertical shear. Gray (1998) proposes that interactions between the initial stage midlevel vorticity center and a second low-level wind surge act to concentrate convergence at low levels resulting in downward extension of the vortex (Fig. 5). All of these researchers agree that a critical step in the genesis process is the transformation of the vortex from a cold-core to a warm-core system, with the resulting increase in low-level vorticity. 5

a

Mid-level Mesocyclone Cooling and moistening

Low-level Anticyclone Warming and drying, iowee

b

Mesocyclone extends down to boundary layer Cold, humidified air fills lower troposphere begins

c

New. downdraft-free convection erupts in the cold, unstable humid air Warm core vortex develops and Intensifies near surface

Figure 3: Conceptual model of tropical cyclogenesis from a preexisting MCS. (a) Evaporation of stratiform precipitation cools and moistens the upper part of the lower troposphere; forced subsidence leads to warming and drying of the lower part. (b) After several hours there is a cold and relatively moist anomaly in the whole lower troposphere. (c) After some recovery of the boundary layer ee convection redevelops. From Bister and Emanuel (1997). 6

300 L p (hPa)

Figure 4a: Radial average ofvorticity for merger ofmidlevel vortices in the baroclinic model with no background vorticity and 5 1 no diabatic heating: (a) 72 and (b) 120 h (contours: 2.0 x 10- s- ). From Simpson et al (1997).

Figure 4b: Radial average ofvorticity for merger of the same midlevel vortices used in Fig. 4a but with a background environment equal to 3/ and with heating turned on at 72 h; (a) adiabatic merger to 72 hand 5 1 b) diabatic intensification to 120 h (contours: 2.0 x 10- s- ). From Simpson et al ( 1997). 7

SECOND WIND SURGE TROP. ••••••••••••• 4 ••• - ••

...... TROP.

TROP. 0

zt

0 pos.-- DIV

Figure 5: A second wind surge impinges upon the region of a prior MCS. Inertial stability of the middle layers causes much of the deep layer wind surge to become concentrated at lower levels. This acts to spin-up the lower levels more than the middle levels and to change the system from a low-level cold core to a warm core. From Gray (1998). 8

c) ITCZ Breakdown

Early modeling studies (Y amasaki, 1989) and theoretical analyses (Hoskins et al., 1985) suggested that tropical cyclones could form spontaneously within the ITCZ as a result of interactions between persistent convection and the large-scale flow. Numerical simulations of the ITCZ indicate that even in the absence of external perturbations, tropical cyclogenesis can occur due to ITCZ breakdown. Guinn and Schubert (1993) used a shallow water model to demonstrate that this breakdown could occur due to barotropic instability. Frank and Wang (1997) used a full-physics mesoscale model to explore the mechanisms involved. They showed that the convection within the ITCZ gradually creates a strip of maximum potential vorticity that breaks down into waves, which subsequently form cyclones (Fig. 6). The disturbances grow initially due to barotropic instability and later by combined barotropic/baroclinic instability. Frank and Wang (1997) also noted that introducing a large-scale SST asymmetry into the simulations could result in formation of a monsoon gyre with subsequent tropical cyclone formation, similar to the system observed by Lander (1994).

Understanding cyclone formation within monsoon troughs is important due to the large number of storms that form there. Although pure breakdown of the ITCZ occurs with some frequency in nature, the studies of Ritchie (1995) and Briegel and Frank (1997) suggest that perturbations of the ITCZ by external circulations are probably involved in the majority ofiTCZ genesis events.

Absolute Vorticity (l.OE-5*1/s) at 850 (hP a)

24

22

20

18

~ 16

i.., 14 e .., 12

110

8

6

4

2

110 120 130 140 150 160 170 180 190 200 LONGl'l'UDE (DEGREES)

1 Figure 6: Absolute vorticity at 850 mb (x 10-s s· ), day 8, t = 06 h. A simulation ofiTCZ breakdown using the Perm State/NCAR Mesoscale Model (MM5). From Frank and Wang (1997). 9

2.4.3 Summary of Major Issues

a) Theoretical Issues

There are many important theoretical issues that need to be addressed to improve our understanding of tropical cyclogenesis. A few of the most important questions are:

What are the relationships between deep convection and vorticity during the genesis period?

By what processes is the low-level vorticity maximum produced?

What are the environmental conditions necessary to form an MCS and MCV, and how can these conditions be specified, and if necessary parameterized, in numerical models?

How is the structure of the forming vortex modulated by the large-scale flow patterns?

It is likely that these and related questions will require both new high-resolution data sets in the core regions of forming tropical cyclones and numerical simulations using models capable of resolving core-environmental interactions without resorting to parameterized convection.

b) Observational Issues

Perhaps the most obvious unresolved observational issue is to develop a clearer definition of the term ''tropical cyclogenesis". When has genesis ended and intensification begun? Researchers tend to employ definitions based on physical processes. For example, the research community tends to classify intensification as that stage of development governed by an internal instability process in which increasing surface winds stimulate greater surface energy fluxes, leading to intensification of the vortex, etc. Genesis is then considered to be the process by which one reaches this self-intensifying stage. This definition is not necessarily useful to a f.orecaster, who has no direct observations suitable for determining the system's energetic state. Forecasters need criteria by which genesis can be defined using available observations. There has been considerable informal discussion of this topic, but it would be valuable to come to some more formal agreement as to what would be the appropriate set of classification criteria for genes1s.

The operational community is becoming increasingly interested in forecasts out to periods of five days or longer. This increases the need for improving genesis forecasts. It should be useful to develop classifications of synoptic-scale flow patterns associated with genesis for use by forecasters, similar to recently developed techniques for track prediction (e.g. - Carr and Elsberry, 1997). 10

2.4.4 Bibliography

Bister, M. And K.A. Emanuel, 1997: The Genesis ofHurricane Guillermo: TEXMEX Analyses and a Modeling Study. Mon. Wea. Rev., 125,2662-2682.

Blier, W. And Q. Ma, 1997. A Hurricane? 22nd Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, Boston, MA, 592-595.

Briegel, L.M. and W.M. Frank, 1997: Large-scale Influences on Tropical Cyclogenesis in the Western North Pacific. Mon. Wea. Rev., 125, 1397-1413.

Chen, S.S. and W.M. Frank, 1993: A Numerical Study of the Genesis ofExtratropical Convective Mesovortices. Part I: Evolution and Dynamics. J. Atmos. Sci., 50, 2401-2426.

Emanuel, K.A., 1995: The Behavior of a Simple Hurricane Model Using a Convective Scheme Based on Subcloud-layer Entropy Equilibrium. J. Atmos. Sci., 52, 3960-3968.

Farfan, L.M. and J.A. Zehnder, 1997: Orographic Influence on the Synoptic-scale Circulations Associated with the Genesis ofHurricane Guillermo (1991). Mon. Wea. Rev., 125, 2683-2698.

Frank, W.M. and H. Wang, 1997. Numerical Simulation ofthe Short-term Variability ofthe ITCZ. 22nd Conference on .Hurricanes and Tropical Meteorology, American Meteorological Society, Boston, MA, 347-348.

Frank, W.M., 1987: A Global View of the Tropical Cyclone: Chapter 3, Tropical Cyclone Formation. University ofChicago Press,R. Elsberry (Ed.), 185 pp.

Fritsch, J.M., J.D. Murphy, and J.S. Kain, 1994: Warm Core Vortex Amplification Over Land. J. Atmos. Sci.,.ll, 1780-1807.

Gray, W.M., 1998. The Formation of Tropical Cyclones. Contribution, Herbert Riehl Memorial Volume, Colorado State University, October 1997, 68 pp.

Gray, W.M., 1978. Hurricanes: Their Formation, Structure and Likely Role in the Tropical Circulation. Meteorology Over the Tropical Oceans, Royal Meteorological Society, Berkshire, p. 155-218.

Guinn, T.A. and W.H. Schubert, 1993: Hurricane Spiral Bands. J. Atmos. Sci., 50, 3380-3403.

Harr, P.A., M.S. Kalafsky, and R.L. Elsberry, 1996a: Environmental Conditions Prior to Formation of a Midget Tropical Cyclone during TCM-93. Mon. Wea. Rev., 124, 1693-1710.

Harr, P.A., R.L. Elsberry, and J.C.-L. Chan, 1996b: Transformation of a Large Monsoon Depression to a Tropical Storm during TCM-93. Mon. Wea. Rev., 124, 2625-2543. Lander, 1994. 11

Holland, G., 1987: A Global View of Tropical Cyclones: Chapter 2, Mature Structure and Structure Change. University of Chicago Press, R. Elsberry (Ed.), 185 pp.

Hoskins, B.J., M.E. Mclntyre and A.W. Robertson, 1985: On the Use and Significance of Isentropic Potential Vorticity Maps. Quart. J. R. Met. Soc., 111, 877-946.

Lander, M.A., 1994: Description of a Monsoon Gyre and Its Effect on the Tropical Cyclones in the Western North Pacific during August 1991. Weather and Forecasting,_2, 640-654.

McBride, J.L., 1995. Tropical Cyclone Formation. Chapter 3, Global Perspective on Tropical Cyclones, Report No. TCP-38, WMO, Geneva, Switzerland, p.63-105.

Molinari, J.S., D. Knight, M. Dickinson, D. Vollaro, and S. Skubis, 1997: Potential Vorticity, Easterly Waves, and Eastern Pacific Tropical Cyclogenesis. Mon. Wea. Rev., 125,2699-2708.

Pytharoulis, I. and C.D. Thorncroft, 1997: The Transformation of African Easterly Waves into Tropical Cyclones in the North Atlantic in 1995. 22nd Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, Boston, MA, 585-586.

Ritchie, E.A., 1995: Mesoscale Aspects of Tropical Cyclone Formation. Ph.D. dissertation, Monash University, 167 pp.

Ritchie, E.A. and G.J. Holland, 1997: Scale Interactions during Formation ._ Mon. Wea. Rev., 125, 1377-1396.

Ritchie, E.A. and G.J. Holland, 1998: Large-scale Relationships and Development ofMesoscale Convective Systems during Tropical Cyclogenesis in the Western Pacific. (Submitted to MWR).

Rogers, R.F., 1997: A Component of Tropical Cyclogenesis: Convective Development within a Warm-core Vortex. 22nd Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, Boston, MA, 555-556. ·

Simpson, J., E. Ritchie, G.J. Holland, J. Halverson, and S. Stewart, 1997: Mesoscale Interactions in Tropical Cyclone Genesis. Mon. Wea. Rev., 125,2643-2661.

Velasco, I. and J.M. Fritsch, 1987: Mesoscale Convective Complexes in the Americas. J. Geophysical Res., 92, 9591-9613.

Yamasaki, M., 1989: Numerical Experiment ofTropical Cyclone Formation in the Intertropical Convergence Zone. J. Meteor. Soc., Japan, 67, 529-540.

Zehr, R.M., 1992: Tropical Cyclogenesis in the Western North Pacific. NOAA Tech. Rep. NESDIS No. 61, Washington, DC. 12

Zhang, D.-L. and N. Bao, 1996a: Oceanic Cyclogenesis as Induced by a Mesoscale Convective System Moving Offshore. Part I: A 90-h real-data simulation. Mon. Wea. Rev., 124, 1449-1469.

Zhang, D.-L. and N. Bao, 1996b: Oceanic Cyclogenesis as Induced by a Mesoscale Convective System Moving Offshore. Part II: Genesis and Thermodynamic Transformation. Mon. Wea. Rev., 124, 2206-2225.

2.4.5 Other Publications From 1993-Present

Harr, P.A., C.A. Finta, R.L. Elsberry, 1997: Role ofmesoscale convective vortices in determining the circulation structure during tropical cyclone formation. 22nd Conference on - Hurricanes and Tropical Meteorology, American Meteorological Society, Boston, MA, 193-194.

Krishnamurti, T.N., H.S. Bedi, D. Oosterhof, and V. Hardiker, 1994. The formation of of 1979. Mon. Wea. Rev., 1050-1074.

Lebedev, S.L. and L.I. Petrova, 1994. Estimation ofthe poissibility oftropical cyclogenesis from instability energy. Russian Meteorology and Hydrology, 5, 44-49.

Liebmann, B., H.H. Hendon, and J.D. Glick, 1994: The relationship between tropical cyclones of the Western Pacific and Indian Oceans and the Madden-Julian Oscillation. Journal of the Meteorological Society of Japan, 401-412.

Mozer, J.B. and J.A. Zehnder, 1994: Cluster analysis of eastern North Pacific tropical cyclogenesis precursors. J. Geophysical Res., 8085-8094.

Petrosyants, 3 M.A. and E.K. Semenov, 1995: Individual Potential ofTropical Cyclone Genesis. Izvestiya, .ll, 327-335.

Ward, G.F.A., 1995. Prediction of Tropical Cyclone Formation in Terms of Sea- Surface Temperature, Voritcity, and Vertical Wind Shear. Australian Meteorological Magazine, 44, 61- 70.

This report concentrates on accomplishments and changes in outlook that have occurred since the previous workshop (IWTC-3), which occurred in 1993. Therefore, references to earlier work are not intended to be comprehensive. WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic: Small- and Mesoscale Interactions (TC structure and intensity)

Rapporteur: Liz Ritchie Naval Postgraduate School 589 Dyer Rd, Room 254 Monterey, CA 93943-5114

Email [email protected] Fax: +408 656 3061

Working Group: P. Harr, M. Lander, and M. Montgomery

2.5.1 Introduction

Small and mesoscale interactions and their effects on tropical cyclone (TC) structure and intensity are often related to interactions between the TC and its environment, that are discussed in Topics 2.1 and 2.2. Often, convection that develops within the tropical cyclone has been triggered by external interactions such as with an upper-level trough or environmental shear that forces a tilt in the tropical cyclone vertical structure. This report examines the subsequent effect of such convection on the intensity and structure of the TC. Some overlap is anticipated with Topic 2.4 since small-scale effects on intensity and structure are intimately related to formation and evolution of the TC vortex. The topic is organised by first considering overall structure type, and then examining evolving and mature TCs separately. Interactions within a mature TC are defined by time scales and relation to basic structural features such as the TC eye.

2.5.2 Dependence on Basic Symmetric Structure

The structural development of a tropical cyclone is highly dependent on the environment in which it is embedded. Harr et al. (1997) find that the relative roles of mesoscale versus environmentally driven processes play an important role in determining the initial size (defined by the outer wind structure), and structure (e.g., RMW to size ratio) of a tropical cyclone. In their conceptual model, a mesoscale driven development results in a relatively small storm (Harr et al. 1996a) whereas environmentally driven formation processes lead to storms with larger outer wind structure that may approach the scale of the monsoon trough (Harr et al. 1996b).

The potential influence of small-scale disturbances on the evolving TC may be highly dependent on this structure. Examples of unusual TC track motions produced by mesoscale convective systems (MCS) differ depending on the size and outer structure of the TC (e.g. Holland and lander 1993; Harr and Elsberry 1996). lt is likely that similar

1 GJH February 23, 1998 considerations based on the basic TC structure and intensity are necessary when examining the effects of such MCS on changes in structure and intensity of Tcs.

2.5.3 Asymmetries and initial development of the TC

The roles that convective asymmetries play during the initial development of a tropical cyclone have been investigated in many recent studies that are based on observations and numerical simulations (e.g. Ritchie 1995; Ritchie et al. 1995; Harr et al. 1996a,b; Ritchie and Holland 1997; Simpson et al. 1997; Montgomery and Enagonio 1997). MCS have been observed to play an important role in developing the midlevel feature about which the general background vorticity (e.g., monsoon trough) and convection organises (Harr and Elsberry 1996). The downward development of this midlevel feature has been linked to vortex interactions with both other similar MCS-generated midlevel vortices and to the background vorticity provided by a monsoon trough, easterly wave, or other synoptic-scale feature (e.g. Ritchie and Holland 1997; Simpson et al. 1997). The re- development of convection on the periphery of such a midlevel . vortex has also been demonstrated to strengthen the initial vortex (e.g. Rogers 1997; Montgomery and Enagonio 1997).

2.5.4 Convective asymmetries within a mature TC

2.5.4.1 Short-lived asymmetries

Recent studies investigating vortex development via short- lived (e.g. 1-2 rotations or < 12 h), asymmetric processes have focussed on the interaction between diabatically:-generated PV anomalies and the tropical cyclone circulation. Some impacts of MCS-generated, midlevel vortices have been investigated in observational studies by Harr and Elsberry (1996), Ritchie and Holland (1997), and in numerical studies by Ritchie and Frank (1998). Montgomery and Enagonio (1997) focus on the impacts of low-level vortices such as those generated in deep convection on the symmetric spin-up of tropical cyclones.

Ritchie and Frank ( 1997; 1998) find that a mid level vortex within the tropical cyclone circulation forces a vertical tilt to develop in the TC structure resulting in asymmetric vertical motions and enhanced convection in the downs hear quadrant of the storm. A long-lived, steadily increasing asymmetry in the surface wind is attributed to down-gradient flow into a localised region of low pressure within the radius of maximum winds associated with the enhanced upward vertical motion and convection. The azimuthally-averaged intensification of the diabatically-driven vortex is less than that of the corresponding symmetric simulation confirming that asymmetries are detrimental to vortex spin-up when conditions are otherwise favourable for symmetric vortex intensification. Corresponding dry simulations suggest however, that when a TC is steady-state with respect to intensification, the triggering of MCSs and their associated midlevel vortices could unbalance the flow enough to produce short-term structural changes, that include regions of higher winds at the surface.

2 GJH February 23, 1998 The process of spin-up via convective asymmetries has been described in terms of vortex-Rossby waves by Montgomery and Enagonio ( 1997) whereby vortex intensification proceeds by ingestion of like-signed PV anomalies into the parent vortex and the expulsion of opposite-sign PV anomalies during the axisymmetrization process. In their simulations, low-level PV anomalies are used as a proxy for convection. lt is found that as the anomalies become distorted due to horizontal shear and are ingested by the basic-state vortex, some small spin-up occurs. An experiment in which pulsing convection over several days is parameterised by the continuous addition of low-level PV anomalies results in a significant spin up of the basic-state, symmetric vortex.

2.5.4.2 Quasi-steady, long-lived asymmetries

Convective asymmetries that are forced by environmental conditions may affect the circulation of a tropical cyclone for much longer time scales than the lifetime of a convective cloud or MCS. Numerical studies have suggested a direct link between the development of vertical motion asymmetries within the storm core and the tilt of the vortex forced by vertical shear (e.g., Jones 1995; DeMaria 1996; Wang and Holland 1996; Frank and Ritchie 1998). Generation of convective asymmetries have also been linked to boundary layer processes (Shapiro 1983; Wang and Holland 1996).

Jones (1995) found that a wave number one vertical velocity pattern developed as a result of vertical shear over a hurricane- like vortex that may influence the convective pattern in a tropical cyclone. DeMaria (1996) suggested that mid level warming over the low-level centre of a tilted vortex would inhibit convection whereas mid level cooling in the direction of tilt would enhance convection away from the core resulting in weakening of the tropical cyclone. Frank and Ritchie (1998) found that the initial adjustment of the vertical motion field was important for triggering convection as Jones (1995) had suggested. However at later stages the convective asymmetry became colocated with the warmest temperatures to the northeast of the storm rather than in the southeast as would be expected from the adiabatic results. Cloud properties also indicated a rotation of the convection with height about the storm core. This is in agreement with observations that discrete convection in the eyewall of tilted tropical cyclones tends to be generated in the same location, but becomes rotationally tilted with height by the rotation (Willoughby 1997- personal communication). Unsurprisingly, Frank and Ritchie (1998) also found that storms in vertical shear do not develop the same steady-state intensity as the same storm in a quiescent environment.

2.5.4.3 Asymmetric eyewall structure

The phenomenon of discrete mesovortices embedded within, or just inside the RMW of tropical cyclones has been studied recently (e.g. Stewart and Lyons 1996; Hasler et al. 1997). These mesovortices appear to be generated within convective supercells and have been observed to last for between 6 and 8 revolutions about the eyewall of the storm. The strongest winds that occur just outward from the mesovortex centre (adding storm circulation to vortex circulation) have been hypothesised to cause extreme wind damage in cases of landfalling hurricanes. In a case study of a weak storm just off the

3 GJH February 23, 1998 island of Guam, Stewart and Lyons (1997) hypothesise that the numerous mesovortices observed with the WD-88 radar may have played an important role in developing the storm eye due to symmetric adjustment of the vortex to the imbalance produced by the mesovortex generation. The development of polygonal eyewall structure that has been observed by satellite and radar and related to oscillations in TC track motion has been considered by Weber (1997) and Schubert et al. (1998). In particular Schubert et al. (1998) consider the development of dynamic instability on the inner edge of an anulus of PV such as might be expected to develop in a symmetric tropical cyclone with heating concentrated in the eyewall. During rearrangement of the PV field, it is hypothesised that part of the low PV air in the eye might mix with the PV in the eyewall producing asymmetric eye contraction in association with a polygonal eyewall.

2.5.6 Summary

The topic of small- and mesoscale forcing on tropical cyclone intensity and structure has been loosely partitioned according to the time scale on which the forcing occurs. Some studies have investigated environmental mechanisms that may modulate asymmetric convection in tropical cyclones. However, it appears that less is understood regarding the processes by which the resulting asymmetric convection affects the cyclone structure and intensity. Progress in this area may be possible with sophisticated numerical models that are better able to resolve both the complex physical processes associated with convection, and the synoptic scale circulations that induce convection within the TC. With the capabilities of current research aircraft that can provide special, high resolution data sets for verification in models, and the future possibilities of autonomous aircraft and remote sensing equipment to collect data that resolves the changes in time as well as space of the evolving tropical cyclone, the questions of small- and mesoscale forcing on tropical cyclone intensity and structure may well be a fruitful area for future research.

2.5. 7 References

DeMaria, M., 1996: The effect of vertical shear on tropical cyclone intensity change. J. Atmos. Sci., 53, 2076-2087.

Flatau, M., W. H. Schubert, and D. E. Stevens, 1994: The role of baroclinic processes in tropical cyclone motion: The role of vertical tilt. J. Atmos. Sci., 51, 2589-2601.

Frank, W. M., and E. A. Ritchie, 1997: Environmental effects on the structure of tropical storms. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Boston, MA 02108, 352-353.

Frank, W. M., and E. A. Ritchie, 1998: Effects of Environmental Flow upon Tropical Cyclone Structure. Mon. Wea. Rev. (to be submitted)

4 GJH February 23, 1998 Harr, P. and R.L. Elsberry, 1996: Structure of a mesoscale convective system embedded in typhoon Robyn during TCM-93. Mon. Wea. Rev., 124, 634-652.

Harr, P.A., M. S. Kalafsky, and R. L. Elsberry, 1996a: Environmental conditions prior to formation of a midget tropical cyclone during TCM-93. Mon. Wea. Rev., 124, 1693-1710.

Harr, P.A., R. L. Elsberry, and J. C.-L. Chan 1996b: Transformation of a large monsoon depression to a tropical storm during TCM-93. Mon. Wea. Rev., 124, 2625-2643.

Harr, P.A., C. A. Finta, and R.L. Elsberry, 1997: Role of mesoscale convective vortices in determining the circulation structure during tropical cyclone formation. Preprints, 22"d Conference on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Boston, MA 02108, 193-194.

Hasler, A. F., P. G. Black, V. M. Karyampudi, M. Jentoft-Nilsen, K. Palaniappan, and D. Chestors, 1997: Synthesis of eyewall mesovortex and supercell convective structures in Hurricane Luis with GOES-8/9 stereo/concurrent 1-min GOES-9 and NOAA airborne radar observations. Preprints, 22nd Cont. on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Boston, MA 02108, 201-202.

Holland, G.J. and M. Lander, 1993: The meandering nature of tropical cyclone tracks. J. Atmos. Sci., 50, 1254-1266.

Jones, S. C., 1995: The evolution of vortices in vertical shear. 1: Initially barotropic vortices. Q. J. R. Meteor. Soc., 121, 821-851.

Moller, J. D., and M. T. Montgomery, 1997: Vortex Rossby-waves and their influence on hurricane intensification. J. Atmos. Sci. (submitted).

Montgomery, M. T., and J. Enagonio, 1997: Tropical cyclogenesis via convectively forced vortex Rossby aves in a three-dimensional quasigeostrophic model. J. Atmos. Sci. (submitted).

Montgomery, M. T., and R. J. Kallenbach, 1997: A theory for vortex Rossby waves and its application to spiral bands and intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc. 123, 435-465.

Ritchie E.A. and G.J. Holland, 1993: On the interaction of tropical-cyclone-scale vortices. 11: Discrete vortex patches. Q. J. Roy. Met. Soc., 119, 1363-1379.

Ritchie E. A., 1995: Mesoscale Aspects of Tropical Cyclone Formation. PhD. Dissertation, Centre for Dynamical Meteorology and Oceanography, Monash University, Melbourne, Australia, 167 pp. [Available from Centre for Dynamical Meteorology and Oceanography, Mathematics Department, Monash University, Melbourne, VIC 3168, Australia.]

5 GJH February 23, 1998 Ritchie E.A, G.J. Holland and J. Simpson, 1995: Contributions by mesoscale convective systems to the development of tropical cyclone Oliver. Preprints, 21st Conf. on Hurr. and Trap. Met., 24-28 April, 1995, Miami, Florida. 570-572.

Ritchie E. A, and G. J. Holland, 1997: Scale Interactions during the Formation of Typhoon lrving. Mon. Wea. Rev., 125, 1377-1396.

Ritchie, E. A and W. M. Frank, 1997: Effects of convective asymmetries on the core structure of tropical cyclones. Mon. Wea. Rev. (to be submitted)

Ritchie, E. A, and W. M. Frank, 1997: Effects of convective asymmetries on the structure of tropical storms. Preprints, 22"d Conf. on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Boston, MA 02108, 199-200.

Rogers, R. F., 1997: A component of tropical cyclogenesis: convective redevelopmnet within a warm-core mesovortex. Preprints, 22nd Conf. on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Boston, MA 02108, 555-556.

Schubert, W. H., M. T. Montgomery, R. K. Taft, T. A Guinn, S. R. Fulton, J. P. Kossin, and J. P. Edwards, 1998: Polygonal eyewalls, asymmetric eye contraction and potential vorticity mixing in hurricanes. J. Atmos. Sci. (submitted).

Shapiro, L. J., 1983: Asymmetric boundary layer flow under a translating hurricane. J. Atmos. Sci., 40, 1984-1998.

Simpson, J., E. A Ritchie, G. J. Holland, J. Halverson, and S. Stewart, 1997: Mesoscale interactions in tropical cyclone genesis. Mon. Wea. Rev., 125, 2643-2661.

Stewart, S. R. and S. W. Lyons 1996: A WSR-880 radar view of Tropical Cyclone Ed. Wea. and Fore., 11, 115-135.

Wang, Y., and G. J. Holland, 1996: Tropical cyclone motion and evolution in vertical shear. J. Atmos. Sci., 53, 3313-3332.

Weber, H. C. 1997: A study of tropical cyclone structure: Polygonal eye walls and other phenomena. Preprints, 22nd Conf. on Hurricanes and Tropical Meteorology, Amer.. Meteor. Soc., Boston, MA 02108, 203-204.

6 GJH February 23, 1998 Table of Contents

TOPIC THREE

TROPICAL CYCLONE MOTION

3.0 Report of the Topic Chairman R. Elsberry (USA)

3.1 Environmental Interaction Rapporteur: J. C. L. Chan ( - China)

3.2 Baroclinic Processes Rapporteur: B. Wang (USA)

3.3 Barotropic Processes (TC Motion) Rapporteur: R. Smith (Germany).

3.4 Small-scale Interactions (motion) Rapporteur: M. Lander (USA)

WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

TOPIC 3.0 Tropical Cyclone Motion

Chair: R. L. Elsberry Naval Postgraduate School Monterey, 93943, U.S.A. E-mail: elsberry®met.nps.navy.mil Fax: 1-408-656-3061

Rapporteurs: J.C.-L. Chan, B.Wang, R.Smith, M.Lander

3.0.1 Introduction

One of the highest priority research objectives of

IWTC-III was continuation of tropical cyclone (TC) motion

studies. Each of the rapporteur reports is related to either a physical interaction (environment and small-

scale) or the barotropic and baroclinic processes

involved in TC motion. Each of the rapporteur reports contains an extensive bibliography. Since most of these papers have been published since 1993, they indicate active research on this topic has indeed continued.

Rather than attempt to summarize the four rapporteur reports, a number of topics in which contrasting viewpoints exist will be highlighted for discussion at the Workshop.

3.0.2 Environment-related issues

As J. Chan points out in section 3.1.4, no unique definition of the TC environment exists for the purpose of separating what portion of the wind field is vortex, environment, or some component that arises from the interaction of the vortex and the environment, eg., the

beta gyres. Chan (section 3.1.2.b) and Smith (section

3.3.4) describe efforts to isolate the beta gyres from

different observation data sets. The so-called TCM-90

analyses did not yield a consistent relationship between

the asymmetric circulation and the inferred propagation vector (difference between environmental steering vector

and actual storm motion) . Glatz and Smith (1996) were not able to calculate the environmental relative vorticity gradient that would explain the propagation vectors in three successive data sets from Hurricane

Josephine. However, Franklin et al. (1996) found the predicted locations of negative and positive vorticity centers in their composite of many cases from the

Hurricane Research Division (HRD) Synoptic Flow

Experiments. A suggested topic for discussion at the

Workshop is what observational evidence is needed to validate the beta-gyre contribution to TC motion, and how such observations might be obtained.

The dispersion that generates an anticyclone in the wake of a TC in barotropic and baroclinic models is considered to be the explanation for the trailing (southeast in and northeast in Southern Hemisphere) anticyclone between larger TCs in global models that have an adequate vortex structure (Carr and Elsberry 1995, 1997). An independent satellite-based observation of subsidence-induced cloud clearing in the region of the trailing or peripheral anticyclone is consistent with the Rossby wave dispersion explanation. However, not all models have this circulation ~ is this because the horizontal structure of the vortex is not defined correctly? Since the TC track is expected to turn poleward if the peripheral anticyclone becomes the dominant steering influence, the evidence for such a Rossby wave dispersion explanation should be discussed by researchers and forecasters.

A linear environmental wind shear has been shown in barotropic models to cause a larger (smaller) deflection from a quiescent flow beta effect if the shear is anticyclonic (cyclonic) . A series of papers (Wang and Li

1995, Li and Wang 1996, and Wang et al. 1997) based on the energetics of the beta gyres has provided an explanation for these differences. However, Elsberry et al. (1993) have suggested that the environmental shear in the 850-300 mb layer is always positive, in which case the difference from the simple beta effect is small.

This topic might also be discussed.

In a binary vortex situation (sections 3.1.2.b and

3.3.3.4), the circulation of the adjacent TC may become the dominant steering influence in a Fujiwhara-type interaction. However, Carr et al. (1997) have defined two other modes (semi-direct and indirect) of TC interactions as well. These modes also cause track deflections even though the separation distances are too large for the TC circulations to overlap. Frequencies of occurrence of these three modes have been calculated for

the western (Carr et al. 1997) and eastern North Pacific

(Boothe 1997) TCs and for the Southern Hemisphere storms

(Bannister et al. 1997). Since objective criteria have been proposed (Carr and Elsberry 1998) for detecting binary TC interactions, this topic appears ready to move from research to forecast centers for testing.

Progress has recently been made in ensemble forecasting for TC tracks, which was one of the recommendations from IWTC-III. As indicated in section

3.3.5, the first tests have been with barotropic models.

In general, the ensemble mean is expected to have smaller track forecast errors simply because random errors will be reduced by the averaging. However, barotropic models may have large systematic errors. Baroclinic model ensemble forecasts have been made with a global model

(Zhang and Krishnamurti 1997) and with the regional GFDL model (S. Aberson, HRD). The key objective in the ensemble forecast approach is to define ensemble members with likely observation errors that contain growing modes. The success of these ensemble approaches in representing the confidence in the track forecasts and the prospects for real-time use of ensemble track forecasts should be discussed at the Workshop.

3.0.3 Baroclinic effects on motion A number of research groups have addressed the

baroclinic effects on TC motion (section 3.2) since IWTC­

III. The effects of vertical shear in the environmental

flow on initially barotropic vortices is to tilt over the

vortex and induce compensating thermodynamic effects to

maintain hydrostatic and geostrophic balance. The

associated vertical motions that lead to convergence and

divergence patterns in these dry vortices are 1-2 orders

of magnitude smaller than when moist physics are

included. With the vertical restoring forces of

convection and cumulus momentum transports included, the model vortex does not have the large tilts as in the dry models, and thus are more like real mature TCs.

Two effects appear to be important in interpreting

these moist model simulations in vertical shear. First,

the convective parameterization scheme, and especially how it is related to the dynamic and boundary layer forcing, affects the vortex structure and the motion.

Second, the vertical structure of the vortex (eg., depth of anticyclone aloft) in the initial conditions also affects the response to the imposed vertical shear. As described in section 3.2, the resulting adjustments are highly nonlinear. These dynamical models are also being used to study TC genesis (thus the vertical structure of the vortex is created rather than imposed as an initial condition) and intensification. Thus, the understanding of motion in relation to wind structure (including intensity) change may be advanced by these numerical simulations, which was one of the priority

recommendations from IWTC-III. Comparisons of the various

simulations with moist physics should be made at the

Workshop to assess the status and suggest future research.

A new tool for understanding factors that contribute to hurricane motion is potential vorticity (PV) inversion

(section 3.2), which has previously been applied in extratropical cyclogenesis studies. Certain assumptions about quasi-geostrophy and conservation of potential vorticity would seem to be questionable in the mature tropical cyclone application. One application of PV reasoning is the vertical penetration depth, i.e., the downward (upward) influence of a potential vorticity anomaly on a circulation at a lower (upper) level.

Without consideration of the near-zero absolut~ vorticity and (likely) static stability characteristics of shallow outflow layers from real mature tropical cyclones, the PV reasoning has been applied to infer downward penetration of outflow layer circulations throughout the lower troposphere. As indicated at the end of section 3.2, the treatment of diabatic effects and convective momentum transports in the PV context needs careful attention.

The extent to which PV reasoning and especially PV inversion techniques are applicable in mature tropical cyclone motion (and intensity change) should be discussed at the Workshop. 3.0.4 Small-scale effects on motion

An active area of research in vortex dynamics

(section 3.3.3.2 and 3.4.3a) for understanding structure changes has been suggested to have an influence on motion as well. Whereas the contracting concentric eyewall cycles are generally accepted as one mechanism for intensity changes, unpublished work by Weber (section

3.3.3.2) evidently indicates associated track deflections of 50 to 100 km. Clearly, such large deflections would be important as the tropical cyclone approaches landfall, and observational confirmation with the long record of eyewall cycles in Atlantic hurricanes should be possible.

A key question with respect to these eyewall cycles is the triggering mechanism. One suggested mechanism is the vortex Rossby wave, which accelerates (decelerates) the flow on the inside (outside) and thus changes locally the vortex structure. It is generally accepted that outer wind structure affects the track deflections from environmental steering via the beta effect. Do the smaller, inner-core structure changes associated with vortex Rossby waves also affect motion, and by what magnitude of track deflections? Similarly, the polygonal eye structures that appear to be reflections of an instability on the inner eyewall flow that leads to small-scale vortices have been suggested by H. Weber

(unpublished -- see section 3.4.3a) to trigger 50 km track oscillations. Again, verification of such track deflections should be possible with Atlantic hurricane data sets where aircraft observations are available. The proposed track deflections associated with these inner vortex structure changes, and validation studies, should be discussed at the Workshop.

Another small-scale circulation that has been proposed to affect TC motion is the mesoscale convective system (MCS) (section 3.4.3c). Numerical simulations in which an assumed vortex structure associated with an MCS is imposed do lead to track deflections. An observational study by Harr and Elsberry (1996) with aircraft observations in a well-developed MCS in

Typhoon Robyn concluded that it was unlikely that an interaction between the typhoon and MVS circulations could be responsible for a marked change in speed and direction of Robyn. Conditions for which the numerical simulations indicate a MCS develops (versus being imposed) in the TC environment, its structural characteristics in space and time, and its subsequent changes in TC track direction and speed need to be validated with observations. The Workshop should be a venue for discussions between modellers and observationalists.

The local winds and precipitation as a landfalling

TC interacts with orography is clearly a critical forecasting problem. In addition, this interaction with topography may also change the track, and thus affect the actual landfall point (section 3.3.3.5). Whereas the models cited in that section have a single layer so that the air is forced to flow over the topography, other baroclinic models allow flow decoupling, and suggest other mechanisms for track deflections (Yeh and Elsberry

1993). A number of modeling groups have attempted to simulate the landfall of Typhoon Herb on Taiwan in 1996. The capability of such models to predict motion changes associated with interaction between the circulation and orography, and the diagnosis of the physical mechanisms, should be discussed at the Workshop.

3.0.5 Future requirements

A common theme in the rapporteur reports is the need for observations to validate the theoretical and numerical studies of TC motion. It_seems the ability to model has surpassed our capability to observe TCs, both in the environment and within the TC vortex. Discussion of how the necessary data sets may be obtained is an appropriate topic for the Workshop.

Each of the rapporteur reports also indicates where progress in understanding TC motion being made and this will lead to planning, coordination of future research.

3.0.6 References (not given in rapporteur reports)

Zhang, Z., and T. N. Krishnamurti, 1997: Ensemble forecasting of hurricane tracks. Bull. Amer. Meteor. Soc., 78, 2785-2795. I--.. WMO/CAS!ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic 3.1: Environmental Interaction

Rapporteur: Johnny C. L. Chan Department ofPhysics & Mat. Sci. City University ofHong Kong 83 Tat Chee Avenue, Kowloon Hong Kong, China

Email: [email protected] Fax: (Hong Kong) 852-2788-7830

Working Group: J. Abraham, J. Callaghan

3.1.1. Introduction

From a synoptic-scale perspective, what is meant by the environment of a tropical cyclone (TC) is obvious: troughs, ridges, monsoon surges, etc. Because the horizontal scales of these systems are generally larger than that of the TC, it is generally assumed that a TC is affected by its environment but not vice versa. However, many recent theoretical studies (see Elsberry 1995, hereafter referred to as E) have suggested a two-way interaction between a TC and its environment. In terms of TC movement, this implies that while the environment generally "steers" the TC, the circulation of the latter can modify that ofthe former. This may then change the "steering current" and hence the movement of the TC. Thus, in discussing environmental interaction, the two-way perspective should be considered. In this report, the effects of the environment on TC movement is first discussed in Section 3 .1.2. Recent studies on the two-way interaction are then summarized in Section 3.1.3. The "roadblocks" that need to be overcome to advance our knowledge in this area are identified in Section 3 .1.4 while possible future study areas are proposed in Section 3 .I. 5.

3.1.2. Environmental effects on TC movement a. The steering concept Steering has always been considered to be the main effect of the environment on the movement of a TC. E has already discussed the different methods used to compute steering and how changes in the environment (which include planetary-scale flow, synoptic-scale circulations and storm.:.scale features) can modifY the steering. The reader is referred to this publication for further details. Dynamically, steering can be interpreted as advection of the relative (cyclonic) vorticity of the TC by the environmental (steering) flow. Although the vorticity equation contains other non-advective terms, Chan (1984) found that their contributions ate generally much smaller than that by the advection term except in the and the upper troposphere. Thus, ifthe advection of planetary vorticity is small (which is a good assumption in most cases, except when the steering flow is weak), steering should, at least to a first approximation, be responsible for the movement of a TC. However, irrespective of how the steering is defined, the movement of a TC is often found to deviate from that specified from the steering vector. Elsberry (1990) labelled such deviation as propagation. This implies that effects other than steering need to be considered.

b. The generalized beta-effect In a number of barotropic numerical studies, the advection of planetary vorticity produces a flow near the TC centre (called the ventilation flow by Fiorino and Elsberry 1989) that appears to correspond to the observed propagation (see E for a review). This has been referred to as the 0-effect. Further numerical experiments with an environment that has shear also produced a similar ventilation flow. Williams and Chan (1994) referred to this as the . "generalized f3-effect". However, studies using the analyzed wind fields associated with TCs that occurred during the Tropical Cyclone Motion Experiment TCM-90 failed to obtain a one-to-one correspondence between the propagation vector and the ventilation flow vector (Elsberry et al. 1993; Ngan and Chan 1995). While it is possible that the analyses did not have a high enough resolution to capture the gyres, it is equally likely that other processes may dominate to an extent as to make its presence undetectable. In fact, Chan and Law (1995) showed from a barotropic modelling study of binary interaction that a strong environmental vorticity gradient can mask the wavenumber-1 gyres. Further, the numerical studies include only barotropic dynamics so that any significant baroclinic processes may overwhelm the barotropic effect so as to render the gyres unobservable. Thus, while the environmental and planetary vorticity distributions should play a role in changing the movement of a TC from that specified by the steering flow, its importance in the overall picture remains unclear. More discussion of this point will be made in Section 3 .1. 4. c. Vertical shear/potential vorticity considerations Since other non-barotropic processes may be important, many recent studies (mostly numerical but some observational) have focused on the vertical variations of wind and/or vorticity. A vertical shear also implies a horizontal temperature (thickness) gradient which then suggests that the horizontal potential vorticity (PV) distribution is not uniform. Most of the numerical studies on the effects of vertical shear and/or PV distributions on TC movement have been discussed in E or in Chapter 3 .3 and will not be repeated here. Recent observational studies (Shapiro 1996; 'Wu and Emanuel 1995a, b) of the effects of a rnidlatitude trough on hurricane motion suggest that a TC tends to move towards an area of maximum PV anomaly. The results also suggest that a midlatitude trough changes the movement of the TC not only through the traditional thinking of steering. However, both studies focused on TCs in the. Atlantic where they occur generally at higher latitudes so that the PV anomalies are appreciable. In the deep tropics, the PV is generally quite uniform in the horizontal except in the upper troposphere. In such cases, the effects ofPV may not be that significant. Verification of the theoretical results using TCs in the tropics is therefore necessary.

d. Momentum transports Li and Chan (1997) proposed to apply the concept of linear momentum (zonal or meridional) conservation to explain TC to recurvature. The idea is that a TC initially moving northwestward and subsequently turning northeastward must either gain westerly momentum from or give up easterly momentum to the environment. They found from a case study that in the upper troposphere, the southerly flow ahead of the trough is advecting the planetary momentum into the TC. In the mid troposphere, this advection is accomplished by the retreating subtropical ridge to the east of the TC. In other words, the important physical process that causes a TC to change from a westward to an eastward motion is the advection of planetary momentum by the environment, and both the trough and the subtropical ridge play equally significant roles. A recent composite analysis of eight recurving TCs by Li (1997) confirmed the results ofthe case study. These results again point to the dominant influence of the environment on the movement of a TC. e. Extratropical transition A TC moving to higher latitudes is likely to be transformed from a warm to a cold core system, a process commonly known as extratropical transition. Its interaction with the environment in terms of changes in structure and intensity is discussed in Chapter 2.3 and will not be addressed here. With regard to motion, while previous studies suggest that the acceleration of the TC is mostly due to the strong westerlies, other types of interactions with the environment have been identified such as the capture by cold fronts (Foley and Hanstrum 1994) and acceleration due to the development of a diabatically-driven downstream anticyclone (Henderson et al. 1997).

3.1.3. Effects of the TC on the environment While the environment is the dominant factor responsible for TC motion, the circulation of the TC may at times modify the environment so that its movement is subsequently altered. The development of a pair of counterrotating gyres due to the 13-effect (Fiorino and Elsberry 1989) represents the simplest example of this effect. These gyres develop m a quiescent environment. If it were not for this effect, the vortex would be stationary. The gyres set up a ventilation flow which then acts as the steering to advect the vortex westward and poleward. Carr and Elsberry (1995) further showed that a large barotropic vortex can develop an anticyclonic flow to its southeast, thus creating a strong southerly flow to the east of the vortex. A small vortex in this area will then be steered by a combination of the flow as.sociated with the vortex and that of the induced anticyclone. Wang and Li (1995) explained the result ofWilliams and Chan (1994) that a vortex in a cyclonic shear moves slower than one in an anticyclonic shear by calculating the energy transfer among the various components of the system. They found that in a cyclonic shear, the wavenumber-1 gyres associated with the vortex give up energy to the environment while the opposite is true under anticyclonic shear conditions. Exchanges of energy among the environment and various wavenumber components have been found from observations by Chan and Cheung (1997). They showed that under different environmental conditions, the vortex and/or its wavenumber components can give up energy to the environment. This then reduce the effects of the latter on the motion of the vortex since the advection process is likely to be altered.

3.1.4. Roadblocks a. Research and understanding

(i) SEPARATING TilE ENVIRONMENf AND THE VORTEX CIRCULATIONS While it is relatively easy to identify qualitatively the environment from a , separating the environment from the vortex quantitatively is a formidable task.. Many previous steering studies averaged the flow around a certain radial band (e.g. George and Gray 1976) and considered this mean flow to be the environment. The residual flow is then the vortex circulation. Kasahara and Platzman (1963) assumed the vortex flow to be axially symmetric so that all asymmetries are contained in the environment. These methods may be acceptable in theoretical studies with simple flows but probably not adequate in real situations. Recently, Kurihara et al. (1993, 1995) have developed a filtering technique to separate the environment and the vortex. Chan and Cheung ( 1997) tested their technique by adding different synthetic environments and vortices and then separating out the wavenumber components. They found that other than a small cyclonic residual, the technique apparently can recover the different wavenumber flows. Using this technique, they obtained results related to the wavenumber flows consistent with those from numerical models. Elsberry et al. (1993) have also used the first 20 modes of a global spectral model analysis to represent the environmental flow. These methods need to be further tested so that one or more techniques can be adopted to separate the environment from the vortex. Without proper separation, it will be difficult to study the interaction between these two circulations.

(ii) OBSERVATIONAL STIJDIES Other than the problem of separating the environment and the vortex circulations, observational studies also suffer from the lack of good observations or analyses. Even the Tropical Cyclone Motion Experiments in 1990, the TCM-90 and SPECTRUM, did not produce a dataset with good temporal or spatial coverage. Because of the relative small number of TCs, composite studies of the observations may not yield any meaningful results. Subsequent analyses by the US National Meteorological Center (Roger et al. 1993) may not have the horizontal resolution to identify the wavenumber-1 gyres, as suggested by the results ofChan and Cheung (1997). Thus, a major stumbling block in increasing our understanding of the interaction between the environment and the vortex in terms of motion is the lack of data and a good analysis. Without observational analyses, it would be difficult to verify the theoretical results.

(iii) THEORETICAL AND NillvfERICAL STIJDIES While the barotropic dynamics is quite well understood, the effects of baroclinic and thermodynamic processes on TC motion and the feedback on the environment is far from clear. Although a number of numerical studies have been made (see E), the results appear to be inconsistent. This is because in each of the models, the physical processes and the parameterizations are different. And without any observational study to verify these results, it is difficult to say which one is correct. Further, all such studies deal only with the effects of the environment on the vortex movement. How the vortex circulation affects the environment has not been addressed. A complete three-dimensional conceptual framework is also lacking. b. Forecasting Although forecasting of TC motion nowadays has generally followed the numerical model guidance, understanding the interaction between the re and the environment may help a forecaster to determine whether an NWP forecast is correct. The systematic approach proposed by. Carr and Elsberry (1997) is probably a good start. The major roadblock then is how such an approach can be made objective so that the forecaster can determine what type of interaction is occurring under a given situation.

3.1.5. Areas for future studies Areas that should be tackled in future studies of environmental interaction in re motion: • Development of a method of separating the environment and the vortex circulations

• Development of a conceptual framework in three dimensions to describe the interaction between a TC and its environment

• Observational studies ofthe interaction using available observations • Development of an objective technique to identifY different environmental flow conditions under which different types of interaction may occur.

• A more extensive and detailed study of the problem of extratropical transition and the interaction between a TC and its environment under such a transition

Acknowledgements. The author would like to thank the City University of Hong Kong, the Research Grants Council of the Universities Grants Committee of Hong Kong and the US Office of Naval Research for their continued support on his research on tropical cyclones.

References Carr, L. E., Ill and R. L. Elsberry, 1995: Monsoonal interactions leading to sudden tropical cyclone track changes. Mon. Wea. Rev., 123, 265-289.

Carr, L. E., Ill and R. L. Elsberry, 1997: Systematic and integrated approach to tropical cyclone track forecasting. Part I: Overview and environment structure characteristics. Wea. Forecasting (submitted)

Chan, J. C. L., 1984: An observational study of the physical processes responsible for tropical cyclone motion. J. Atmos. Sci., 41, 1036-1048.

Chan, J. C. L. and A. C. K. Law, 1995: The interaction of binary vortices in a barotropic model. Meteor. Atmos. Phys., 56, 135-155.

Chan, J. C. L. and K. K. W. Cheung, 1997: Characteristics of the asymmetric circulation associated with tropical cyclone motion. Accepted for publication in Meteor. Atmos. Phys.

Elsberry, R. L., 1990: International experiments to study tropical cyclones· in the western North Pacific. Bull. Amer. Meteor. Soc., 71, 1305-1316.

Elsberry, R. L., 1995: Tropical cyclone motion. Chap. 4 in Global Perspectives on Tropical Cyclones. R. L. Elsberry (Ed.), WMO/TD-No. 693, World Meteorological Organization, Geneva, Switzerland, 15 8-197.

Elsberry, R. L., P. H. Dobos and D. W. Titley, 1993: Extraction oflarge-scale environmental flow components from the TCM-90 analyses: Implications for tropical cyclone motion studies. Preprints, 20th Conf Hurr. Trap. Meteor., Amer. Meteor. Soc., Boston, MA, 485-488.

Fiorino, M. and R. L. Elsberry, 1989: Some aspects of vortex structure in tropical cyclone motion. J. Atmos. Sci., 46, 979-990.

Foley, G. R. and B. N. Hanstrum, 1994: The capture of tropical cyclones by cold fronts off the west coast of Australia. Wea. Forecasting, 9, 577-592. George, J. E. and W. M. Gray, 1976: Tropical cyclone motion and surrounding parameter relationships. J Appl. Meteor., 15, 1252-1264.

Henderson, J. M., G. M. Lackmann and J. R. Gyakum, 1997: An analysis ofHurricane Opal's forecast track errors using quasi-geostrophic potential vorticity inversion. Submitted to Mon. Wea. Rev.

Kasahara, A. and G. W. Platzman, 1963: Interactions of a hurricane with the steering flow and its effect upon the hurricane trajectory. Tellus, 15, 321-335.

Kurihara, Y., M. A. Bender and R. J. Ross, 1993: An initialization scheme of hurricane models by vortex specification. Mon. Wea. Rev., 121, 2030-2045.

Kurihara, Y., M. A. Bender, R. E. Tuleya and R. J. Ross, 1993: Improvements in the GFDL hurricane prediction system. Mon. We a. Rev., 123, 2791-2801.

Li, Y. S., 1997: Momentum transports associated with tropical cyclone recurvature. M.Phil. Thesis, City Univ. Hong Kong, 100pp.

Li, Y. S. and J. C. L. Chan, 1997: Momentum transports associated with tropical cyclone recurvature. Preprints, 22nd Conf. on Hurricanes and Trap. Meteor., Fort Collins, CO, Amer. Meteor. Soc., 19-23 May, 366-367.

Ngan, K. W. and J. C. L. Chan, 1995: Tropical cyclone motion - steering vs. propagation. Preprints, 21st Tech. Conf Hurricanes and Trop. Meteor., Miami, Amer. Meteor. Soc., April 24-28, 23-25.

Rogers, E., S. L. Stephen, D. G. Deaven and G. J. DiMego, 1993: Data assimilation and forecasting for the Tropical Cyclone Motion Experiment at the National Meteorological Center. Preprints, 20th Tech. Conf HurricaJZes and Trop. Meteor., San Antonio, Amer. Meteor. Soc., 329-330.

Shapiro, L. J., 1996: The motion of Hurricane Gloria: A potential vorticity diagnosis. Mon. Wea. Rev., 124, 1465-1475.

Wang, B. and X Li, 1995: Propagation of a tropical cyclone in a meridionally-varying zonal flow: An energetics analysis. J Atmos. Sci., 52, 1421-1433.

Williams, R. T. and J. C. L. Chan, 1994: Numerical and analytical studies of the beta-effect in tropical cyclone motion. Part II: Zonal mean flows. J. Atmos. Sci., 51, 1065-1076.

Wu, C. C. and K. A. Emanuel, 1995a: Potential vorticity diagnostics of hurricane movement. Part I: A case study ofHurricane Bob (1991). Mon. Wea. Rev., 123, 69-92.

Wu, C. C. and K. A. Emanuel, 1995b: Potential vorticity diagnostics of hurricane movement. Part II: Tropical Storm Ana (1991) and Hurricane Andrew (1992). Mon. Wea. Rev., 123,93-109. WMO/CAsncsu

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic 3.2:Baroclinic Processes

Rapporteur: Bin Wang Department of Meteorology, University of 2525 Correa Rd., Honolulu, HI 96825, USA E-mail: bwang@ soest.hawaii.edu Fax: 1-808-396-2877

Working Group: J. Evans, W. Frank, W. Gray, L. Shapiro, Roger Smith

3.2.1 Introduction

Both the tropical cyclones (TCs) and their environments are baroclinic. A number of baroclinic forcing affecting TC propagation (defined as TC motion relative to the surrounding steering flow) have been identified which include (a) vertically differential beta-drift arising from TCs' baroclinic structure, (b) ambient potential vorticity (PV) gradients, (c) ambient vertical shear, (d) heating which can modify PV structure and vertical interaction, and (e) a horizontally varying vertical shear across a TC track. Most studies so far have focussed on examination of the effects of these forcings in isolation with simple mechanistic models except the last mechanism which was examined using empirical approach.

3.2.2 Adiabatic vortices

When the cyclonic circulation decreases with height, the tendency of beta-drift decreases with height as well, because of its dependence on the strength of the outer vortex circulation (DeMaria 1985). Wang and Li (1992) found that such a baroclinic vortex can propagate with a vertically uniform speed against the distortion due to vertically differential beta drift. This was attributed to the vertical coupling of the vortex circulation by a secondary circulation required by vorticity conservation and hydrostatic balance (Eisberry 1995).

On an f-plane, the interaction between vortex circulation and environmental PV gradients can induce vortex propagation to the right of the vertical shear vector if the downshear displacement of the upper negative PV anomalies plays minor roles (Shapiro 1992).

Even without beta effect or ambient PV gradients, vertical shears can create vertical interaction between the lower and upper cyclonic circulations by inducing a vertical tilt of the vortex (Jones 1995). The vertical interaction allows the upper- and lower-level vortex centers rotate around the mid-level center. The rotation rate depends on the parameters that enter the Rossby penetration depth (Khandekar and Rao 1974; Jones

3.2.1 1995; Smith et al. 1998). As a result of the rotation, the vortex does not move with the surface wind, but with the environmental wind at some higher "steering" level whose height depends on the vertex structure and the height of the positive PV-anomalies (Jones 1998). The PV-anomalies at the surface and upper-levels can also be considerably deformed, leading to asymmetries in the PV which can then affect the motion and vertical tilt. Using an idealized two-level quasi-geostrophic model, Wu and Emmanuel (1993) examined the contribution of upper-level negative PV anomalies on the flow that steered the hurricanes below, which results in a leftward drift relative to the vertical shear (see also Flatau et al. 1994).

3.2.3 Potential vorticity inversion approach

The vertical interaction may be conveniently visualized in terms of PV thinking. Given an appropriate balance condition, boundary conditions, and background state, a PV anomaly can be inverted to derive the 3-D wind distribution associated with the PV (Hoskins et al. 1985, Davis and Emmanuel1991). The circulations at different levels interact via penetration flows associated with the PV anomalies in different levels. The penetration flow depth depends on the strength and horizontal scale of the anomaly, the Coriolis parameter, and static stability (Hoskins et al. 1985; Shapiro and Montgomery 1993). lt can be more than 10 km forTCS (Jones 1995).

Studies by Wu and Emmanuel (1995a,b) and Wu and Kurihara (1996) have used a piecewise inversion technique to evaluate the influence of upper- and lower-level PV anomalies on the motion of several hurricanes (also see Moeller 1995, Moeller and Jones 1998).

A new piecewise inversion technique was recently developed by Shapiro (1996) and applied to Hurricane Gloria of 1985. Motivated by the weak asymmetries in the core of Gloria (Shapiro and Montgomery 1993), his inversion technique is based on decomposition of the horizontal wind into symmetric (vortex) and asymmetric (environmental) components from which PV anomalies can be better defined. Because the non linear terms in the PV and balance equation were found to be small, a linear decomposition was made. Thus, the ad hoc nature. of the previous piecewise decomposition is avoided. Moreover, removal of hurricane vortex and use of a climatological basic state were also avoided.

In a set of benchmark analyses, Shapiro (1996) found that the wind that steered Gloria was primarily attributable to the PV anomalies confined within a cylinder of radius 1000 km and levels 500 hP a and above. The results imply that in order to improve prediction of short-term changes in the environmental flow, measurements of upper-level winds and heights are required at least out to 1000 km. A set of nine synoptic-flow cases has been recently used to evaluate how representative the result for Gloria is (Shapiro and Franklin 1997, 1998).

3.2.2 3.2.4 Diabatic vortices

When a cyclonic vortex is capped by anticyclonic circulations, the vertically differential beta-drift or vertical shears may induce a vertical shearing of the vortex. Diabatic heating becomes necessary for maintaining the baroclinic vortex structure through transporting negative PV upward. Wang and Holland (1996b)found that the beta-drift in this case tends to be more poleward. This occurred because the upper-level anticyclonic PV anomalies induced a cyclonic rotation of the low-level beta gyres, and resulted in more poleward secondary steering flow over the low-level vortex core. In the presence of both the earth vorticity gradient and the vertical shear, the processes are highly nonlinear (Fiatau et al. 1994, Wang and Holland 1996c).

Diabatic processes can modify PV structure of a TC, affecting its motion. Wang and Holland (1996c) suggested that the presence of vertical shear leads to a sloping absolute vorticity vector and thus to asymmetric diabatic PV modification which could affect the motion. However, in their calculation of the slope of absolute vorticity vector they omitted the TC's relative vorticity. As this can be more than an order of magnitude greater than the Coriolis parameter, the effect they discussed is likely to be significant only in extremely strong vertical shear (Smith et al. 1998}.

The interaction between heating and circulation may also result in convective asymmetry which induces asymmetric divergent flow over the vortex core, affecting vortex motion by deflecting the vortex to the region of maximum convection (Wang and Holland 1996b). They suggested three possible mechanisms: the vortex-tube stretching arising from the relative flow crossing the high relative vorticity core in the lower troposphere and the boundary layer; the divergent circulation associated with the vertical tilt of the vortex; and the asymmetric heat fluxes from the ocean arising from the asymmetric boundary layer flow.

Since the development and structure of the upper-level anticyclone depend on the diabatic heating and the momentum transports associated with convection, different parameterization of cumulus heating and momentum transports can result in different upper-level which in turn affect significantly TC motion (Dengler and Reeder 1997). The change in motion in this case is related to changes in the intensity ahd structure of the TC.

3.2.5 Empirical evidence of baroclinic influences on TC propagation

Gray and McEiroy (1998) showed that TCs form and move in directions along which the is the warmest and the deep vertical wind shear reverses sign with westerly (easterly) shears on poleward (equatorward) side of the cyclone's track. They argue that this causes the surrounding right and left of track deep layer mean wind profile to possess weaker parallel flow in the direction of cyclone motion than the mean deep layer flow over the cyclone center, causing TCs to propagate faster than their surrounding steering flow. They also attributed the leftward propagation of most TCS to their movement through tropospheric layers in which their forward environment is colder than rear environment. This causes the deep layer thermal wind of the environmental flow to be directed to the right of the cyclone movement, resulting in a leftward propagation.

3.2.3 REFERENCES

Chan, J. C.-L., and R. T. Williams, 1987: Analytical and numerical studies of the beta effect in tropical cyclone motion. part 1: zero mean flow. J. Atmos. sci., 44, 1257-1265.

Davis, C. A., and K. A. Emmanuel, 1991: Potential vorticity diagnostics of cyclogenesis. Mon. Wea. Rev., 119, 1925-1953.

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Fiorino, M., and R. L. Elsberry, 1989: Some aspects of the vortex structure related to tropical cyclone motion. J. Atmos. Sci., 46, 975-990. Flatau, M., W. H. Schubert and D. E. Stevens, 1994: The role of baroclinic processes in tropical cyclone motion: The influence of vertical tilt. J. Atmos. Sci., 51, 2589-2601.

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Willoughby, H.E., J.A. Clos, and M.G. Shorebah, 1982: Concentric eye walls, secondary wind maxima, and the evolution of a hurricane vortex. Journal ofApplied Science, 39:505-514.

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Gray, W.M., 1990: Strong association between West African rainfall and U.S. landfall of intense hurricanes. Science, 249, 1251-1256.

Handmer, J. and D.I. Smith, 1992: Cost-effectiveness of Flood Warnings. Report prepared for the Australian Bureau of Meteorology by the Centre for Resource and Environmental Studies. Canberra, Australia: Australian National University. 50 pp.

Jensen, R.C. and G.R. West, 1986: Input-Output for Practitioners: Theory and Applications. Aus­ tralian Regional Developments No. 1. Canberra, Australia: Australian Government Pub­ lishing Service. 133 pp.

Joy, C.S., 1991: The cost of natural disasters in Australia. Paper presented at the Climate Change Impacts and Adaptation Workshop, Climatic Impacts Centre. Sydney, Australia: Mac­ quarie University.

Landsea, C.W., N. Nicholls, W.M. Gray, andL.A. Avila, 1996: Downward tren~s in the frequency of intense A~lantic hurricanes during the past five decades. Geophysical Research Letters, 23:1697-1700.

Molinari, J. and D. Vollaro, 1989: External influences of hurricane intensity. Part I: Outflow layer eddy angular momentum fluxes. Journal of , 46: 1093-1105.

Noonan, B., 1993: Catastrophes: The new math. Best's Review, August: 41-83.

Pielke, Jr., R.A., 1997: Reframing the U.S. hurricane problem. Society and Natural Resources, 10(5): 485-499.

Pielke Jr., R.A. and C. W. Landsea, 1998: Normalized hurricane damages in the United States 1925-1995. Submitted to Weather and Forecasting.

Pielke, Jr., R.A. and R.A. Peilke, Sr., 1997: Hurricanes: Their Nature and Impacts on Society. Chichester, UK: John Wiley and Sons Press.

33 Changnon, S.A., J.M. Changnon, and D. Changnon, 1995: Uses and applications of climate fore­ casts for power utilities. Bulletin of the American Meteorological Society, 76: 711-720.

Coates, Lucinda, 1996: An Overview ofFatalities from Some Natural Hazards in Australia. Paper presented to the Conference on Natural Disaster Reduction, Surfers Paradise, 29 Septem­ ber-2 October, Queensland, Australia.

DeMaria, M. and J. Kaplan, 1994: A statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic basin. , 9: 209-220.

Diaz, H. and R.S. Pulwarty, 1997: Hurricanes: Climate, Social, and Economic Impacts. Berlin, Germany: Springer-Verlag.

·Dixon, A., L.F. Scura, R.A. Carpenter, and P.B. Sherman, 1995: Economic Analysis of Environ­ mental Impacts Second Edition. London: Earthscan Publications Limited.

Dlugolecki, A.F. et al., 1996: Chapter 17, Financial Services. In: Climate Change 1995: Impacts, Adaptations and Mitigation of Climate Change: Scientific - Technical Analyses. Contri­ bution of Working Group II to the Second Assessment Report of the IPCC (Intergovern­ mental Panel on Climate Change). London: Cambridge University Press.

Dvorak, V.F., 1984: Tropical cyclone intensity analysis using satellite data. NOAA Tech. Mem. NES 11, 47 pp.

Dwyer, L., 1986: Environmental policy and the economic value of human life. Journal of Envi­ ronmental Management, 22: 229-243.

Englehardt, J. and C. Peng, 1996: A Bayesian benefit-risk model applied to the south Florida building code. Risk Analysis, 16(1): 81-91.

Field, B.C., 1994: Environmental Economics: An Introduction. Sydney, Australia: McGraw-Hill Inc. 482 pp.

Flavin, C., 1994: Storm warnings. Worldwatch, 7(6): 10-20.

32 quent in-person classwork. Indeed, no WMO Training Course is scheduled for 1998 currently. It is hoped that this will resume again in 1999.

Bibliography Abelsdn, P., 1996: Project Appraisal and the Environment. London, UK: Macmillan Book Lim­ ited. 304 pp.

Anaman, K.A., S.C. Lellyett, L. Drake, R. Leigh, A. Henderson-Sellers, P.F. Noar, P.J. Sullivan, and D.J. Thampapillai, 1998: Benefits of meteorological services: Evidence of recent re­ search in Australia. Meteorological Applications, 5(1) in press.

Anaman, K.A. and S.C. Lellyett, 1996: Contingent valuation study of the public weather service in the Sydney metropolitan area. Economic Papers, Economic Society ofAustralia, 15(3): 64-77.

Anaman, K.A., D.J. Thampapillai, A. Henderson-Sellers, P.F. Noar, and P.J. Sullivan, 1995: Methods for assessing the benefits of meteorological services in Australia. Meteorological Applications, 2: 17-29.

ATFHCI (Academic Task Force on Hurricane Catastrophe Insurance), 1995. Restoring Florida's paradise. Tallahassee, FL: State of Florida.

Banham, R., 1993: Reinsurers seek relief in computer predictions. Risk Management, August: 14- 19.

Blong, R., 1992: Impact of Climate Change on Severe Weather Hazards- Australia. Department of the Arts, Sport, the Environment and the Territories. Canberra, Australia: Australian Government Publishing House. 35 pp.

BTFFDR (Bipartisan Task Force on Funding Disaster Relief) 1995: Federal Disaster Assistance: Report of the Senate Task Force on Funding Disaster Relief U.S. Senate, 104-4. Wash­ ington, DC: United States Government Printing Office.

Chang, S., 1983: Disasters and fiscal policy: hurricane impact on municipal revenue. Urban Af­ fairs Quarterly, 18(4): 511-523.

31 5.4.8. Issues Past Efforts: In the past few years, education for tropical cyclone forecasters around the world have focussed upon attendance of World Meteorology Organization (WMO) sponsored Training Courses and Workshops. For example, in April and May 1996 the WMO Training Course on Tropical Meteorology and Tropical Cyclone Forecasting was held in Miami, U.S.A. in conjunction with additional classwork held at Florida State University in Tallahassee, U.S.A. This Training Course was for tropical cyclone forecasters from all parts of the world and included lec­ tures as well as "hands-on" applications in all aspects of tropical meteorology/tropical cyclone analysis and forecasting. A similar, but shorter WMO Workshop was also held in Miami for the WMO RA-IV region (Atlantic, Gulf of Mexico and Caribbean Sea countries) in March 1997. In addition to classes, the WMO (through past International Workshops on Tropical Cyclones) has produced a thorough handbook entitled "Global Guide to Tropical Cyclone Fore­ casting" ( 1993). This is the first publication that has attempted to address analysis and forecasting for tropical cyclones around the world. Overall, it has been considered an outstanding success in its breadth and applicability for forecasters throughout the globe. Current and Future Directions: One obvious resource that should be taken more advan­ tage of is the Internet Providing freely accessible material on tropical cyclones via the Web can . be a boon to both forecasters and the general public. As an example of such educational opportu­ nities (primarily geared toward the general public) is the Hurricanes, Typhoons and Tropical Cyclones Frequently Asked Questions (FAQ) file maintained by Chris Landsea . On this Website, there exists basic answers to questions on tropical cyclone formation, movement, frequency, impacts, etc. along with illustra­ tive diagrams and links to additional helpful sites. Such a FAQ can, and should, be developed with providing tropical cyclone forecasters with the most up-to-date techniques and tools. (A . . suggestion would be to make the second edition of the "Global Guide" a completely Web accessi- ble document.) However, it is recommended that the Web not replace person-to- person classes and workshops. There is nothing like the experience of being "in-person" to best interact as teacher and student. It may be though that the availability of the Web may reduce the need for very fre- ·

30 It is suggested that digital databases of all of the types listed above (and more) be made available free of charge to whomever would like via an easily accessible Web or Ftp site. As suggested above in the first section, a comprehensive listing of such data should be kept up to date in an "official" site to allow for the easiest and most widespread accessibility of such data.

5.4.7. Communication Issues within the Research Community

The world has become a smaller place with the advent of readily available Internet access. The use of email (communication from one individual to another), telnet (remote login capabilities for computer work stations), ftp (remote access of computer systems for uploading and downloading files), newsgroups ("bulletin boards" of a huge variety of interests open to the general public) and the Web are now widespread not only in developed nations, but throughout the world.

One way that the tropical cyclone community has taken advantage of such advances is to provide an email "bulletin board" solely for tropical cyclone issues. The Tropical Storms mailing list is for those who are active in either the research or forecasting of tropical . storms worldwide. This email group helps to foster communication amongst the research and forecasting communities. Current email traffic is fairly low - only on average one email per day. There are currently 190 members in the Tropical Storms mailing list representing the countries of Australia, , Canada, China, , France, Germany, , Israel, Mexico, New Zealand, United Kingdom, and United States of America. However, the list is currently composed primarily of researchers and primarily from developed countries. It is recommended that an effort be made to encourage more forecasters and more individuals from developing countries to join the Tropical Storm mailing list. Another way that the Internet has been utilized beneficially is through weekly global tropical cyclone summaries. These had been previously compiled by Jack Beven for the period August 1991 through June 1996. Having a weekly summary of tropical cyclones has proven to be a wonderful resource of a "first-look" at what had happened in the tropics for a particular slice of time. Fortunately, these summaries are about the recommence due to efforts ofGary Padgett ofthe U.S. . These summaries will be posted on the Tropical Storms mailing list, the wx-talk and sci.geo.meteorology newsgroups and archived on a Web-accessible location. The 1991 through 1996 summaries will also be made available via the Web shortly.

29 defined radius of any point. Narratives for all tropical storms for the 1980-1995 period are included along with basin-wide tropical storm statistics.5

Limitations of these "best track" databases is that they usually only provide either maxi~ mum sustained winds and/or central pressures and no information on the radial distribution of winds. Additionally, all of these databases have biases/systematic errors in the positions and intensities and require their own "re-analysis" using today's analysis techniques upon previous year's data. Just as important can be the access of real-time assessments of intensity and position as determined by the various forecasting centers. This information is valuable in determining the uncertainty in knowing the intensity and positions in real-time. However, obtaining this informa­ tion is difficult and not always archived - especially not in a digital format. Opportunities to improve our archives would be to expand the "best track" databases to include (where available) information on the radial extent to various thresholds of winds (i.e. 34 kt, 50 kt, 64 kt, 100 kt, etc.) as well as to systematically digitally save all of the real-time analyses of TC position and intensity. Observations of Tropical Cyclones -·Aircraft, ships, buoys, rawindsondes, land stations, satellite imagery, radar imagery, etc.: Many groups have collected large quantities of observa­ tions within tropical cyclones in support of research projects. Observational platforms include a large variety of data including, but not exclusive to: aircraft flight level data, ships, buoys, raw­ indsondes, land stations, satellite imagery, and radar imagery. An example is the huge collection of rawindsonde data at Colorado State University in support of the compositing and other work on tropical cyclone structure, motion, etc. However, very little of any of these data are readily acces­ sible in digital form off of the Internet. (Though, of course, most groups have provided data when requested.) One project begun at the NOAA Hurricane Research Division is to provide a surface wind analysis for past hurricanes combining various observations of widely varying heights and averaging times into a common analysis framework. Such a synthesis of data can be quite useful for many uses (general public, emergency managers, etc.), but should not be a substitute for also providing the original digital databases as well.

5. Contact: National Climatic Data Center, Federal Building, Asheville, NC 28801, USA. 704/271-4800, email [email protected].

28 Typhoon Committee reports, these disagreements can carry over into the best tracks and thus into climatology.

5.4.5. POSSffiLE solutions to ease the confusion: • Change all TC warning centers to a common wind standard. • . Better coordination and sharing of data between warning centers. • Better education of the public to the uncertainties of TC analysis and forecasting. This would include the need for those who are disseminating real-time information on the WWW to clearly identify themselves, and apply appropriate caveats or preambles to the information, so that users understand that there might exist a more "official" source of information. • A need for a central, Web site - perhaps based at the World Meteorological Organization - that would act as a reference page for providing the "official" TC advisories for a certain region. This Web site could also be used as a pointer toward tropical cyclone imagery, archive data, educational material, etc.

5.4.6. Archiving of Tropical Cyclone Observations, Analyses and Forecast Fields "Best Track" and "Real-time" Tropical Cyclone Data: A common format for tropical cyclone intensity and position information has been the "best track" databases, named for the p6st-analysis of available observations to create the most reliable intensity and position estimates. ·These after-the-fact assessments often can differ considerably from those made in "real-time" by tropical cyclone forecasters. Global "best track" databases are now easily accessible. For the Atlantic and Northeast Pacific basins, the data is available via the Web through the National Hur­ ricane Center at: . For the remaining basins, the data is available via a CD-ROM: Global Tropical and Extratropical Cyclone Climatic Atlas (GTECCA) 2.0 CD-ROM: This single volume CD-ROM contains global historic tropical storm track data for five tropical storm basins. Period of record vary for each basin, beginning as early as the 1870's, with 1995 as the latest year. Northern hemispheric extratropical storm track data are included from 1965 to 1995. Tropical track data includes time, position, storm stage (maximum wind, central pressure when available). The user has the capability to display trac;:ks, and to track data for any basin or user-selected geographic area. The user is also able to select storm tracks passing within a user-

27 hourly files with a 3-7 day storage time. Other useful information (such as aircraft reconnaissance data) are found elsewhere on this gopher server.4

5.4.4 Obstacles and Opportunities As operational advisories become available on the Internet, it is becoming increasingly easy for people to access warnings from agencies and organizations other than those that have official responsibility for the area. The potential for conflicting information to reach the commu­ nity then becomes quite real. As an example, strike probability information is available in real­ time from the University of Hawaii tropical cyclone page (at http://www.solar.ifa.hawaii.edu/ Tropical/StrikeProb.html) and TC warnings can be found at the above mentioned U.S. Joint Typhoon Warning Center site. Emergency service agencies in Australia are aware of these sites and have accessed them, potentially creating difficulties when the Australian Bureau of Meteorol­ ogy is trying to coordinate informq.tion with the emergency managers. Confusion in differences amongst warning centers comes from several sources:

• The difference in wind averaging-times: The U.S. warning centers use a 1-minute average wind, while most of the rest of the world uses a 10-minute average. This can cause differ­ ent intensity estimates between warning centers even if they agree on all other meteoro­ logical parameters (an unlikely scenario). • Analysis and forecast disagreements between warning centers: Each warriing center per­ forms their own TC analyses and forecasts, and these do not always agree with other cen­ ters. The reasons for this could include different interpretations of satellite imagery, different surface observations, different forecast models, or a differing understanding of the underlying meteorology. • A difference in TC status between warning centers caused by either or both of the above points. For example, it is not unusual in the Western Pacific to see one center warning on a depression where other centers have no cyclone, or one center calling a system a typhoon when others are still calling it a tropical storm. As shown in several WMO

4. An important note: WWW sites occasionally suffer from reliability problems. Thus, they are not reliable substitutes for product reception via the GTS.

26 (1) ~11 URLs start with "http://". Note that URLs are often subject to change without notice.

(2) Shows the site contain's either (a)rchive material of past storms or tropical cyclone (e)ducational material.

(3) Not all the Australian tropical cyclone advisories available over the U.S. Family of Services system appear on the web site.

A combination of Tables 1 and 2 shows some significant omissions. Most notably, the Indian Meteorological Department has no WWW presence. The regional TC warning centers at Honolulu, U.S.A., Fiji, Mauritius, and Reunion are also unrepresented. Additionally, several ser­ vices do not make their tropical cyclone products directly available via the Web. However, some of these products can be found on the WWW by other means.

5.4.3. The U.S. Family of Services The United States provides several meteorological data feeds to public, private, com­ mercial, and education interests under the collective title "Family of Services" (FOS). The ser­ vices include two feeds of text information containing many TC products. These include: • All TC products from the U.S. warning centers, civilian and . • The Japanese high seas/tropical cyclone product. • TC warnings from the Hong Kong . • TC warnings and outlooks from the Indian Meteorological Department. • TC warnings.from the Fiji Meteorological Service. • TC warnings from the Bureau of Meteorology, Australia. Several other TC products have appeared intermittently on the FOS. Their current sta­ tus is uncertain. These include: • TC w,amings from the Philippines (PAGASA). • TC warnings form Reunion • TC warnings from Mauritius. Many web sites use FOS data. Currently, the best TC text data site is the hurricane sec­ tion of the Florida State University Meteorology Gopher (URL gopher://metlab 1.met.fsu.edu:70/ 11/weather/HURR). TC warnings and other tropical meteerology information are sorted into

25 5.4.2. National Weather Services from TC-affected regions on the WWW Table 4 gives a list of known web sites of other national weather services with interests in or in regions affected by tropical cyclones, along with a breakdown of available information.

Table 4: Known web sites of national weather services in TC-affected areas or with TC interests

TC Archive/ Real-time Satellite/ Country URL (1) Education TC data Radar (2) Imagery

Australia, Bureau of Meteorol- www.bom.gov.au Some (3) yes Yes ogy (e)

China, Chinese Meteorological www.cma.go.cn ? ? ? Agency

France, Meteo France www.meteo.fr No No ?

Indonesia, Meteorological arid www.cbn.neLid/ No Yes ? Geophysical Agency commerce/bmg/englishlbmgin- dex.htm

Japan, Japanese Meteorologi- www.goin.nasda.go.jp/ No No Some cal Agency GOIN/JMA (e)

Macau, Meteorological www.smg.ctm.net No No No & Geophysical Services

Malaysia, Malaysian Meteoro- www.kjc.gov.my Some No Yes logical Service

Mexico, Servicio Meteorolog- smn.cna.gob.mx/ Yes Yes Yes ico N acional SMN.html (a)

New Zealand, Meeorologifal www.met.co.nz ? Yes Some Service of New Zealand, Ltd. (a)

Philippines, PAGASA max.ph.net/ -pagasa/ ? No No pagimg.htm

Singapore, Meteorological ww.gov.sg/metsin/ Yes Yes No Service

South , Korean Meteoro- 203.247.66.42/index.htm No Yes No logical Admin.

Taiwan, Central Weather www.cwb.gov.tw Yes Yes ? Bureau

United Kingdom www.met-office.gov.uk No No Yes (a)

24 vices were often preferred to general services. This was probably an underlying factor explaining the development of tropical cyclone warning services as specialist services often targeted at clearly-defined geographic areas.

5.4 Information Issues

5.4.1. Availability of Tropical Cyclone Advisories Table 3 shows tropical cyclone warning center web sites as of January 1998. As there are only five entries in the table, it is apparent that most WMO tropical cyclone warning centers are not yet connected to the World Wide Web.

Table 3: Known tropical cyclone warning center web sites

Archive/ Graphic Satellite/ URL Text Educa- Center Products Radar (1) Products tion (2) Imagery (3)

Canadian (4) Hurricane www.ns.ec.gc.ca/ Yes No Yes Yes Center weather/hurricane (a, e)

Hong Kong Observatory, www.info.gov.hk/hko/ Yes ? Yes Yes China enter30.htm (a, e)

Joint Typhoon Warning www.npmocw.navy.inil Yes Some Yes Some Center, USA (a)

National Hurricane Center, www.nhc.noaa.gov Yes Some No Yes USA (e)

Naval Pacific MeteoroJogy www.npmoc.navy.mil Yes Some ·Yes No and Oceanography Center, USA

(1) All URLs start with "http://". Note that URLs are often subject to change without notice.

(2) Graphics products include graphical versions of track forecasts, strike probabilities, or similar information. It does not include satellite or radar imagery.

(3) Shows the site contains either (a)rchive material of past storms or tropical cyclone (e)duca­ tional material.

(4) It is not known whether this is an official WMO tropical cyclone warning center.

23 ditions for people in potential disaster-prone areas through the provision of timely, reliable and accurate information for households and national disaster and emergency authorities. Tropical cyclone warning services are used as inputs in production processes by firms in industries such as mining, tourism, , aviation and offshore oil drilling. These services are also used by the general public to provide personal safety information about the timing and possible physical impacts of tropical cyclones and the related precaution needed to be undertaken to reduce their vulnerabilities to impacts. Coates ( 1996) provided information indicating the con­ siderable decline of human fatalities due to tropical cyclones in Australia over the last 100 years partly due to improved warning services. The costs of running publicly-funded tropical cyclone warning services are often well known, sometimes published in relevant agency annual reports. However because such services tend to be non-market goods carrying no direct price, their bene­ fits have been derived only indirectly using methods such as the contingent valuation procedure (Anaman and Lellyett, 1996). The valuation of benefits of tropical warning services is underpinned by two important economic concepts: indivisibility and non-exclusiveness (Randall, 1981). A good is indivisible (or is considered non-rival in consumption) if its use by one individual does not reduce its avail­ ability in terms of the quality and/or quantity of the service to others. A good is non-exclusive if individuals cannot be excluded or it is very difficult to exclude others from using the good or it is considered not desirable by society (possibly through its elected leaders) to exclude anybody from using the good. Tropical cyclone warning services are indivisible and are generally considered to be non-exclusive because Governments often deem it necessary to provide such information to the . . general public for safety reasons and also to minimize public disaster payments in the event of the events (Anaman et al., 1995). This explains why they are often provided and financed by Govern­ ments using public funds. Like other information products and services, the quality of tropical cyclone warning ser­ vices can be evaluated based on desirable attributes required for public or private decision mak­ ing. These quality attributes include accuracy, timeliness, convenience and ease of understanding. Based on surveys of users in Australia, Anaman et al. (1998) provides a comparison of the quality attributes of three types of meteorological services, including the public weather services used by the mining industry which includes tropical cyclone wami~g information. The ease of under­ standing of information was consistently ranked the highest valued attribute while specialist ser-

22 an appropriate manner, as misuse of information might actually increase vulnerability. Because appropriate use of information is not always clear, research on this topic is needed as well.

5.3.5. Case Study: Use of tropical cyclone warnings in China China has a great need for typhoon forecasts and warnings. While a tropical cyclone is approaching China, social response includes the following three steps: When a tropical cyclone warning is issued (i.e., a typhoon may land on China 72h later), ships and oil-drilling platforms are informed to prepare to return. Sea dikes and river water-levels are checked. Flood water diver­ sion schemes are drafted. When a typhoon warning is issued (typhoon may land on China 48h later), the water level of certain rivers is decreased to a controlled level in order to receive poten­ tial floodwaters. Emergency squads are organized. Emergency materials and finances are pre­ pared. Population in the area that may be inundated will be informed to prepare for evacuation. When an urgent warning is issued (typhoon may land on China 24h later), emergency staff and

. materials are dispatched to the are~ that will be affected by the approaching tropical cyclone. Weak parts of the dikes and dams will be reinforced. Movable properties, people in the low-lying areas are evacuated. In densely populated areas, horizontal evacuation is very difficult. It is possi­ ble to carry out vertical evacuation through building local shelters, which can significantly reduce evacuation time. For example, in July, 1986, typhoon Peggy (8607) landed on Guangdong, China. With an accurate forecast 72h in advance, thousands of fishing boats were called back. As a result, no fisherman was killed by the storm. Dikes and dams were reinforced, which allowed 35 large and middle-sized reservoirs within the affected area to survive the storm, even though it results in more than 1000 mm rainfall. Large amount of materials were removed. People were evacuated and, as a consequence, the injury and death rate was decreased significantly. It was estimated that an economic loss of more than 1000 million RMB were avoided in Guangdong Province alone.

5.3.6. Benefits of tropical cyclone warning services As the China case study suggests, given the enormous amount of damage that tropical cyclones impose on human societies, it is not surprising that there has been growing interest in the measurement of their impacts and related costs and benefits of tropical cyclone warning services (Anaman et al., 1995). Tropical cyclone warning services <;:an dramatically change the living con-

21 variability on annual and decadal time scales. Consequently, it is important that scientific knowl­ edge be connected to specific processes of decision in a form that is useful to decision makers. A central question underlying decisions such as those faced by the Florida Task Force is the following: What if the past is not a reliable guide to the future? It has been frequently observed that the societal dimensions (e.g., demographics) of tropical cyclone impacts are under­ going constant change. It has also been appreciated that the climatological dimensions of tropical cyclone impacts may undergo significant variability on annual and decadal time scales (and per­ haps even longer scales related to changes in climate) (Diaz and Pulwarty 1997). In recent years scientists have begun to demonstrate some skill in interannual climate forecasts. The seasonal tropical cyclone forecasts issued by the team led by William Gray are the most well known. Related research has focused on extreme events related to El Nino (i.e., ) (e.g., Gray, 1990). such research may prove useful to decisions based on expectations of future tropical cylone risk. Improved understanding of climate variability and ability to anticipate changes in event frequency, magnitude, and location has two important implications for efforts to better undersand tropical cyclone impacts. First, variability on annual and decadal time scales means that data available on the climatology of hurricanes may not be a reliable guide to the immediate future. Thus, there is a need to support research that extends the historical record as far back as possible using innovative methods from disciplines normally not associated with tropical cyclones (e.g., , , etc.). In addition, an appreciation of climatological variability ought to instill a degree of caution in those who use models based on historical storm events to assess future soci­ etal risk to tropical cyclones. The presence of forecasts of tropical cyclone activity provides risks and opportunities for decision makers. They present opportunities for decision makers to alter their decision routines in such a fashion as to incorporate new information into their plans and policies. The forecasts also present risks because they are largely untested and the costs of an improperly used prediction can be very large. By themselves, improved long-term forecasts of tropical cyclone activity will not neces­ sarily lead to better decisions and reduced vulnerability. Decision makers need to incorporate new information into their decision processes and perhaps even reconsider the structure of those processes themselves (e.g., long-term forecast information ~ay allow for decisions that in the past were not available). Reduced vulnerability will largely be a function of using new information in

20 5.3.3. Early warnings Early warnings (beyond 72 hours out to 120 hours) are a very important, but also a very complex issue. Medium range forecasts have increase their effectiveness in the last few years. Nowadays, a prognostic chart for 5 days in advance has as much quality as 72 hour forecasts had 10 years ago. This encouraging fact leads to the idea of issuing early warnings for tropical cyclones. However, these early warnings have to be taken in a different way than regular warn­ ings. They should be issued only for governmental and Civil Defense use, as an approximate idea

of which will b~ the tendency of tropical cyclone movement for the 72 to 120 hour period, and not as an exact track_forecast. The lack of accuracy and uncertainty of these medium-range forecasts has to be very clear to users. But early warnings may still be very useful and helpful, as they increase the feeling of danger and focus the attention of persons who have to take important deci­ sions, some which require significant lead time. A good example of this was the case of hurricane "Lili" in . This hurricane crossed the central portion of the Island on October 18, 1996. Four days earlier, on the 14th, an early warning was issued for Governmental authorities and the Civil Defense. This early warning stated that a tropical cyclone was forming in the Gulf of Honduras with great possibilities of becoming a tropical storm or hurricane during the following 3-4 days, development that could be very dangerous for western and central Cuba. Early warnings were also issued during riext two days, updating the situation. Then regular warnings began to be issued publicly,as usual, 24 hour before the hurricane made landfall. By this time of regular warning issuances, everything was prepared and only the very last protection measures, such as evacua­ tion, had to be done, but everything was prepared for that and all officials were aw;:tre of the neces­ sity of protection. The result was that no one died and material losses were largely minimized.

5.3.4. Seasonal (and longer-term) Warnings Many decisions related to topical cyclone impacts are explicitly or implicitly based upon somo notion of what the future will hold. For example, a recent study of the costs and benefits of revisions to the South Florida Building Code is based upon hurricane incidence of 1960-1995 (Englehardt and Pend 1996). The Task Force created by the Florida legislature to look at hurri­ cane catastrophe insurance assumed a "300-year storm damage maximum of US$76 billion" (ATFHCI 1995, 43). Decisions based on assumptions such as these may be sensitive to improved knowledge of both the historical record of tropical cyclones and of probabilistic projections of

19 ment community, mainly with respect to the different levels of evacuations along the coast, are closely tied to the intensity forecasts.

5~------

0~--~---.----,---,----r------~ 90 91 92 93 94 95 96 97 year

Fig. 7. Atlantic basin average of the absoh.1te NHC official intensity forecast errors for the 1990-1997 period.

200 190 LINOO 190 Offical 170 0SIIT Forecast ._,.., • SliT .... 160 + Sll T • 3H 08.1 T-NJM • 72 Hour ..):, 1:50 - "lAC: Sf'C 24 Hour V * AAC:f'LT LVL 1'10 OAC:DUI< P-+W Q 130 X SURF'A CE: OB w 120 w Q..·110 0 en 100 0 !l Q 90 * - z 80 ..... '::! 70 •• 60 •

~0 • 40 30 OD 20 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 9 9 10 11 12 13 14 1, 16 17 DATE-TIME

Fig. 8. Preliminary best track intensity for indicated by a solid line. Stars and circles denote the 24-h and 72-h official NHC intensity forecast, respectively.

18 state intensity or weakening, is determined by the location of tl:ie low-level cloud circulation cen­ ter with respect to the deep convection. Eyewall replacement cycles are commonly associated with intensity fluctuations (as indicated by Willoughby et al. 1982). It is difficult to detect such small-scale eyewall cycles unless there is nearly constant radar or aircraft surveillance. Moreover, the time scale of these cycles appears to vary from one hurricane to another. For example, the time period in between eyewall replacements during Hurricane Alien in 1980 was a day or two, while in Hurricane Andrew in 1992, it was less than 12 hours. There are numerous models available to forecast the track of tropical cyclones. However, . only a few models are operationally available to address the intensity" problem. In addition to SHIFOR, SHIPS (Statistical Hurricane Intensity Prediction Scheme) model predicts intensity up to 72 hours and uses climatology and persistence as well. It also includes predictors such as the SST, the vertical shear of the horizontal wind, the 200-mb eddy flux convergence of relative angu­ lar momentum and upper-tropospheric temperature. Details of this model can be found in DeMaria and Kaplan, 1994. The Geophysical Fluid Dynamic Laboratory (GFDL) models are also used as guidance to predict changes in TC intensity associated. In most cases, the initial state of the models do not correctly represent the tropical cyclone. The NHC 24- and 72-hour official forecast intensity errors for the Atlantic basin since 1990 are shown in Fig. 7. An inspection of the figure clearly shows that no significant improve­ ment has been made during that period· and, in general, the average errors are about 10 and 20 knots for 24- and 72-hour periods, respectively. These have shown, however, little to no skill above SHIFOR. Overall, these average errors are relatively small and indeedthey are. Thus, intensity forecasting of hurricanes has shown no skill. However, the real challenge is to forecast · those few cases of rapid intensification or decay, particularly if they occur just prior to or at land­ fall. Figure 8 shows the 24- and 72-hour intensity forecasts and the best track intensities for eastern Pacific Hurricane Linda during September 1997. Note that none of the official forecasts captured the rapid intensification of the hurricane which occurred between 1200 UTC 11 Septem­ ber and 0000 UTC 12 September. In fact, the 72-hour intensity forecast valid for 1800 UTC 12 September was underestimated by 100 knots. There are numerous examples of tropical cyclones which exhibit rapid changes in intensity. Comm~nly, these cases are difficult, if not impossible, to predict with present knowledge and tools. Actions taken by the emergency mange-

17 Fig. 6. Example of the area of warning is selected using 24-hour average track forecast errors.

Intensity problems: The forecaster is required to determine the initial TC maximum inten­ sity, which for the United States is defined as the .maximum 1-min sustained wind at the 10-m level. (For much of the rest of the world, it is a 10-min maximum wind standard.) In spite of advanced satellites, aircraft, and other new technology like Global Positioning System (GPS) dropwindsondes, the measure of the initial intensity is highly uncertain.2 The problem of forecasting tropical cyclone intensity change is even more difficult than determining the initial intensity. There are only a few parameters which the forecaster uses to determine the potential for intensity changes. Most common are sea surface temperature (SST), variation in surface pressures, and vertical shear of the horizontal wind. However, many of these parameters are, in general, difficult to measure due to the lack of observations.3 Forecasters can also infer short-~ange changes in intensity from the cloud pattern observed· in satellite images or radar. For example, the shear patterns, which is a general sign of steady-

2. Powell et al. (1996) have worked extensively on the surface wind structure associated with TC.

3. Upper-tropospheric trough interactions, which can be traced back to Riehl (1954), are features which are also taken into consideration during the preparation of the intensity forecast. This theory is once again becoming rejuvenated within the hurricane community (see MC?linari and Vollaro, 1989). The problem here is that it is hard to determine subjectively and operationally when the trough interaction will produce a favorable or unfavorable environment for the tropical cyclone to intensify.

16 5.3.2 Roadblocks: Track and intensity

Track problems: Figure 5 show~ the errors in the initial position, 12, 24, and 72-hour fore­ casts issued by the National Hurricane Center over the last few decades. The largest improvement is associated with the 72-hour forecast, for which the mean error has been reduced from 400 to 300 nautical miles. The improvement in the 72-h forecast is probably a result of advances in glo­ bal models. 500

197D-74 400 1980-84

"ii) 1975-79 .E~ 300 1985-89 ili 1990-94 CJ s 1995 ..sttl .... 200 1996 0 t: UJ 100

0 12 24 36 48 72 Forecast Period (hours) . Fig. 5. NHC Atlantic track forecast errors

Because the average 24-h forecast error is roughly 100 n. mi. and considering the area of hurricane force winds, the length of the coastline selected for hurricane warning is near 300 n mi. Figure 6 illustrates how the area of warning is selected, using the historical 24-h errors. The area of warning could be extended considerably, for example in the cases of a TC moving parallel to the coast or for larger tropical cyclones. Those cases of land-falling hurricanes with 24-h forecast errors much larger than the aver­ age could be potentially dangerous because a portion of the coast would have only a short time to respond. An ambitious goal of the NHC is to have all 24-h errors smaller than 200 n mi. This could tend to a reduction of the area of warning and actions could be concentrated within that smaller region.

15 Table 2: Major Factors Covarying with Warning Response Direction: As factor Level of Empirical Factor increases response ... Support Physical cues Increases High Social cues Increases High Perceived risk Increases Moderate Knowledge of haiard Increases High Experience Mixed High Education Increases High Family Plan Increases Low Fatalistic beliefs Decreases Low Resource level Increases Moderate Family united Increases High Family size Increases· Moderate Kin relations (number) Increases High Community involvement Increases High Ethnic group member Decreases High Age Mixed High Socioeconomic status Increases High Gender (female) Increases Moderate Having children Increases Moderate Channel: Electronic Mixed Low Channel: Media Mixed Low Channel: Siren Decreases Low Personal contact Increases High Proximity to threat Increases Low Message specificity Increases High Number of channels Increases Low Frequency Increases High Message consistency Increases High Message certainty Increases High Officialness of source Increases High Fear of looting Decreases Moderate Time to impact Decreases Moderate Source familiarity Increases High

14 Fig. 4. Example of forecast track guidance

Warnings in Content: Of course, a warning is of little value if it not used effectively by decision makers. Social scientists have developed a large body of research which, in varying degrees, has demonstrated which factors are associated with warning response. Table 2 illustrates many of these factors, the direction of influence each has on warning response, as well as the level of empirical support for the covariance (source: Sorensen 1993). The table suggests, quite accu­ rately, that the warning process is a complex one.

13 Hurricane Canny 1 030 UTC, 18 July '97 Max. 1-min sustained suHace winds (kt) for marine exposure Analyail based on AFRES C-130 Recon dill& 1rom 975 mb adj\Jiillld to tile sfc 1rom 0200 • 0830: shlps, buoys arid c-mans from 0300 • 0900 0811 UTC poarticn exrrapctBit!ld 1mm 0811 fix USU10 050 dt!JQ at -4 let

Fig. 3. Example of the wind analysis received at the NHC during TC events.

These above parameters are then the input for numerical track prediction models. An ensemble constructed from the output of all different models is then used to provide guidance for the official track forecast which, in general, tends to be within the swath as illustrated in Fig. 4. The official intensity forecast (with the exception of a few subjective rules described below) is prepared based on extrapolation or using model guidance, which is based on climatology and per­ sistence. NAWIPS (National Centers Advanced Weather Interactive Processing System) has pro­ :vided a very efficient way to display the output of all available numerical models. This has helped the forecaster tremendously in assessing, in a short amount of time, the various scenarios from different models which could impact the forecast.

12 inland and submerging two islands. 300,000 people were killed. This typhoon-induced storm surge resulted in the largest loss of life due to a tropical cyclone this century.

5.3 Warning Process

5.3.1 The operational forecast process at the U.S. National Hurricane Center The Hurricane Specialist at the National Hurricane Center (NHC) issues track, intensity, and surface wind structure forecasts which extend up to 72 hours. The package is released every six hours and includes forecast and public advisories, a tropical cyclone discussion and strike probabilities.1 The forecast process begins by locating the center of the tropical cyclone and estimating its initial motion and current intensity. In most of the world's basins, the location and the intensity of tropical cyclones are estimated from satellite using the Dvorak ( 1984) technique. This method is primarily based on cloud pattern recognition and is highly subjective. Such estimates are received at the NHC operationally from various independent meteorological agencies (e.g., Air Force Global Weather Center). However, when tropical cyclones are a threat to land in the Atlan­ tic basin, data from reconnaissance missions become available and become the primary and most reliable information used to determine the location and the intensity of the tropical cyclone. In addition, meteorological analysis of surface and upper-air data is a common practice. It is also required to determine the initial distribution of the surface wind field (e.g., the 34-, 50- and 64-kt wind radii). This paramter is difficult to estimate over data-sparse areas and is even much more difficult, if not impossible, to forecast with the present technology. The wind distribution is very important for decision-making officials, because preparation to save life and property is expected to be completed before the onset of the 34-kt winds. To estimate the initial structure, a state-of-the-art wind analysis provided operationally by the Hurricane Research Divi­ sion (HRD) as shown in Fig. 3 is commonly used. This produce integrates wind data from all the observing platforms. For details, see Powell et al. (1996).

1. A detailed summary of the different issuances is included in the Hurricane Operational Plan, WMO Report TCP-30.

11 age, in China each tropical cyclone of typhoon strength results in losses of more than US$500 million. Disastrous events caused by tropical cyclones in China and other Asian countries are mainly due to strong winds, torrential rain and storm surges. Some tropical cyclones spawn torna­ does. Strong winds and around tropical cyclones' core region may destroy all houses, build­ ings, telecommunications, electric systems in a certain area. Its damages and casualties are equivalent to those of an . A typhoon at sea can also cause serious impacts to ocean­ going vessels and island communities. For instance, hundreds of fishing boats and off-shore oil­ drilling platforms are often thre~tened by tropical cyclone winds in the South China Sea (SCS). For example, on August 7, 1988, Typhoon Bill (8807) landed on Zhejiang province, China, with a maximum wind of 35m/s. In Hangzhou city, the capital of Zhejiang province, 80% of the power syste~s were destroyed, which caused a power failure of the entire city. About 100 factories stopped work. The beautiful west lake scenery was destroyed seriously in the twinkling of the eye. The direct economic loss amounted to 400 million yuan. Tropical cyclone-induced torrential rain can also cause severe catastrophe. Damages caused by floods could be more serious than that caused by wind. Flash floods caused by tropical cyclones inundate large areas of land, leaving fields waterlogged and covered with a large amount of , wash out crops and railways, and destroy transportation and communications. For example, Typhoon Nina (7503) stagnated in province in Aug., 1975 and caused extremely severe heavy rain and flood disaster in Henan, which shocked the country. Heavy rain covered about 1/3 of the whole Henan Province. The lh, 3h, 6h (830mm, which broke world 6h rainfall), 12h, 24h (1061.8mm) and 3 day (1605m) precipitation amounts were all the highest records in the history of mainland China. Two huge reservoirs and more than ten small reservoirs collapsed at almost the same time. A of more than ten meters flowed downstream. Fields were inundated, the main railway from Beijing to Guangzhou was stopped for 18 days. Tens of thousands of people lost their lives. The direct economic losses amounted to 10 billion yuan. Storm surges can also have large societal impacts. A storm surge can wash out sea dikes and coastal development. For example, in 1970, a severe tropical cyclone landed on the and caused a rare strong storm surge. Sea dikes were broken with sea water washing

10 • A standardized database on the cost of extreme events would be extremely useful

• Improved techniques are required for applying knowledge of actual events to hypothetical situations to evaluate risks (i.e., simulation).

Each of these recommendations points to a need to better understand the impacts of extreme events. More specifically, in the context of tropical cyclones, these recommendations point to a need to address interrelation of physical and societal dimensions of impacts. Such knowledge of impacts is important from the standpoint of decision makers who face policy deci­ sions over short (single event) and long (annual or decadal) time periods and over local; national, regional, and global scales. Multidisciplinary Research: Overall, there is a need to promote research which in addi­ tion to examining the direct physical and economic impacts of tropical cyclones, looks at the com­ plex social and political issues affecting relationships between the many networks mating up 'at · risk' communities.

5.2. Societal Impacts: A Chinese Case Study

5.2.1. Social disruption, caused by tropical cyclones Statistics published by the (Shah 1983; Smith 1989) have shown that the death toll caused by tropical cyclones during 1947-1980 was almost 500,000 people, which ranks first among disasters. With the improvement of monitoring techniques and enhanced capability in preparedness and combat natural disasters, there has a significant decrease in the death toll caused by tropical cyclones. However, with the development of coastal industry and agriculture and enor­ mous growth in the population, more buildings and other structures are exposed to tropical cyclone damage, which is making societal disruption more and more serious. Damage estimates collected by the Asian and Pacific Typhoon Committee of the WMO have shown economic loss caused by tropical cyclone exceeds US$4 billion annually to the countries along the rim of .the western North Pacific. For example, China is one of the countries seriously affected by tropical cyclones. About seven typhoons make landfall in China annually. From 1990-1996 tropical cyclones affected an average of more than 4.6 million hectares, more than 700 people were killed, more than 480,000 houses collapsed, and the direct economic loss was beyond US$36 billion. On aver-

9 Parallelisms · In cases where data on or experience with extreme events is lacking in a particular region, knowledge of a similar place or time might substitute. As Glantz ( 1988, 2) has suggested, "in order to know how well society might prepare itself for a future change in climate (the characteristics of which we do not yet know), we must identify how well society today can cope with climate variability and environmental impacts." This approach is also called "forecasting by analogy."

5.1.3 Issues Estimation and Validation: Even in the immediate aftermath of Andrew many estimates of damages were off by significant amounts (Noonan 1993). West and Lenze (1994, 145) find that "at present, most evaluation in regional impact analysis is confined to the fairly simple and non­ rigorous step of asking whether the results look 'reasonable'". There is clearly a need for further research in the area of impacts assessment. Trends: Normalized data indicate clearly that the United States has been fortunate in recent decades with regard to storm losses. Caution must be urged in interpreting statements regarding the increasing number of "billion-dollar losses" (e.g., Flavin 1994), because it may be that increases in hurricane losses have resulted largely from changes in society, not in climate fluctuations. A greater understanding of the vulnerability of all at risk communities is required. Insurance: The role of the insurance industry is always under scrutiny. In the aftermath of Hurricane Andrew, rates for property insurance rose considerably in Florida and the Caribbean area (Tomlin, 1994 ). Such increases have resulted in many being underinsured or uninsured. The · potential for interaction between the insurance industry and regulatory bodies, in providing differ­ ential premium rates for individual enterprises that use better designs and construction codes, must be explored. The 1995 Assessment of the Intergovernmental Panel on Climate Change, in its chapter on financial services, made a number of recommendations in the areas of property insurance and related activities, reproduced below (Dlugolecki et al 1996, 555).

• Better information is required on the past and future location, severity, and frequency of extreme events, especially storms and precipitation.

• The areas most vulnerable to extreme events need to be identified.

• Better information is required on the damage-response curve for the various hazards.

8 risk of tropical cyclone impacts on people and property (Dlugolecki et al. 1996). The first three are based upon the documented historical record of tropical cyclone impacts.

Loss record Bases loss estimates on experience, but its weakness is that "experience" is a relatively short record and may not be representative (cf. Wamsted 1993). It may not even be representative of climatological trends. Stage damage The people or property subject to risk is inventoried based on a number of key dimensions (e.g., number, type, and location of structures) and then, based on the inventory, a computer model is created to estimate losses from a particular event's impact. Simulation Modeling is used here as well, but the focus is not on a particular event but instead on a family of events and the corresponding fryquency and magnitude distribution of impacts. Companies such as Applied Insur­ ance Research and EQE International run these sorts of models (Banham 1993).

The stage damage and simulation models are only as good as the assumptions which underlie them. For instance, prior to Hurricane An drew, models such as -these led hurricane loss experts to conclude that the worst-case scenario for a hurricane impact along the U.S. coast would be around $10 billion (e.g., Sheets 1992). Even in the immediate aftermath of An drew, many esti­ mates of damages were off by significant amounts (Noonan 1993). The primary reason that the model estimates were off for this particular event was a number of important factors not included in the models that only became apparent in the wake of the disaster. Current estimates indicate that if An drew had happened just 100 kms further north, the insured losses are estimated to have been US$60 billion. There are two additional methods of risk assessment that seek to utilize information beyond that available in the historical record (Dlugolecki et al. 1996).

Event prediction With reliable information of future events, decision makers can imple­ ment policies to take advantage of that knowledge. There is a large liter­ ature on the use of climatological information by decision makers in, for example, agriculture and utility industries (cf. Changnon et al. 1995), but very little (if any) in the area of tropical cyclones.

7 canes "have become increasingly frequent and severe over the last four decades as climatic condi­ tions have changed in the tropics." In fact, there has been a decrease in the frequency of intense tropical hurricanes (Landsea et al. 1996) but society has become more vulnerable (Pielke 1997). Interpreting the hurricane loss record is, therefore, fraught with difficulties and requires careful attention.

millions 1995$ 40,000 .-.------,

/ 1926 $$7 4.4 billion

30,000 -

20,000 -

10,000 -

0 l!) CXJ ---.::t ['-QC') (00) N LD CO .....- 0 (V) <.0 m.N L.DCO ..q- NN ('() (V) C')..q-...q- -.:;t-.::t l!) LD l!) <.0

Fig. 2. Time series of US hurricane-related losses (direct damages in millions of dollars) froms 1925 to 1995 in normalized 1995 damage amounts (utilizing inflation, coastal county population changes, and changes in wealth). From Pielke and Landsea, 1998.

A recent study by the US based Insurance Information Centre estimated that a hurricane costing between US$50 billion and US$1 00 billion could result in over one third of all US based insurers failing. There are at least five different methods that decision makers utilize to assess the

6 5.1.2 Valuation of impacts of tropical cyclones Consider the case of the United States where losses have been quickly increasing within the last two decades, even after considering inflation (Fig. 1). However, to best capture the year­ to-year variability in tropical cyclone damage, consideration must also be given toward two addi­ tional factors: coastal population changes, and changes in wealth. Both population and wealth have increased dramatically over the last several decades and act to enhance the recent hurricane damages preferentially over those occurring previously. More appropriate trends in the United States hurricane damages can be calculated when a normalization of the damages are done to take into account inflation and changes in coastal population and wealth (Pielke and Landsea 1998).

10 30

a Deaths Damage 25 (1990 Dollars)

20 ~6 1:: Ill ~ 15 15' "'::J :::1 0 Ul ~4 10

2 5

0 0 1900 10 20 30 40 50 60 70 80 90 1900 10 20 30 40 50 60 70 80 90 Fig. 1. Deaths and damage from hurricanes

With this normalization, the trend of increasing damage amounts in recent decades disap­ pears (Fig. 1). Instead, substantial multi-decadal variations in normalized damages are observed: the and 1980s actually incurred less damage than in the preceding few decades. Only dur­ ing the early 1990s does damage approach the high level of impact seen back in the 1940s through the 1960s, showing that what has been observed recently is not unprecedented. Over the long term, the average annual impact of damages in the continental United States is about $4.8 billion (1995 $)--substantially more than previous estimates. Of these damages, over 83% are accounted for by the intense hurricanes (Saffir-Simpson 3, 4, and 5), yet these make up only 21% of the U.S.-landfalling tropical cyclones. Understanding trends in impacts is important, but it is not always a straightforward task. Consider a 1995 U.S. Senate report (BTFFDR 1995, 23), which mistakenly asserted that hurri-

5 imposed by a tropical cyclone on individual entities - either businesses or households. For soci­ etal economic analysis, the analysis is done for all members of a defined society affected by a tropical cyclone. The society needs to be geographically defined as a city, region or a nation state. Market values are sometimes adjusted to reflect economic scarcity of resources due to market imperfections (including government-based distortions) (Abelson, 1996). Intangible impacts of tropical cyclones include human deaths or fatalities and anxiety, stress and induced ill-health. Losses of human lives are difficult to value in monetary terms. There is however a literature developed by economists that places monetary value on human lives (Field, 1994; Dwyer, 1986). Although a life is often considered "priceless," a group or society through its revealed actions is often prepared to pay only a finite amount of money to reduce by a certain amount an individual's risk of death. Therefore a "contingent valuation" method based on individuals' willingness to pay to take measures to reduce probability of death could be used to put a value on life based on the concept of "statistical life." A contingent valuation study might involve describing the environmental improvement being evaluated (such as advance warning meteorological information) to reduce the probability of death by a certain amount; identifying and selecting respondents (especially from the target group at risk from the event) using scientific sampling procedures; designing and applying a survey questionnaire through interviews; analyz­ ing the results statistically and aggregating individual responses to represent a value for the group (Field, 1994, Dixon et. al., 1995). Anxiety and stress-related sickness may be valued by the increase in health-related costs from the cyclone. Regardless of whether an individual economic unit or the whole society is be{ng considered, impacts of tropical cyclones are measured as the dif­ ference between the occurrence of the event and what would have been expected to occur without the occurrence of the event. Research often focuses on impacts of cyclones in developed nations, since the absolute values of damages are very large. But it is often the case that impacts in less developed, or smaller countries can have a proportionally much greater impact on a regional or national economy. In cases where cyclones strike small island nations, for example, in the South Pacific or in the Carib­ bean, where the economies are based largely on tourism anclior agriculture, the magnitiude of the damages may be small compared to the impact of Hurricane Andrew, but the impact on the nation's economy can be devastating.

4 An important indirect impact is what is normally called in economic jargon, secondary economic impact on a regional or local economy. A secondary economic impact of a tropical cyclone affects a local or regional economy beyond the economic sectors initially affected by the event. These effects occur on different sectors of the economy over various periods of time and can often be quantified and measured. Sometimes the secondary impact of a tropical cyclone on a regional economy can actually be positive even though its effects on the whole national economy are negative. For example, Chang (1983) demonstrated that Hurricane Frederick caused about 1.6 billion dollars of property damage and other direct losses in the State of in the United States. However this hurricane also led to the declaration of the directly-affected region as disaster area by the Federal Government. This brought an influx of about $670 million in Govern­ ment and private sector recovery funds and resulted in a net $2.5 million increase in the municipal funds for the city of Mobile within 12 months. Secondary economic impact is estimated using input-output analysis or variations of this technique such as social accounting matrix or comput­ able general equilibrium analysis (Jensen and West, 1986; West, 1993). Impacts of tropical cyclones can also be classified as tangible or intangible .. Tangible impacts are those which can be easily measured in monetary terms. Tangible benefits or positive impacts include, for example, the increased rainfall in inland areas which can be measured. Tan­ gible losses are also easily quantifiable, for example, damages to buildings from floods related to the cyclones. Intangible impacts are often difficult to measure in monetary terms because they are ·not purchased or sold in markets and hence direct market values may not exist. Intangible losses include anxiety and fear of future severe events, inconvenience and stress-induced illness. Intan­ gible benefits or positive impacts of tropical cyclones include improved preparedness by the pop­ ulation for future severe events. Impacts can also be classified as short-term or long term depending on the duration of the after-effects. Once the effects of a tropical cyclone are classified as tangible or intangible, it allows for a more systematic valuation of the impacts on society. Valuation also requires the separation of the impacts on society in general from those impacts on an individual, household or business. The distinction is necessary to determine financial losses relevant to individuals and societal losses attributed to society as a whole. The effects of a tropical cyclone on a single economic unit such

as a mining firm is dealt with via analysis in changes in ne~ income when financial losses are con­ sidered. For tangible impacts, market prices are often used to approximate changes in income

3 Impacts of tropical cyclones can be classified as either direct or indirect. Direct impacts arise from physical or direct contact of the phenomenon associated with tropical cyclones with people, their property and equipment and and . For example, tropical cyclones can cause :flooding leading to deaths of and/or destruction of property as occurred with in 1997 in Mexico. Indirect impacts of tropical cyclones are those induced by these direct events. Indirect impacts often occur away from the scene of the tropical cyclone landfall and typ­ ically after the occurrence of the event. Indirect impacts include permanent removal of people from an affected area, disruption to household and leisure activities, loss of basic community ser­ vices, stress-induced sickness and anxiety of future tropical cyclones (Handmer and Smith, 1992). Table 1 illustrates direct and indirect impacts associated with Hurricane Andrew's landfill in Flor­ ida in 1992 (Pielke and Pielke 1997).

Table 1: Damage Estimates in South Florida Associated with Hurricane Andrew. Current dollar estimates of $30 billion in damages directly related to Hurricane Andrew in South Florida. Original sources are located in Pielke and Pielke (1997).

Amount Type of Loss ($billions) Common insured private property 16.5 Uninsured homes 0.35 Federal Disaster Package 6.5 Public infrastructure State 0.050 County 0.287 City 0.060 Schools 1.0 Agriculture Damages 1.04 Lost Sales 0.48 Environment 2.124 Aircraft 0.02 Flood Claims 0.096 Red Cross 0.070 Defense Department 1.412 Total 30.0

2 IWTC-IV Report

Section 5.0: Societal Impacts

5.0 Introduction Tropical cyclones are frequent in tropical regions of the world and frequently result in large societal impacts, especially in Australia and the South Pacific, the United States, and the Indian subregion. For example, in 1997 along the Mexican coast Hurricane Pauline resulted in the death of over 400 people, destroyed many farms and businesses, caused hundreds of millions of dollars damage and displaced over 50,000 people (Economist Magazine, 18th-24th October 1997 edition). In the United States, Hurricane Andrew ( 1992) resulted in US$30 billion in losses, and the United States experiences more than US$5 billion in losses annually (Pielke and Landsea 1998). In Australia, tropical cyclones are one of the most economically significant natural disas­ ters responsible for the highest insurance payouts. The Insurance Council of Australia indicated that the average cost of tropical cyclone in terms of insurance payouts was about 75 million Aus­ tralian dollars (A$) over the 20-year period from 1970 to 1989. The total net costs of tropical cyclone to the public have been estimated to be about four times as large as insurance payouts suggesting that the average annual total costs could be about A$300 million (Joy 1991). In the Indian subregion, annual cyclones cause extensive damage to property and result in human casu­ alties including fatalities especially in and .

5.1 Economic Aspects

5.1.1 EconomiC classification of impacts from tropical cyclones An economic understanding of the impacts of tropical cyclones on human societies requires knowledge of the interrelation of the and the social system created by human beings. Impacts from tropical cyclones on human societies are often negative. But there are also positive impacts or benefits from tropical cyclones on some aspects of human soci­ eties. These benefits include increased rainfall to inland areas and the maintenance of the fertility of fioodplain soils due to periodic flooding associated with tropical cyclones (e.g., Blong, 1992; Ryan, 1993).

International Workshop on Tropical Cyclones IV

Report of Section 5.0

Societal Impacts

Lead Authors: Roger Pielke, Jr., Kendall McGuffie, Lixion Avila, and Chris Landsea

Contributing Authors: Kwabena Anaman, Jay Baker, J. Beven, Jeremy Collymore, J. Gill, J. Hawkins, Tang Huifang, Jerry Jarrell, Pierre-Yves Lemee, Betty Hearn Morrow, Jose Ru­ beria, David Sargeant, and Meng Zhiyong

Section 5.1: Economic Aspects Section 5.2: Societal Impacts Section 5.3: Warning Process Section 5.4: Information Issues

DRAFT of 22 January 1998

Prepared as a contribution to IWTC-N, World Meteorological Organization, Haikau, China, Aprill998

Report production: D. Jan Stewart and Jan Hopper, Environmental and Societal Impacts Group, National Center for Atmospheric Research

Table of Contents

TOPIC FIVE

TROPICAL CYCLONE IMPACTS

5.0 Report of the Topic Chairman R. Pielke (USA) K. McGuffie (Australia)

5.1 Economic Impacts Rapporteur: K. McGuffie (Australia)

5.2 Societallmpacts Rapporteur: R. Pielke (USA)

5.3 The Warning Processes Rapporteur: L. Avilia (USA)

5.4 Information Issues Rapporteur: C. Landsea (USA) · i) Chapter 9: Ready Reckoner The idea here was to provide a range of useful information in a readily accessible form. Comments from a number of people indicate that this is one of the more useful parts of the Guide and should be retained. A range of additional information that has become available should also be incorporated. For example the wealth of information available from the frequently asked questions Internet site maintained by Chris Landsea. c) Chapter 3: Tropical Cyclone Motion Considering the contents of this chapter in comparison to that on structure clearly reflects the greater sophistication of forecasting techniques for motion. Some of the details require updating, particularly with regard to new or revised methods that have been produced recently, and the views provided by the Systematic Approach. However, the overall chapter is in reasonably good shape.

d) Chapter 4: the Habitation Layer This chapter by Chester Jelesnianski focuses on the surface conditions in tropical cyclones and in particular on the processes occurring near landfall. As such it may be prudent to hold substantial revision until the World Weather Research Programme initiative on tropical cyclone landfall has matured.

e) Chapter five: Seasonal Forecasting Potential approaches to forecasting seasonal activity off tropical cyclones are summarised in this chapter. There is a strong concentration on the North Atlantic, arising from the extensive work done there by Bill Gray and his collaborators. Since the chapter was written there has been a significant increase in development of longer-range forecast techniques. The chapter would benefit from a revision to include these new techniques and to describe the accuracy of the more established ones.

f) Chapter 6: Operational Strategy Gary Foley contributed this chapter to provide an overall indication of good forecast office practice on a range of procedures from pre-season preparations through interactions with the media and general operational strategy. The forecast community at the workshop should provide advice on the usefulness of this chapter and any proposed revisions.

g) Chapter 7: Warning Strategies This chapter attempted to provide strategies for optimising the preparation, issue, understanding and response to tropical cyclone warnings. lt was derived substantially from the extensive experience of Bob Southern. Since that time there has been a major increase in the investigation of social and economic aspects of tropical cyclones, including: the Australian tropical cyclone coastal impacts project, the U S weather research programme hurricane landfall project, and the session ·on social and economic aspects at this workshop. These aspects should be incorporated into the revised chapter.

h) Chapter 8: Numerical Track Prediction Models. This chapter attempted to provide basic information on numerical model techniques together with an overview of available operational models. The operational modelling section, in particular, is now badly dated and in need of revision. There is also the question of whether a separate chapter on modelling should exist or whether the relevant components should be incorporated into the other chapters. For example track prediction modelling could appear in the revised chapter 3. 4.8.2 Next Edition of the Guide

The Guide is in a loose - leaf format to facilitate changes and additions. However, the reality is that the logistics of printing changes and distributing them to a, largely unknown, list of addresses effectively inhibits other than major structural changes.

We considered producing the first edition as a web page, but at that time Internet access by forecasting offices was still fairly limited. There is now widespread access to the Internet and we should consider producing the second edition of the Guide on the web. Advantages of a web edition are many, multi-media capacity and ease of update being just two. Local hard - copy also can to be made to provide ease of access in operational conditions. The disadvantages are that there is a substantial amount of work involved in both the web edition and in providing an ongoing maintenance. lt is recommended that consideration be given to producing the second edition of the Guide on the web.

Another significant advance that has occurred since publication of the Guide is the Systematic Approach developed at the Naval Post Graduate School. The workshop should review this approach and recommend on how it might be incorporated into the revised Guide. One way would be to completely restructure the Guide along the lines of the Systematic Approach. A second method would be to incorporate the Systematic Approach within the current structure of the Guide. This is recommended, as there ar many more parts to the Guide than are currently covered by the Systematic Approach.

4.8.3 Chapter by Chapter Assessment

A brief assessment of each chapter is provided in this section. No attempt is made to consider ways in which the Systematic Approach may be incorporated.

a) Chapter 1: Global Overview. This chapter contains an excellent coverage of the various regions of responsibility, naming conventions, problems with different measuring systems and units, observational practices, and an extensive climatology of tropical cyclones around the globe. Whilst there are some additional climatological characteristics that could be added, and there may be some minor changes to the observation practices, this chapter requires very little revision. b) Chapter 2: Tropical Cyclone Structure. In preparing this chapter, Bob Merrill provided a combination of known information and attempts to provide methods for interpretation and classification of poorly documented conditions, such as extratropical transformation. We seek the views of the forecasting community on the usefulness and degree of application of Bob's recommended methods of analysing and predicting in particular outer structure, extra tropical transition and rainfall. Significant revision of these sections would seem to be in order. WMO/CAS/ICSU

FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

Topic 4.8: Upgrade to the Forecasters Guide to Tropical Cyclone Forecasting

Rapporteur: G.J. Holland Bureau of Meteorology Research Centre PO Box 1289K Melbourne, Vie 3001, Australia

Email [email protected] Fax (Australia) +613 9669-4660

Working Group: R. Elsberry, F. DeiSol, K. Kuroiwa

4.8.1 Introduction

The Global Guide to Tropical Cyclone Forecasting was published in 1993 on information gathered from the first two international workshops on tropical cyclones. The goal was to a usable operational manual that could improve the capacity of forecasters to discriminate and objectively judge the best forecast approach a) by contributing logical methods for proceeding through analysis and forecasting; b) by providing insights into forecasting approaches; and c) by clearly indicating the limitations of forecast techniques. · The Guide was provided in a loose - leaf format for ease of use and to encourage insertion .of local information and forecast guidelines.

·The Guide contains nine chapters covering: 1. Global Overview (C. Neumann), 2. Tropical Cyclone Structure (R. Merrill), 3. Tropical Cyclone Motion (G. Holland), 4. The Habitation Layer (C. Jelesnianski), 5. Seasonal Forecasting (W. Gray), 6. Operational Strategy (G. Foley), 7. Warning Strategy (R. Southern), 8. Ready Reckoner (G. Holland), and 9. Bibliography. Each chapter was written by the author's listed in brackets under the overall editorial direction of G Holland. Approximately 30 others contributed as direct contributors, technical reviewers or those providing a perspective from individual ocean basins.

Since completion, the Guide has attracted favourable comment from a wide range of sources, Including Tropical Cyclone Programme meetings in the various Regional Associations. lt obviously fills a need and has a role in forecasting offices. However, even after a relatively short period of five years several of the sections are coming dated and are in need of revision. This brief rapporteurs report addresses the future of the Forecast Guide and suggests a change to an electronic medium. Future of the Guide will be debated during IWTC IV and decided at a meeting of a selected sub committee at the end of the workshop.

4.7 Rainfall Pred Teq - Xu Xiangde of National Workshop on Tropical Cyclones , Chinese Academy of Meteorological Sciences (in Chinese with English abstract), 225-229.

Internet Universal Resource Locators (URL's) http://www .hurricanehunters.com (aircraft reconnaissance) http://www.mmm.ucar.edu/pm/satellite/geo-coverage2.html (Geostationary sattelite coverage) http://www.pmel.noaa.gov/toga-tao Tropical Atmosphere Ocean (TAO) Array http://www.wmo.ch/hinsman/satsun.html WMO Satellite Activities http://www.goes.noaa.gov NOAA's Geostationary Satellite Browse Server http://trmm.gsfc.nasa.gov Global eye on tropical rainfall http://www.bom.gov.aulbmrc/meso/Project/Aerosonde/aerodev .html Aerosonde Information

Page 11 4.7 Rainfall Pred Teq- Xu Xiangde observational evidence. 22 nd AMS Conf. On Hurr. And Trop. Meteor., Ft Collins, CO, May, 1997.

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Velden, C.S.,: Can we observe the parameters and processes involved in TC intensity change adequately?. Special session on TC Intensity change at the AMS 1998 Annual Meeting, Phoenix, AZ, January 1998.

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Velden, C.S., C.M. Hayden, S. Nieman, W. P. Menzel, S. Wanzong and J. Goerss, 1997: Upper-tropospheric winds derived from geostationary satellite water vapor observations. Bull. Amer. Meteorological Society, 78, 173-195.

Velden, C.S., T. Olander, and R. Zehr, 1998: Development of an objective scheme to estimate tropical cyclone intensity from digital geostationary satellite infrared imagery. Submitted to Weather and Forecasting.

Velden, C.S., and T. Olander, 1998: Evaluation of an objective scheme to estimate tropcial cyclone intensity from digital geostationary satellite infrared imagery. To appear in issue of Weather and Forecasting.

Velden, C.S., T.L. Olander and S. Wanzong, 1998: The impact of multispectral GOES-8 wind information on Atlantic tropical cyclone track forecasts in 1995. Part 1: Dataset methodology, description and case analysis. To appear in Spring issue of .

Velden, C.S., and S.Wanzong: Application of multi-spectral data from GMS-5 to the analysis of western North Pacific typhoons. 8th AMS Conf. On Satellite Meteorology, Atlanta, GA, February, 1996.

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Zehr, R, and R. Phillips, 1997: Expert system for tropical cyclone intensity forecasts. Preprints, 22nd AMS Conf on Hurricanes and Tropical Meteorology , Fort Collins, CO, Amer. Meteor. Soc,

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Page 10 4.7 Rainfall Pred Teq- Xu Xiangde

Meteorology, Miami, FL, Amer. Meteor. Soc., 511-515.

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Page9 4.7 Rainfall Pred Teq- Xu Xiangde

Landsea, C. W., W. M. Gray, P. W. Mielke and K. 3. Berry, 1992: Long term variations of

Western Sahel monsoon rainfall and intense U.S.landfalling hurricanes. J. Climate, 5,

1528-1534.

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North Atlantic basin hurricane location. Mon. Wea. Rev., 125, 1780- 1791.

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Morrison, R. P., L. M. Leslie, and G. D. Hess, 1996: Development of tropical cyclone landfall strike probabilities using Monte Carlo simulations. Preprints, 11th Conf Num. Wea. Pred. , Norfolk, V A, Amer. Meteor. Soc., Boston MA 02108, 414-415.

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Holland, G.J., 1980: An analytic model of the wind and pressure profiles in hurricanes. Monthly Weather Review, 108, 1212-1218.

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Jarvinen, B. R., and C. J., Neumann, 1979: Statistical forecasts of tropical cyclone intensity (SHIFOR), NOAA Technical Memorandum NWS NHC-10, 22 pp.

Jones, W. L., J. Zec and L. Rice, 1997: NSCAT geophysical algorithm for inferring hurricane ocean surface winds. 22nd AMS Conf. on Hurr. and Trop. Meteor., Ft Collins, CO., 53-54.

JMA, 1997. Outline of the operational numerical weather prediction at the Japan Meteorological Agency. Appendix to Progress Report on Numerical Weather Prediction . Numerical Prediction Division, JMA.

Jury, M., 1993: A preliminary study of climatological associations and characteristics of tropical cyclones in the SW . Met. Atmos. Physics, 51, 101- 115.

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789-804.

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Lander, M.A., 1995: Unusual tropical cyclone motion associated with reverse orientation of the monsoon trough of the western North Pacific. 21st Conf. on Hurricanes and Tropical Meteorology, April24-28, 1995, Miami, FL, pp 213-215.

Lander, M., 1997: On the ability of operational dynamic models to predict tropical cyclone intensity. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology , Ft. Collins CO, Amer. Meteorol. Soc., 621-622.

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North Pacific tropical cyclones. Preprints, 21st AMS Conf on Hurricanes and Tropical Meteorology , Miami, FL, Amer. Meteor. Soc., 94-96.

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4. 7.3 Acknowledgments

I want especially to thank and acknowledge Prof. Lianshow Chen o IWTC-IC eo-chairman for his positive suggestion and helpful comments in preparing for the report. I also want to thank Mr. Zhihui Yan in NMC, China to provide some data of operational forecasts in August, 1997. While drafting the report, Mr. Di Kang and Ms Zhiyong Meng and Mr. Xuejin Zhang prepared many materials for it.

Fitzpatrick, P. J., 1995: Forecasting tropical cyclone intensity change in the West Pacific. Preprints, 21st AMS Conf. on Hurricanes and Tropical Meteorology , Miami, FL, Amer. Meteor. Soc., 389-391.

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Gray, W. M., 1984a: Atlantic seasonal hurricane frequency. Part I: El Nino and 30mb

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Guard, C. P., 1995: Some synoptic and dynamic aspects of the processes involved in rapid intensification of

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Name Description Place Date Duration

Victor (9712) Landing and Hong Kong 30Z12, Aug 2 Jul31- Aug Turning 3 Northward

Winnie Landing twice Wenling, 30Z13, Aug Aug 10-Aug (9713) Zhejiang 18 20

Yingkou, 30Zl5, Aug Liaoning 20

Zita (9715) Landing and Leizhou, 30Z05, Aug Aug21-Aug Decay Guangdong 22 23

Amber (9716) Landing twice Hualian, 30Z03, Aug Aug22-Aug Taiwan 29 29

Fuzhou, 30Z08, Aug 29

Table 4. Tropical cyclones landed on China coast in August, 1997

1-4 1.398 .344 1.306 .207 .152 .024

11-21 .487 1.579 .45 1.263 .291 .821 .187 .5 3 .129

22-29 .480 ' 2.014 .287 1.359 .149 .657 .072 .301 .000 . 0

.051 . 37

Table 5. High-LAFS Precipitation Verification (Aug 1997)

In CAMS, a nested TC rainfall model was developed and got a satisfactory result of TC rainfall (Fig 15). A two-way model boundary was employed. The impacts of different scale weather systems and topography were considered as well. Over China offshore area, this model can give us a reasonable prediction in spite of less observation over the sea. (Song, et al, 1996). Besides, the employment of satellite observation information to revise the model initial condition will also contribute to the good result of numerical prediction (Xu, et al, 1996).

Page4 4.7 Rainfall Pred Teq - Xu Xiangde

.N~

Fig.$ The Ilow cbort of1lu: oou"'l nctwOdt system

Figure 13. The flow chart of the neural network system

.... · .·

Fig.4 The typho(m heavy rain area(shaded area)

• 4. 7.2.3 Numerical Model Prediction

• Many operational and research numerical models have been developed in order to predict rainfall in China. A High-LAPS model were developed to predict precipitation in NMC, China. It is based on a limited area multivariate-3D optimum interpolation analysis system and a primitive equation system with 15-sigma vertical levels. It is run every day at 20 UTC for 24-48 hours rainfall forecasts. Then the products are nationally distributed.

August 1997, 4 tropical cyclones landed on China (Table 4). Heavy rainfalls were brought by these tropical cyclones, The rainfall alleviated the of North China but caused great damage in East China. The verification results of High-LAPS are shown in Table 4. The High-LAPS gives its 24 hour forecast on the TC rainfall location after the TC landfall.

Page3 4. 7 Rainfall Pred Teq - Xu Xiangde

.. Fig.3 The synoptic C()ricept model of,typhoon · heavy rainfall location

Also, the diagnosis factor such as water vapor transportation, vertical motion and instability will help forecaster correctly estimate the rainfall area. The isentropic potential vorticity (IPV) analysis showed that the heavy rainfall will occur in the area between ·cold advection and high PV advection areas.

• 4.7.2.1 Statistic Forecast

• Chinese meteorologists chose 37 factors, such as moisture, humidity, stability, flow convergence and related environmental conditions and established five successive regression equations for the maximum of process rainfall causes by tropical cyclones in certain area, which can forecast the area and time of TC-caused torrential rain. Its forecast effect is rather valuable.

• 4. 7.2.2 Statistic Dynamic Prediction

• Based on the dynamic analysis, the artificial neural network system was installed to predict the rainfall related to TC (Fig. 13). The system can predict heavy rainfall automatically and the forecasting network of neural catalogue in the corresponding area (Cao et al, 1996).

Page2 4.7 Rainfall Pred Teq - Xu Xiangde

4.7 Rainfall Prediction Techniques- Xu Xiangde

4.7.1 Overview

Tropical cyclone (TC) disaster, as one of the most severe disasters, is mainly caused by strong winds, torrential and storm surges (Chen, 1995). Due to torrential rains associated with the flooding and great damages, for example, typhoon Nina (1975) which caused torrential rains of 1060 mm in 24 hours on the third day after landfall and great lives and properties were lost over the middle of China, many attentions are attracted to improve the rainfall prediction techniques.

On the view of rainfall prediction, the accurate TC motion prediction is an essential prerequisite of TC rainfall forecasting, especially the prediction of landfall position and motion over land. Moreover, there are 5 factors related to TC landfall: (1) water vapor transportation, (2) vertical motion, (3) potential instability, (4) upper level divergence, (5) mesoscale system activities (Chen, et al, 1979). These factors are bound up with the TC's intensity and structure changes, topography or synoptic systems of environmental field such as trough, mid-lower tropospheric jets, subtropical highs and tropical systems, and TC inner shear lines which will cause the asymmetric distribution ofTC rainfall (Fig 10, Fig 11).

Fig. I. The rainfall asymmell)' during typhoon arid mid-latitude trough interaction

· Fig.2 The rainfall asymmetry related to typhoon reverse trough. Thin lines indicate isatach, unit::rnls

This section will focus on the following 4 aspects about TC rainfall prediction: (1) rainfall amount, (2) location, (3) distribution, and (4) duration. From the state of the arts in this area, many forecasting techniques were employed to forecast tropical cyclone rainfall. The major parts are as followed.

4.7.2 Synoptic Diagnoses

As the factors mentioned above, the forecaster can estimate the location or area where the rainfall will occur according to the synoptic diagnoses of the real-time weather processes. Fig. 12 is concept synoptic model to show the rainfall area. The theoretical studies and case analyses show that rainfall area should be located between the upper tropospheric jet and the lower tropospheric jet. The two jets coupled by TC circulation will result in the heavy rainfall and its location.

Page 1 Jury, M., 1993: A preliminary study of climatological associations and characteristics of tropical cyclones in the SW Indian Ocean. Met. Atmos. Physics, 51, 101-115.

Knaff, J. A., 1997: Implications of summertime sea level pressure anomalies. J. Climate, 10, . 789-804. Lander, M., 1994: An exploratory analysis of the relationship between tropical storm for­ mation in the Western North Pacific and ENSO. Mon. Wea. Rev., 122, 636-651.

Landsea, C. W., 1998: El Niiio-Southern Oscillation and the seasonal predictability of tropi­ cal cyclones. Accepted as a chapter for El Niiio: Impacts of Multiscale Variability on Natural Ecosystems and Society, edited by H. F. Diaz and V. Markgraf(forth­ coming).

Landsea, C. W. and W. M. Gray, 1992: The strong association between western Sahel monsoon rainfall and intense Atlantic hurricanes. J. Climate, 5, 435-453.

Landsea, C. W., W. M. Gray, P. W. Mielke and K. J. Berry, 1992: Long term variations of Western Sahel monsoon rainfall and intense U.S. landfalling hurricanes. J. Climate, 5, 1528-1534.

Lehmiller, G. S., T. B. Kimberlain and J. B. Elsner, 1997: Seasonal prediction models for North Atlantic basin hurricane location. Mon. Wea. Rev., 125, 1780-1791.

Nicholls, N., 1979: A possible method for predicting seasonal tropical cyclone activity in the Australian region. Mon. Wea. Rev., 107, 1221-1224.

Nicholls, N., 1984: The Southern Oscillation, sea-surface temperature and interannual fluc­ tuations in Australian tropical cyclone activity. J. Climatal., 4, 661-670.

Nicholls, N., 1992: Recent performance of a method for forecasting Australian seasonal tropical cyclone activity. Aust. Met. Mag., 40, 105-110.

Shapiro, L. J ., 1982: Hurricane climate fluctuations. Part 11: Relation to large-scale circula­ tion. Mon. Wea. Rev., 110, 1014-1023. REFERENCES

Chan, J. C. L., 1994: Prediction of the interannual variations of tropical cyclone movement over regions of the western North Pacific. Intl. J. Climatal., 14, 527-538.

Chan, J. C. L., 1995: Tropical cyclone activity in the Northwest Pacific in relation to the El Nifio/Southern Oscillation phenomenon. Mon. Wea. Rev., 113, 599-606.

Chan, J. C. L., 1995: Tropical cyclone activity in the western North Pacific in relation to the stratospheric quasi-biennial oscillation. Mon. Wea. Rev., 123, 2567-2571.

Elsner, J. B. and C. P. Schmertmann, 1993: Improving extended-range seasonal predictions of intense Atlantic hurricane activity. Weather and Forecasting, 8, 345-351.

Evans, J. L. and R. J. Alien, 1992: El Nifio/Southern Oscillation modification to the struc­ ture of the monsoon and tropical activity in the Australian region. Int. J. Climatal., 12, 611-623.

Gray, W. M., 1968: Global View of the Origin of Tropical Disturbances and Storms. Mon. Wea. Rev., 96, 669-700.

Gray, W. M., 1979: Hurricanes: their formation, structure and likely role in the tropical circulation. Supplement to Meteorology Over the Tropical Oceans. Published by RMS, James Glaisher House, Grenville Place, Bracknell, Berkshire, RG 12 1BX, D. B. Shaw, ed., 155-218.

Gray, W. M., 1984a: Atlantic seasonal hurricane frequency. Part I: El Nino and 30mb Quasi-Biennial Oscillation influences. Mon. Wea. Rev., 112, 1649-1668.

Gray, W. M., 1984b: Atlantic seasonal hurricane frequency. Part II: forecasting its variability. Mon. Wea. Rev., 112, 1669-1683.

Gray, W. M., 1990: Strong association between West African rainfall and U.S. landfall of intense hurricanes. Science, 249, 1251-1256.

Gray, W. M., C. W. Landsea, P. Mielke and K. Berry, 1992: Predicting Atlantic seasonal hurricane activity 6-11 months in advance. Wea. Forecasting, 3, 440-455.

Gray, W. M., C. W. Landsea, P. Mielke, and K. Berry, 1993: Predicting Atlantic basin seasonal tropical cyclone activity by 1 August. Wea. Forecasting, Vol. 1, No. 8, 73-86.

Gray, W. M., C. W. Landsea, P. Mielke, and K. Berry, 1994: Predicting Atlantic basin seasonal tropical cyclone activity by 1 June. Wea. Forecasting, 9, 103-115.

Hess, J. C., J. B. Elsner and N. E. LaSeur, 1995: Improving seasonal predictions for the Atlantic basin. Wea. Forecasting, 10, 425-432. 4.6 Seasonal Pred Teq - Bill Gray

hurricanes and greater the eight major hurricane days. This is to be contrasted with the 10 of 41 years when our 1 December forecast predicted less than 1. 7 intense hurricanes and 2.0 intense hurricane days. Note the very large differences in observed intense hurricane tracks for these 10 high versus 10 low hurricane hindcasts. This is an impressive result considering that this forecast is issued six months before the start of the "official" hurricane season and eight months before the active portion of the hurricane season. We are now working on the possibilities of predicting the probability of seasonal US landfall one to three seasons in advance.

4.6.6 Summar:y

Seasonal hurricane prediction is only beginning to come into its own. This appears to be a very promising area for new research. We encourage others to investigate precursor global atmospheric and ocean conditions which are associated with active and inactive TC seasons in their individual storm basins. It is necessary that one consider a variety of global-ocean predictors in combination. The key is to look at global data sets. We find that we improve seasonal forecasts when we maximize four to six different hindcast precursor signals during 45-50 past seasons. For the Atlantic basin these hindcast predictions are ENSO, Qbo, African rainfall, SLPA, Atlantic SSTA, etc.

When 1 Dec .. Fcsl. wos for •.:; .. IH > '3.5 (Ob.~) HO > 8 fOb. ii.:4 l.

10 Seasons

•\ - 1,, ,.,~ .. · ·~ \ . -- ·--~· ,. \ -\ .

When 1 Dec. Fcst. was for

'\,~ 10 Seasons \ .,/'\. \

Figure 9: Contrast of observed intense hurricane tracks between 1950 and 1990 from the 10 most active hindcast (> 3.5 intense hurricanes [IH] and > 8.0 days of intense hurricanes [IHD] occurring) seasons versus the 10 calmest hindcast seasons (< 1.7 IH and < 2.0 IHD from a 1 December of the previous year initial forecast date. The ratio of observed IHD between the two composites is 9.5 to 1 (from Landsea et al. 1994).

PageS 4.6 Seasonal Pred Teq- Bill Gray usually found in conjunction with a more stable troposphere, higher SLPA and increased vertical wind shear. Interannual SSTA variations are, by themselves, are not a primary cause of seasonal TC variations. Other factors such as the ENSO, Qbo and SLPA are more dominant. The SSTA influence on seasonal TC activity has often been overemphasized.

Seasonal tropical cyclone activity can sometimes be related to the strength on the monsoon trough. Evans and Alien (1992) found that variations in the Australian monsoonal flow can be associated with changes in tropical cyclone activity such that a strong monsoon circulation during La Nina events is accompanied by more tropical cyclones while fewer cyclones with a weaker monsoon trough. Over the Atlantic basin, June through September rainfall in 's Western Sahel has shown a very close association with intense hurricane activity (Gray 1990). Wet years in the Western Sahel are accompanied by increases in the incidence of intense (category 3-4-5) hurricanes, while drought years are accompanied by a decrease in intense hurricane activity. Other meteorological features can also be related to seasonal variations in tropical cyclone activity .

..... WAVE a

Figure 8: Idealized schematic of an Atlantic easterly wave disturbance moving westward from Africa into the central and western near 15°N latitude during late summer. In the top diagram (a) only weak surface pressure and weak subsidence drying conditions exist. A weak trade wind inversion is present. The top of the moisture level is high enough such that the wave's upward vertical motion is able to overcome the subsidence drying and a hurricane is able to form. The top diagram was typical of 1995. In the bottom diagram (b) surface pressure and subsidence are stronger than normal, the height of the moist level is lower than normal and the trade wind inversion is stronger. The wave's upward vertical motion cannot overcome these adverse influences. The westward moving easterly wave disturbance of diagram (b) is not able to transform itself into a tropical storm or hurricane and continues to move to the west as a cloud cluster. The bottom diagr! am is more typical of 1994 and 1997.

• 4.6.5.4 Real-time Seasonal Forecasts • The issuing of real-time seasonal forecasts of tropical cyclones has begun. Nicholls (1979 1984 1992) was the first to develop seasonal forecasts for the amount of the Australian basin. J. L. Chan (1994) has recently started issuing seasonal forecasts of tropical cyclone frequency for the Northwest Pacific. Atlantic basin predictions of Atlantic Basin tropical cyclones have been issued by the author and his colleagues since 1984. Forecast methodology is described by Gray et al. (1984a 1984b), Gray (1992 1993 1994). In addition, Elsner and Schmertmann (1993), Hess et al. (1995) and Lehmiller et al. (1997) have been developing Atlantic seasonal hurricane prediction schemes. Fig. 9 has been constructed to show an example of the Gray project 1 December hindcasts of the amount of next year's major or category 3-4-5 hurricane activity. These diagrams show the observed differences in intense hurricane tracks for 10 of 41 yearly hindcasts when our 1 December statist! ical forecast model predicted more than 3.5 intense

Page7 4.6 Seasonal Pred Teq- Bill Gray

·70

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Figure 6: Number of Atlantic basin seasonal hurricane days during non-El Nino years (top) and El Nino years (bottom) between 1900 - 1997. El Nino years have only about 40 percent as many hurricane days as do non-El Nino years.

• 4.6.5.3 Other Variables • Interannual tropical cyclone variations have also been well linked to more localized, cyclone basin-specific variations in tropospheric vertical shear, sea level pressures (SLP), SSTA, monsoon and tradewind strength, regional-global rainfall patterns, and mid-latitude circulation features. For instance, it has been shown that prior season Sea Level Pressure Anomalies (SLPA) changes in the Atlantic (Shapiro 1982; Gray 1984b) and Australian region (Nicholls 1984) often carry through to the hurricane season and can induce alterations in tropical cyclone frequency. Lower SLPA are typically associated with more convective activity, higher moisture at middle levels and weaker vertical wind shear (see Fig. 8). Higher SLPA is usually associated with enhanced subsidence drying, and a higher wind shear. This SLPA association has recently been discussed by Knaff (1997).

HURRICANE DAYS PER YEAR I·~ r 30 mb Zonal Win-·-- I Jl -~ ..;..,.;.._ lrnm EAST I . . .T 1. T . t 4S . 40 § I : I T T . T . I i, . rl·:·l() .' rJ I. rj"r·L Ir I I r .,1 ,1 i ··i r1·! I . -.0. 1~WJ_LL J_ 1JJJJLtLU.tltl___u_ J~j__1 _:_ J l 11 illl L I I I I l l 1 1-11 111. ll·l '1 ' • .1 1.1 ~ • l 1 ,-y·,-~l·r·_. ... r,-,-r-r-~r··T- <9 5.2 ~ ro ~ ~ n .u oo ~ ~ ~ ~ ~~~~~ fl

1900's

Figure 7: Relationship between 30mb stratospheric east vs. west wind direction and seasonal number of Atlantic hurricane days from 1949 - 1997. Years with no observations are those in which the 30 mb zonal wind is changing direction or is very weak during the hurricane season. On average there are twice as many hurricane days during Qbo west as compared with Qbo East years.

Sea Surface Temperature Anomalies (SSTA) in TC genesis regions have both a thermodynamic and dynamic effect on tropical cyclones. In general, positive SSTAs are associated with decreased moist static stability, lower than average surface pressures, and reduced tropospheric vertical wind shear. Cooler than average waters are

Page6 4.6 Seasonal Pred Teq -Bill Gray

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B TIME-·-~ RICH HISTORICAL DATA BASE -- Forecast. Reality ·········••.11··( LOT OF CULTURE ) ..... ·..... ······ .. _...... ·- t = 0 STATISTICAL CLIMATE PREDICTION

Figure 5: Illustration of the two methodologies of climate prediction diagram. The top (A) initial value numerical prediction and the bottom diagram that of the empirical diagram illustrates methodology. Method A lacks past information while Method B is rich in past information as to how the atmosphere has previously functioned.

4.6.5 Cyclone Basin Seasonal Predictors

• 4.6.5.1 ENSO • Global tropical cyclone activity is influenced by variations in the El Nine-Southern Oscillation (ENSO). ENSO variations can cause a general rearrangement of global circulation patterns, rainfall, and surface pressure. During warm ENSO conditions Atlantic and Australian basin TC activity are typically reduced. In the Northwest and South Pacific TC activity are enhanced east of 150°E and reduced to the west of 150°E. Northwest Pacific TCs typically have more northward tracks during warm ENSO years and Japan is more threatened. In cold ENSO (or La Nina) years, Northwest Pacific TCs tend to have a more westward movement. The Philippines and China are more vulnerable in these years. Other storm basin changes occur with ENSO variations. A more thorough discussion ofthis topic is given by Landsea (1998). The forecaster usually has a good idea whether a warm or cold ENSO event will be present during the coming hurricane season.

• 4.6.5.2 QBO • In addition to ENSO, three basins (the Atlantic [Gray 1984a], Southwest Indian [Jury 1993], and Northwest Pacific [Chan 1995]) show systematic alterations of tropical cyclone frequency by the stratospheric Quasi-Biennial Oscillation (Qbo), the east-west oscillation of stratospheric zonal winds that encircle the globe 20° latitude either side of the . The physical associations between these stratospheric winds and seasonal tropical cyclone activity is being researched and are too involved to discuss here (see Gray et al. 1992). The Qbo can be easily extrapolated by up to a year in advance.

The importance of both ENSO and the Qbo for the alteration of Atlantic basin seasonal hurricane activity is shown in Figs. 6 and 7. A full documentation of ENSO and Qbo influences in the other TC basins is in various stages of observational documentation.

PageS 4.6 Seasonal Pred Teq- Bill Gray

• 4.6.4.1 Empirical Versus Modeling Based Approach to Climate Prediction • The best methodology for detecting seasonal to multi-seasonal predictive relationships is based on the simple assumption that changes in future atmospheric-ocean conditions will closely mimic those atmospheric changes which have been observed in the past. The likely superiority of this approach over the initial-value numerical modeling approach is due to the complex nature of the atmosphere/ocean system (Fig. 4) and to the inability of the numerical models to realistically represent and skillfully forecast all of these complicated nonlinear processes for thousands and thousands of time steps into the future. Any initial value measurement errors or imperfect physical parameterization of the complicated atmosphere-ocean physics quickly degrades (the so-called 'butterfly' influence) into unrealistic future states (Fig. 5). There is no question but that global numerical modeling, out to time scales of one to two weeks, has been a great success story. But this sh! ort range success in global modeling forecasting does not mean that an extension of this methodology to seasonal and beyond time 'scales will prove as successful as the statistical forecast schemes. Modeling based seasonal forecasts (typically more expensive to develop than empirical schemes) must show improvements surpassing the empirical schemes before they can be considered to be of genuine value. So far this demonstration has not been made. This is not an indictment of numerical prediction, per se, but only of the extension of numerical prediction to seasonal and beyond time scales.

·cloud · cov<'r -- Rollltive humldily

Figure 4: Flow diagram for climate modeling showing all the complicated feedback loops (from A. Robock).

One of the important advantages of the empirical-based seasonal forecast method is that one does not have to understand the details of the complicated atmosphere-ocean climate system (such as air-sea interaction, cumulus-scale turbulence, , etc.) in order to develop a skillful forecast scheme. One only has to learn of past physical associations. By contrast, numerical modeling requires full knowledge of the atmosphere and ocean's complicated physics processes and the ability to predict these changes for long periods in the future.

• 4.6.4.2 Coming Inundation of Numerical Climate Outputs • Despite these apparent methodological problems, initial value global climate modeling is developing at a rapid pace. There are now over 30 global numerical models in various stages of development, testing, and operational prediction. Many of these global forecast centers are extending model predictions forward in time to seasonal, yearly, decadal, and century time scales. Most predictions are, so far, not showing consistent forecast success at seasonal or greater time frames. The TC forecaster should expect to see an increasing variety of seasonal global circulation and SSTA climate forecasts coming forth in the next few years. He should be cautious in utilizing such predictions. Most have not been tested over many years. Their multi-year reliability is unknown. Statistical seasonal forecasts, by contrast, are typically developed from data sets extending backward in time for 20-50 years or more. Such forecasts have a known hindcast track record. Their skill ! and shortcomings are already known.

Page4 4.6 Seasonal Pred Teq - Bill Gray

• 4.6.3.2 Requirements for the Development ofSeasonal Prediction Schemes

• The development of seasonal statistical prediction of TC activity requires that one analyze as many past years of 'global' atmospheric and ocean information as possible and determine their association and correlation with seasonal TC activity. The global atmosphere- ocean system beats in unison and must be treated as a whole. Many of the prior difficulties which have been encountered in developing individual TC basin seasonal prediction have arisen because developmental data sets from which the prediction have developed have been confined to local or regional information. For instance, in the Atlantic basin one cannot look only at tropical Atlantic SSTA and SLPA patterns and ignore considerations of the more important global influences of ENSO, the stratospheric Qbo, Western Sahel rainfall, and North Atlantic SSTAs. Only marginal skillful prediction is possible from prior knowledge of only Western Atlantic climate factors. The rapid development of computer ! technology and the new advancements in data storage and retrieval have greatly improved the potential for the development of new seasonal prediction schemes utilizing full global data sets.

4 .6.4 The Atmosphere's Long Term Predictive Signals

It is surprising that there are many long period predictive signals observable within the atmosphere itself. This is particularly the case in those conditions where atmospheric predictive signals result as a consequence of or a proxy response to the ocean. There has been a general belief that it is only the ocean itself (not the atmosphere) which offers the only available long-term predictive signals. This belief is in part based on the know ledge that the atmosphere's hydrologic cycle and kinetic energy dissipation rates operate on a time scale of only 8-10 days. This implies that the atmosphere's memory or conservatism with time is short. Most prior statistical climate forecast schemes have accepted this premise and have turned to the use of SSTA (and SLPA - a typical inverse function of SSTA) information as the primary predictive component. This view ofthe atmosphere as having no long term memory and much predictive potential is incorrect.

Although long period atmospheric changes may be primarily driven by long-term changes in the ocean, this does not mean the atmosphere itself does not have a long term memory as a consequence of ocean driven influences. Ocean measurements such as the thermocline depth, upwelling-downwelling effects, etc. are less accessible and usually do not extend back in time as long as do atmospheric measurements. Consequently, we can often better detect trends originating in or maintained by the ocean within the atmosphere than we can within the ocean itself. The atmosphere appears to reflect more about the future state of the combined atmosphere-ocean system than does the ocean surface by itself (see Fig. 3). There are strong multi-season predictive signals for Atlantic seasonal hurricane activity contained in prior-year West African rainfall, the strength of the anticyclone, the SOl and the stratospheric Qbo. Combined atmospheric precursor signals allow us to explain (in hindca! st) as much as 40-60 percent of the year-to-year variance of Atlantic seasonal hurricane activity from one to four seasons in advance. Further research will undoubtedly lead to the uncovering of more atmosphere-ocean predictive relationships and higher forecast skill. WIND Temperature lPressure -Height+ Sfc. Pressure Anomaly • • ' . ' ". '· ' . \ " . ' ' ' ' . ·, ' '. '. '. '. \ \ .. ~. Rainfall .,.;,- \ . \ ' " .JIII" .. \. -"' ' '\ .·" ., ...... · '\ ~ . . - \

Figure 3: Illustration of how strongly varying ocean conditions can drive long period atmospheric wind, temperature, surface pressure, and rainfall changes which can cause the atmosphere to have a long period memory.

Page3 4.6 Seasonal Pred Teq -Bill Gray Tropical ..i lrit!:!nse. st.('orm inten;it:y Hnrricane intensity hurricane intensity 'IJ:opical .Cyclone .Basin Ave. (S;O;) %of total Ave.(S.D.' -~ oftotaJ. Ave. (S.D•) %of total Atlantic (1~44-95) · • 9.8{3.0) 11.4~ 5.7(2.2) 12;1% 2.2(U) 10.9% NE Pa.cifk {1970.91)) 17.0( -4.4:) 19:79'{ 9;8{3.1) . 20.7%. . 4.6{2:5) 22.9% NW Pacific. (1979-g5) . 26.9(4..1) 32.11]1( 16.8(3.6) 35.5% 8.3(3.2) 41,3% ·North ~ndian (1970:!15) 5.4(2.2) 6;3% 2.2(L.Sl 4.6% 0:3(0.5) 1.5% SW Indian ( < lOO~E) (19G9/70'-94/91i) . 10.3(2;9) 12.0% 4.£-{2·4} 10.4% 1.8(1;9) 9.0% Austra!ia.n/SE IiuUan (100 :_142°EJ(196£/70-94/95) ~.5(2.6) 7.5% 3.3(1.9) 7.0% 1.2(1.4) ().0% Australian/SW Pacific . ' (> l42"E)(l969/70~94/9U 10;2(3;1). 11;89{. \ 4.6(2.4) 9.7% 1.7(1.9) 8.5% ---· ------Global (lf>70-!l5) S6.1(8.0) i 17.3{6.5't 20.1(5.7)

Table 4: Average, standard deviation and percent of global total of the number of tropical storms (winds at least 18 ms '), hurricane-force tropical cyclones (winds at least 33 ms '), and intense hurricane-force tropical cyclones (winds at least 50 ms ').The first value provides the long-term average from reliable records, the second value (in the parenthesis) is the corresponding standard deviation, and the third is the percentage of the global total. Dates in parentheses provide the nominal years for which accurate records are currently available. (from Landsea 1998)

4,6.3 Theory of Seasonal TC Variability

The net radiational cooling of the troposphere in all seasons and regions is approximately (1.1 °C/day). This daily cooling is balanced by surface to atmosphere sensible heat transfer (about 25 percent) and by condensation heating from surface evaporation (about 75 percent). Condensation heat release requires there be an average global rainfall of about 2.7 mm/day. In the tropics (30"N to 30°S) rainfall amounts average about 4.0 mm/day.

Depending on a variety of global atmosphere and ocean conditions (such as the presence or absence of El Nino, ocean SST patterns, the phase of the stratospheric Qbo, middle latitude zonal or blocking wind conditions, the strength of the Atlantic ocean thermohaline circulation, and other slowly varying global atmosphere-ocean conditions, individual seasonal rainfall and tropical cyclone patterns in the various storm basins are always undergoing seasonal alteration from their background climatology. Some storm basins will experience above average seasonal rainfall amounts during an individual year while other storm basins, due to net global rainfall compensation, will receive less. There is a strong tendency for yearly global tropical cyclone activities to be conservative. Table 1 shows that the standard deviation of yearly global named storm activity ( 86 per year) is only about nine percent while the typical individual basin seasonal standard deviations are ! 2 to 10 times higher. Each year enhanced TC activity will occur in some of the global basins while reduced activity will take place in the other basins.

Each hurricane season experiences alternating day-to-day variations in favorable and unfavorable TC wind conditions from its background climatology. For instance, in the Atlantic basin there are about 50-60 easterly waves which move westward from Africa between June through September. These wave perturbations occur during both active and inactive hurricane seasons. In those seasons or months when background Atlantic wind and thermodynamic conditions are more favorable for TC formation, a higher percentage of these 50-60 easterly waves will develop into hurricanes. When seasonal background conditions are less favorable, a smaller number of formations occur, Easterly waves, cloud clusters, etc. occur in all seasons. It is typically not the day to day variation of the number and strength of individual tropical disturbances which determine whether a season is to be active or not (these come and go in all seasons) but rather the monthly or seasonal background con! ditions into which these multi-day disturbances form and move which is the crucial determinant.

• 4.6.3.1 Seasonal Sea Level Pressure, Rainfall and Tropical Cyclone Associations

• There is typically more seasonal tropical cyclone activity when cyclone basin sea level pressure anomalies (SLPAs) are below normal. Rainfall and SLPA are typically negatively associated on a seasonal basis. Values of seasonal SLPA as small as ± 0.5 ~o 1.0 mb are closely related to seasonal TC frequency variations. Lower than normal storm basin SLPAs are associated with more seasonal basin mean upward vertical motion, rainfall, and TC activity. Positive SLPA results in lower middle level moisture contents, stronger temperature inversions, more inhibiting of deep cumulus convection, and less seasonal TC activity (Knaff 1997).

Page2 4.6 Seasonal Pred Teq- Bill Gray

4.6 Seasonal Prediction Techniques -Bill Gray

4.6.1 Introduction

The more one studies Tropical Cyclones (TC) the more one becomes aware of the fundamental role of environmental influences on TC formation, structure, intensity and motion. Tropical cyclones typically do not evolve in climatologically fixed environments but rather in environments exhibiting various short-term favorable deviations from monthly and seasonal background climatology.

Although the role of the TC environment has been known and accepted in varying degrees by tropical cyclone specialists for many years, the full extent and the frequent dominant role it plays is only now being fully appreciated. Environmental conditions can vary from storm to storm and with time during the lifetime of a cyclone. They also vary from season to season and by region. For instance, the formation of Atlantic TCs deep within the presents a different type of formation environment than that typical of the monsoonal trough environments in which the majority of the globe's other TCs develop.

4.6.2 The promising potential of climate prediction

• 4.6.2.1 Climate Prediction Methodology

• There is a surprising degree of season to multi-season predictability residing within historical global atmospheric and oceanic data sets. This predictability is just now beginning to be exploited, but much more development is necessary to fully make use of the existing potential. The most promising avenue for the development of new climate prediction schemes are based upon how the atmosphere-ocean has behaved in the past. This hindcasting based methodology appears to offer a greater likelihood for advances in seasonal predictive skill than do those seasonal forecasts made from initial value numerical modeling. Accumulating knowledge of past years of seasonal climate and tropical cyclone events appears more useful for the development of skillful seasonal forecasts at this time (and likely in the foreseeable future) than what can be obtained from numerical climate modeling which requires a precise knowledge of how the atmosphere and ocean function in unison and how this int! eraction manifests itself through thousands and thousands of future time steps.

Our knowledge of the environment-tropical cyclone interaction in the global's hurricane-typhoon basins is becoming better defined as the years of reliable satellite and convectional data, continue to grow. Major interest rests with knowing how the coming season's environmental flowpattems and net seasonal tropical cyclone activity will deviate from the long-term climatology.

The Atlantic cyclone basin offers the best possibilities for seasonal tropical cyclone prediction. This is because the Atlantic is the most climatologically sensitive of the . It has the highest year-to-year variations of cyclone activity. Recent year variations in numbers of Atlantic hurricanes has ranged as high as 12 (as in 1969), 11 (as in 1950 and 1995), and nine (as in 1980, 1955, 1996), and as low as two (as in 1982) and three (1997, 1994, 1987, 1983, 1972, 1962, 1957). Recent research has shown that a sizable amount ( 40-60 percent) of this variability can be forecast two to nine months in advance (Gray et al. 1992, 1993, 1994). With more research and a larger historical data base it is to be expected that further improvements in forecast skill in. this and in other storm basins will be possible.

Year-to-year variability ofTC activity in monsoon trough regions is less dependent upon seasonal variations of the global ocean-atmosphere circulation such as the El Nino/Southern Oscillation (ENSO), the stratospheric Quasi-Biennial Oscillation (Qbo), than is the Atlantic storm basin. The interannual standard deviation of named storms in the Atlantic to the long-term yearly average is about 0.39 while in the Northwest Pacific basin it is only half this amount- 0.21 (see Table 4). This does not mean that skillful and meaningful statistical forecasts cannot be developed and in the non-Atlantic storm basins. Nicholl's (1984) and J.C.L. Chan (1994) have been making seasonal TC forecasts by statistical methods for the Australian and Northwest Pacific region in recent years. It is likely that further improvement in these statistical forecasts can be developed if more efforts are made to exploit the availability of the ever expanding global atmosphere-ocean data ! sets. It is likely that applying similar statistical techniques on the remaining cyclone basins of the North and South Indian Oceans, the South Pacific, and Northeast Pacific will result in the development of at least marginally skillful forecasts of interannual seasonal TC variability.

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4.5 Int Pred Teq - Chip Guard

checklists that weight the various factors and provide a measure of the potential for intensification, weakening, rapid intensification, etc. (e.g., WMO 1993). It is often valuable for a warning center to review the checklists of other warning centers for potential checklist cross-feed and improvements.

Doppler radar techniques

• Doppler has shown tremendous value in terms of identifying the distribution of the wind structure of TCs. While the maximum range for Doppler wind acquisition is generally about 125 nm ( km) from the radar, the accuracy, resolution, and timeliness of the data allow for very good short-term intensity prediction. The radar also shows that we still have much to learn about small scale changes such as eye wall cycles, convective asymmetries, mesoscale eddies at the eye-eye wall interface, low-level wind "jets", spin up and spin down of the average wind, and the gustiness.

4.5.3 Operational concerns

TC techniques, in general, need to be reliable and quick/simple to use. These are the key reasons why there is resistance to using new techniques at operational warning centers. Forecasters are constrained by time, and will generally not use a technique if it is time consuming. This is especially the case with intensity prediction tools, because most of the time is devoted to developing the track forecast. A case in point is the complexity of applying the Dvorak intensity forecast scheme, which requires the knowledge and application of a multitude of rules (page 2.21, WMO 1993). This difficulty has led to misapplication of the technique in some cases and total abandonment of the technique in others. The ultimate goal is to have reliable automated techniques that require little manual input or to have the necessary manpower to devote a forecaster to the intensity prediction problem. The latter is not likely.

There are several issues that need to be resolved in order to develop accurate intensity prediction techniques. First, there needs to be an accurate best track data base on which to develop and evaluate the techniques. This means that we must resolve some basic questions so that these best track data bases can be improved.

1. What is the relationship between TC near-surface wind speed and its spin-up and spin-down with relation to changes in cloud patterns? 2 .. What is the true relationship between near surface maximum winds and conventional flight-level wind data? 3. What is the best ratio to use relating over-water and over-land sustained winds to peak gusts (gust factor)? 4. How do we get peak gust and landfall information into the operational data and into the best track data bases?

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Table 4. Measured minimum central pressures of tropical cyclones in the western Australian region compared to estimated minimum central pressures using the Dvorak technique. Pressures are in hPa.

Observed Central Pressure Estimated Central Pressure u~(Yr) (hPa) (hPa) 905 915 IOrson ~~~ 955 955 EJEjl 1Annettell199511 933 927

941 IKirsty 11199611 938 Olivia ~I 925 927 Microwave satellite sensor techniques

• Velden et al. (1991) presented a method for deriving western North Pacific TC intensity from satellite passive microwave observations. Vertical profiles of gleaned from remotely sensed radiances from the Microwave Sounding Units (MSU) on the NOAA satellites were used to reveal upper tropospheric warm anomalies associated with the TCs. A nonlinear statistical relationship between the satellite-derived warm anomalies and the near-surface intensities as measured by reconnaissance aircraft. The technique had a standard error of 13 hPa and 15.4 kt. Merrill (1995) developed a physical retrieval algorithm for ascertaining the upper level TC thermal structure using 55-GHz passive microwave observations. The coarseness of the current microwave sounders (MSU and DMSP microwave sounder (SSM!T)) prevents the results from improving on the Dvorak technique. Results should be greatly improved when the Advanced MSU is made available on future T! IROS-N satellites.

May et al. (1997) showed the potential for using the DMSP microwave imager (SSMII) to determine TC intensity. Satellite analysts at the JTWC have used microwave imager data to qualitatively improve intensity estimates where the Dvorak technique is inconclusive. The SSMII data show good potential for providing a Dvorak-like quantitative intensity estimation technique.

Because of the irregular frequency of polar-orbiting satellite passes over TCs, microwave intensity . techniques must exceed the accuracy of the Dvorak technique in order to have utility. The goal is to develop a technique that is sufficiently accurate to be used to calibrate the Dvorak technique when the polar-orbiting data are available. ·

Pattern recognition techniques

• Several investigators (e.g., Neumann 1979; Chen and Gray 1986; Merrill1988; Guard 1995) have shown the upper level environmental influences on intensity change. These pattern-recognition techniques have been valuable in providing qualitative assessments of the potential for TC intensification, weakening, rapid intensification, and rapid over-water weakening. Guard (1983) also related the process of intensification-after-recurvature to favorable synoptic patterns, seasonal sea surface temperature distribution, and the monthly changes in the mean latitude of recurvature. Such older studies should be updated and reevaluated.

The Systematic Approach to Tropical Cyclone Forecasting (Carr and Elsberry 1994) provides a well-structured format for evaluating the interactions between a TC and its environment, and the feedback among TC motion, intensity, and wind distribution. The Systematic Approach can be used to consolidate the intensity prediction knowledge-base and integrate it into the intensity forecast development process.

At most TC warning centers, the satellite and pattern recognition techniques have been incorporated into

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Mesoscale prediction models

• The Japan Meteorological Agency (JMA) is now taking the lead in the Case Ill ofthe Comparison of Mesoscale Prediction and Research Experiments (COMPARE) intercomparison project. Super , which occurred during the coordinated set of field experiments (SPECTRUM!fCM-90trYPHOON-90), has been chosen as the target of the simulation study. The main theme of the third case is the explosive development of Typhoon Flo (T9019), which showed a marked deepening rate of -80 hPa in 48 h leading to a minimum central pressure of 890 hP a. The major goal of the third case is to assess the ability of current mesoscale models to simulate and even predict explosive intensification (personal communication with M. Nagata).

The Colorado State Regional Atmospheric Modeling System (RAMS) was used to duplicate the rapid intensification of Hurricane Andrew (Eastman 1995). The RAMS includes explicitly resolved convection, where the convection is resolved by employing 2-way interactive meshing, with a horizontal grid interval of 5 km on the finest mesh. The mesh moves with the storm center so that the areas of deep convection are always resolved by the finest grid. The RAMS model accurately simulated the rapid intensification of Hurricane Andrew, and produced central pressures that were only a few hPa different than those measured. The track was also representative, with the simulation missing landfall by 200 km and 12 hours after a 72-h integration. Regional models are showing good potential for producing TC intensity prediction guidance.

4.5.2.4. Synoptic, satellite, and Doppler radar techniques:

Dvorak technique

• Perhaps the most life-saving meteorological analysis technique ever developed is the Dvorak TC intensity estimation technique (1975; 1984). The technique relates satellite cloud patterns in the visual and infrared spectra to near-surface maximum wind intensity. Despite the success of the technique, it does have some shortfalls that need to be improved upon. While the technique has a 24-h intensity prediction component based on past and anticipated future trends of strengthening and weakening, the intensity forecast component can be complicated to apply and is often mis-applied (see page 2.21 in WMO 1993). Foley and Holland (personal communication) compared central pressure estimates for recent TCs using the Dvorak technique with measured central pressures from land stations and oil rigs. The comparisons are shown in Table 3.

One of the shortfalls of the Dvorak technique is that it weakens TCs moving over land too slowly. Koba et al. (1991) examined TCs crossing the Philippine Islands, and found that the Dvorak Current Intensity (Cl) number tends to over-estimate the intensity of a TC which shows sudden weakening after landfall. This was attributed to the time lag between the Cl number and the T number. The authors applied three new criteria that significantly reduced the over-estimation. The basic premise is that when a TC moves acr.oss a large island such as Luzon, the Cl-number is the same as the T-number, and does not lag the T-number as it does over water.

Zehr and Phillips (1997) are using an expert system approach to predict TC intensity. The system uses the current intensity and past trend as the first guess, then produces a prediction after assessing potential vertical shear effects, TC structure effects calculated from TC data and structural changes in the past 12 hours, and sea surface temperature distribution and landfall effects. The vertical shear is determined qualitatively from satellite imagery and quantitatively from 850 and 200 hPa analyses at 8 degrees latitude as described by Mundell (1991).

Page 5 4.5 Int Pred Teq - Chip Guard only statistic evaluated was the simple comparison of the model's intensity trends with the intensity trends derived from the JTWC best tracks. At the 0- to 24-h and the 0- to 48-h forecast intervals! (Fig. 2a), most of the models only have modest skill over random choice. At the extended 48- to 72-h period, most of the models show no skill over random choice (Fig. 2b). During 1996 in the Atlantic, the official forecast beat all intensity forecast models at all forecast periods. The dynamic prediction model GFDL model was only comparable at 72 hours (Table 1), while an intensity-guidance derivative of the GFDL model, GFDI, was comparable to the official forecast.

Figure 2(a)

(0 TO 48 HOUR TREND)

1- 0 w a.: 0: 50 0 0 '({!.

0 JTWC NGPS JTVM NCEP EO EGRR JGSM GFDN

(b)

(48 TO 72 HOUR TREND)

t; LUa: a: 50 0 0

0 JTWC NGPS .ITYM NCEP EC EGRR JGSM GFDN

Page4 4.5 Int Pred Teq- Chip Guard

• Kaplan and DeMaria ( 1995) developed an empirical model for predicting the maximum wind of landfalling TCs, which is based on the observation that the wind speed decay rate after landfall is proportional to the wind speed. The wind speed is determined from a two-parameter exponential decay model that uses wind speed at landfall and the time after landfall. The model explains up to 93% of the variance of the best track intensity changes. Figure 1 illustrates the predicted inland penetration of hurricane and storm force winds for a 135-kt TC moving at various speeds. Similar techniques need to be developed for other cyclone-prone locations.

The Maximum Possible Sustained Inland Wind Speed (kt) For Any Slow-(top), Medium-(middle), or Fast-(bottom) moving Category 4 (135 kt at landfull) Storm

ldAX WIND=l35 KT SPEED OF MOTION=B KT

.:1. ..$•. :... '•. '.'95 ' . : .· 65:

. ' g;. 35 ~~;·l~',-. <\'" ')~ •..

MAX WJND=l35 KT SPEED OF MOTION=12 K'r

;;;, 35 --'). 't ~- ')' MAX WIND=l35 KT SPEED OF MOTION=l6 KT

• 4.5.2.3. Dynamic:

• Dynamic TC prediction models

It is generally accepted that current global spectral dynamic models lack the resolution necessary to produce accurate TC intensity predictions. Krishnamurti and Oosterhof (1989), using a T170 triangular truncated global spectral model ( -73 km), showed that as the horizontal resolution of a global model is increased there is a marked improvement in the track and structure of the storm. Improvements in the model physics can also improve the model performance. Lander (1997) performed a cursory evaluation of the capability of seven dynamic models to predict TC intensity. The results presented were based on a preliminary examination of the seven dynamic model's ability to predict the 24-, 48-, and 72-h intensities of several1996 western North Pacific TCs. The

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Pacific. Its 1996 performance is shown in Table 1. All warning centers should have a similar technique on which to base-line their TC intensity forecast skill.

Model 24 1100 1112 11 1136 1148 1172 I OFFICIAL ,3.8 (2.0) 117.6 (4.0) 110.9 (5.7) 1113.1 (6.8) 1115.5 (8.1) 18.8 (9.8) I SHIFOR 3.8 (2.0) 19.0 (4.7) 12.1 (6.3) 14.4 (7.5) 1117.1 (8.9) 21.4 (11.1) 1

SHIPS 3.8 (2.0) 9.0 (4.7) 11.8 (6.1) 13.6 (7.1) 1116.5 (8.6) 21.0 (10.9) 1 GFDI 3.8 (2.0) 9.5 (4.9) 112.1 (6.3) 14.9 (7.7) 117.5 (9.1) 118.9 (9.8) I GFDL 3.8 (2.0) 17.5 (9.1) 1117.7 (9.2) 17.3 (9.0) 17.7 (9.2) 1118.7 (9.7) I I#CASES 128 126 11116 107 199 118o I

Table 1. 1996 North Atlantic average model absolute wind speed error in knots and (m/sec) for a homogeneous sample of models used at the Tropical Prediction Center (OFCM 1997, page B-24).

Statistical Typhoon Intensity Forecast (STIFOR) (Chu 1994) is the equivalent of SIDFOR, but uses western North Pacific climatology as a predictor. It is used by the JTWC, but its performance has not been formally evaluated.

• 4.5.2.2. Statistical-dynamic:

Statistical Hurricane Intensity Prediction Scheme (SHIPS), developed by DeMaria and Kaplan (1994), uses sea surface temperature and synoptic fields as predictors in addition to predictors derived from climatology and persistence. SHIPS is used at the National Hurricane Center; its 1996 performance is shown in Table 1.

Typhoon Intensity Prediction Scheme (TIPS), developed by Fitzpatrick (1995) for the western North Pacific, uses predictors similar to those in SHIPS, but also uses a number of satellite derived parameters as well. Fitzpatrick's work is also based on the research ofMundell (1991), who developed a statistical and satellite technique to predict the peaking day of TC intensity and to predict rapid intensification. The technique was used operationally at the JTWC, but has been abandoned due to its complexity and the false alarm rate attributed to it. When compared with 1983 JTWC forecasts, TIPS showed an average improvement of 19% and an improvement of 25-30% for rapidly intensifying TCs. However, more evaluation on more current, high-confidence independent data sets is needed. The predictor for the scheme are listed in Table 2.

Table 2. List of 24-hour regression predictors used in the TIPS routine.

24h Regression Predictors 1) POT Maximum Possible Intensity (MPI) minus initial intensity (Vma$)- MPI is averaged over the forecast track (ms-J). 2) SHEAR Magnitude of 200-850 mb vertical shear averaged over a. 5° circle a.nd 1 averaged over the forecast track (ms- ). 3) TREND Indicator variable tha.t parameterizes the existence of a. well-formed, contracting eye. If Vma:r: ~ 55 kts and .6-V.,..,,.. > 0 the past 12 h, TREND = 1; otherwise, TREND = o. 4) PIX Percent of pixels for T, < -ssvc in a. 0-4° circuh1.r a.rea.. 5) DPIX 12 h cha.nge of pixels (%) for T~o < -70°C in a 0-1° circular area. 6) VSM Northward component of storm motion vector (ms- ). 7) LONG Initial storm longitude (deg E).

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4.5.1 Discussions at previous IWTCs

The topic of operational tropical cyclone (TC) intensity prediction has been largely ignored in previous IWTC venues. In A Global View of Tropical Cyclones (Elsberry et al. 1987), most emphasis was placed on tropical cyclone track prediction. Discussions relating to operational TC intensity prediction were largely limited to the formative stages or genesis. Structure change was addressed in a theoretical sense, but there was ·no discussion of operational applications to structure change. Me Bride and Holland (1987) also conducted a worldwide survey of techniques and verification statistics on TC forecasting, which revealed that there were very few TC intensity prediction techniques. A new survey should be conducted at the IWTC-IV.

In Global Perspectives on Tropical Cyclones (Elsberry et al. 1995), discussions of operational TC intensity prediction were still largely limited to the genesis phase. A small section of the document was devoted to strong wind forecasting, but the text concentrated on the potential of upcoming technology rather than on actual techniques used in operational intensity prediction.

In 1993, the WMO (1993) published the Global Guide to Tropical Cyclone Forecasting , which did discuss some aspects ofTC intensity prediction. These are below.

Up until the early 1990's, most intensity prediction was based on trends established either by data from reconnaissance aircraft (Atlantic; eastern, central, and western North Pacific (until1987)) or from satellite imagery analysis, primarily using the Dvorak tropical cyclone intensity estimation technique (1975; 1984). As TC track prediction improved, especially beyond 24 hours, largely as a result of improved basic understanding from dedicated TC research programs (e.g., TCM-90, SPECTRUM), improved dynamic models, and expanded computer power, emphasis began to turn to TC intensity change. At the 19 th AMS Conference on Hurricanes and Tropical Meteorology in Miami, Florida, a panel discussion addressed the question: "Is there any hope for tropical cyclone intensity prediction?" (Elsberry et al. 1992). While opinions spanned the spectrum from extreme pessimism to extreme optimism, the event did represent the first forum to address th! e intensity topic. Interest was accelerated by the massive costs ofTC wind damage incurred from Hurricane Andrew, , and in 1992. The 1993 NOAA National Disaster Survey Report on Hurricane Andrew pointed out that hurricane intensity forecasts lack skill and urged scientists to "redouble their efforts to develop models and operational techniques to forecast tropical cyclone intensity changes more effectively" (Fitzpatrick 1995).

One of the problems with intensity prediction is that the techniques have been based on poor intensity data bases. Guard and Lander (1997) illustrated that current tropical cyclone intensity data bases are not of sufficient accuracy for quality intensity research and technique development. A comparison of JTWC best track data with maximum intensity data obtained after-the-fact for 30 land falling TCs in Japan showed that there was a difference of about 15 kt (7 .8 m/sec) between the two maximum intensity values. Guard and Lander are in the process of developing a research-quality, confidence-based TC intensity data base for the western North Pacific (Guard 1997).

4.5.2 Operational tropical cyclone intensity techniques

Operational TC intensity techniques can be divided into several different classifications. These include: (1) statistical, (2) statistical-dynamic, (3) dynamic, and (4) synoptic/satellite. Each type has shown various levels of success, but none, to date, has shown skill at 24 hours significantly better than the Dvorak satellite intensity estimation technique. While there has been some cursory evaluation of the spectral models, most TC warning centers have not established a formal method for evaluating the ability of dynamic models to predict intensity.

• 4.5.2.1. Statistical

Statistical Hurricane Intensity Forecast (SHIFOR), developed by Jarvinen and Neumann (1979), is a simple statistical model that uses predictors derived from persistence and climatology. It is comparable to the CLIPER motion model, and is used at the National Hurricane Center in the Atlantic and in the eastern

Page I Davidson, N. E., and K. Puri, 1992: Tropical prediction using dynamical nudging, satellite-defmed convective heat sources and a cyclone bogus. Mon. Wea. Rev., 120,2501-2522.

Davidson, N. E., Y. Kurihara, T. Kato, G. Mills, and K. Puri, 1998: Dynamics and prediction ofmesoscale, extreme rain event in the Baiu front over Kyushu, Japan. Mon. Wea. Rev. (in press)

Goerss, J. S., 1997: NOGAPS 1996 tropical cyclone forecast performance. Preprints, 22st Conf. Hurr. Trop. Meteor., Miami, FL, Amer. Meteor. Soc., Boston, MA 02108,465-466.

He Zhong, Hu Situan and Chen Duo, 1996: A diagnostic prediction method on the sudden changes of the tropical cyclone track. Extended Abstract ofTenth Session ofNational Workshop on Tropical Cyclones, Chinese Academy of Meteorological Sciences (in Chinese with English abstract), 22-28.

Heming, J. T., J. C. L. Chan, and A. M. Radford, 1995. A new scheme for the initialisation of tropical cyclones in the UK Meteorological Office Global Model. Meteorological Applications, 2, 171-184.

Jiang Leyi and Feiliang, 1996: An artificial intelligence forecasting system about tropical cyclone track. Extended Abstract ofTenth Session ofNational Workshop on Tropical Cyclones, Chinese Academy of Meteorological Sciences (in Chinese with English abstract), 248-250.

JMA, 1997. Outline of the operational numerical weather prediction at the Japan Meteorological Agency. Appendix to Progress Report on Numerical Weather Prediction. Numerical Prediction Division, JMA.

LeMarshall, J. F., L. M. Leslie, and A. F. Bennet, 1996: Tropical cyclone BETI- an example ofthe benefits of assimilating hourly satellite wind data. Aust. Met. Mag., 45,275-279.

Kurihara, Y., R. E. Tuleya, and M. A. Bender, 1997. The GFDL Hurricane Prediction System and its performance in the 1995 hurricane season. Mon. Wea. Rev. (accepted)

Mao Shaorong and He Zhong, 1996: Fuzzy probability forecasting method for TC tracks. Extended Abstract ofTenth Session ofNational Workshop on Tropical Cyclones, Chinese Academy of Meteorological Sciences (in Chinese with English abstract), 108-1 09.

Morrison, R. P., L. M. Leslie, and G. D. Hess, 1996: Development of tropical cyclone landfall strike probabilities using Monte Carlo simulations. Preprints, 11th Conf Num. Wea. Pred, Norfolk, VA, Amer. Meteor. Soc., Boston MA 02108, 414~415. Peak, J. E., L. E. Carr, III, and R. L. Elsberry, 1997: Systematic approach to tropical cyclone track forecasting: Development of a prototype expert system. Preprints, 22st Conf Hurr. Trop. Meteor., Ft. Collins, CO, Amer. Meteor. Soc., Boston, MA 02108,465-466.

Puri, K., G. Dietachmayer, G. Mills, N. E. Davidson, R. Bowen, L. Logan, andL. M. Leslie, 1997: The BMRC Limited Area Prediction System, Aust. Met. Meg. (in press)

Rennick, M. A., 1998: GFDN performance in NWPAC during 1996. Mon. Wea. Rev. (submitted)

Wang Xiaohong and Sun Songqing, 1995: The tropical cyclone observing and forecasting workstation. Atmospheric Science Research and Application (in Chinese with English abstract), I, 108-112.

Xue Zongyuan and Li Zuofeng, 1995: A statistical dynamical model (SD-95) for the prediction of tropical cyclone track and its test. Atmospheric Science Research and Application (in Chinese with English abstract), 1, 59-64.

Zhong Yuan et al., 1996: Comprehensive discriminatory model for predicting medium range tendency ofTC track over West Pacific. Extended Abstract ojTenth Session ofNational Workshop on Tropical Cyclones, Chinese Academy of Meteorological Sciences (in Chinese with English abstract).

Zhou Zengkai et al., 1996: An artificial nerve network TC prediction system. Extended Abstract ofTenth Session of National Workshop on Tropical Cyclones, Chinese Academy of Meteorological Sciences (in Chinese with English abstract), 225-229. employs a "model traits" knowledge base that identifies which objective TC track forecast models tend to perform well or poorly according to the meteorological situation (Carr et al. 1997). The expert system includes objective algorithms to help the forecaster identify the best classification for the meteorological situation including detection of different modes ofTC interaction (Carr and Elsberry 1998),

4.4.6 Roadblocks and Opportunities

With regard to improved dynamical TC track prediction, the most frequently cited roadblock was the need for better initial representation of the environment and vortex of the TC. New and tailored sources of data such as high resolution cloud/vapor winds and targeted observations via manned or unmanned aerial reconnaissance, combined with implementation of variation data assimilation techniques, represent a significant opportunity to improve the representation of environment in operational models.

Improved representation of the TC vortex represents probably the toughest roadblock. Even if new sources of aerial reconnaissance can adequately sample (and what is adequate?) the structure of the real TC, limitations in model resolution and insufficiently accurate representation of physical processes may still tend to generate track forecast errors in spite of (and perhaps partly as a result of) an accurate representation in the model of the structure of the real TC. Furthermore, basic understanding of the processes controlling and changing the outer structure ofTCs is very incomplete, making it difficult to judge how well models represent TC structure and structure change. There was a clear consensus among the working group members that a quantum jump in TC track forecast accuracy occurred when TC synthetic observations of some kind were incorporated into numerical TC prediction models. A key question is whether another quantum jump can be achieved by refming the accuracy with which the synthetic observations represent the structure of the TC, particularly on a case by case basis. For example, will model errors in maintaining proper TC structure throughout the prediction play a greater factor in determining track forecast accuracy than improvements in the initial representation of TC structure?

Although ensemble methods represent significant opportunity to attach some measure of confidence to TC track forecasts, serious questions remain as to how best to generate ensembles for TC track forecasting (i.e., perturb the environment, and/or TC structure, and/or TC position?).

Finally, TC forecasting systems currently under development represent a significant opportunity to improve official TC track forecasts by insuring forecasters make optimal, situation-dependent utilization of available objective TC forecast guidance and other resources.

4.4.7 References

Aberson, S. D., S. J. Lord, M. D~Maria, and M. S. Tracton, 1995: Short-range ensemble forecasting ofhurricane tracks. Preprints, 21st Corif. Hurr. Trop. Meteor., Miami, FL, Amer. Meteor. Soc., Boston, MA 02108,494-496.

Cai, Yiyong, and Xia Lihua, 1996: The study and forecast on sudden change ofTC moving speed. Extended Abstract of Tenth Session ofNational Workshop on Tropical Cyclones, Chinese Academy ofMeteorological Sciences (in Chinese with English abstract), 101-104.

Carr, L. E., Ill, and R. L. Elsberry, 1994: Systematic and integrated approach to tropical cyclone track forecasting. Part I. Approach overview and description of meteorological basis. Tech. Rep. NPS-MR-94-002, Naval Postgraduate School, Monterey, CA 93943-5114, 273 pp.

Carr, L. E., Ill, and R. L. Elsberry, 1998: Objective Diagnosis of binary tropical cyclone interactions for the western North Pacific basin. Mon. Wea. Rev. (accepted)

Carr, L. E., Ill, R. L. Elsberry, and M. A. Boothe, 1997: Systematic approach to tropical cyclone track forecasting: Scenario-specific model and objective guidance traits. Preprints, 22st Conf Hurr. Trop. Meteor., Ft. Collins, CO, Amer. Meteor. Soc., Boston, MA 02108,461-462.

Chan, J.C.L., and R.H.F. Kwok, 1997: A diagnostic study on the improvement in tropical cyclone motion prediction by the UK Meteorological Office Global Model. Meteorological Applications, 4, 1-9.

Cheung, K. K. W., and J. C. L. Chan, 1997: Ensemble forecasting oftropical cyclone motion using a barotropic model. Part 1: Perturbation if the environment. Mon. Wea. Rev. (submitted) developed some time ago, and do not seem to be emphasizing development of new statistical models, but rather are placing heavy reliance on numerical TC prediction. An important exception in this regard is China, which, in addition to pursuing the typical advances in numerical TC prediction reported by other working group members (e.g., increased resolution, improved parameterizations, better TC representation), is actively involved in developing new, and improving existing, statistical/empirical TC prediction models (e.g., Xue e! tal. 1995; Zhong et al. 1996). A particularly promising development is a diagnostic technique for predicting whether a TC in the South China Sea will be steady or right/left turning in the next 48 h, which achieved a prediction accuracy of 87.8% (43 of 49 cases) for TCs in 1994-1995 (He et al. 1996; Mao et al. 1996). Work to develop similar diagnostic schemes for other areas is in progress (Cai and Xia 1996).

4.4.4 Ensemble track forecasting approaches

Application of ensemble forecasting techniques to the TC track forecasting problem is a comparatively recent development. Based on inputs from the working group, ensemble TC tracks forecasts are not employed operationally anywhere as of yet, but some preliminary research work has been accomplished. The major limitation is the lack of computer power to do ensemble forecasts with sophisticated baroclinic models of sufficient resolution to adequately represent the TC. Instead, barotropic models, which are initialized and receive boundary conditions from vertically-averaged fields from operational baroclinic model analyses, must be used.

Using ensemble fields from the U.S. National Meteorological Center (NMC) global model, Aberson et al. (1995) ran the barotropic model VICBAR to sixteen-member TC track forecast ensembles. On average the ensemble mean track was found to be more accurate than any individual member, and the actual track of the TC usually fell within the ensemble envelope. However, Aberson et al. noted that in individual cases the TC track did not always lie near the greatest clustering of the ensemble tracks, and sometimes was located well outside the ensemble envelope.

Cheung and Chan ( 1997) tested bow well three different methodologies for perturbing the TC environment improved barotropic model track forecasts for TCs that occurred during the Tropical Cyclone Motion 1990 (TCM-90) field experiment. Whereas the Monte Carlo method provided no overall improvement, both the lagged-average and breeding of growing modes methods of generating ensembles resulted in "improved" ensemble mean track forecasts in about 40% of the cases. Cbeung and Chan found that track forecast improvements tended to occur when the TC environment was undergoing significant evolution, including transitions in environment pattern, rapid changes in subtropical ridge strength, recurvature, and multiple TC situations.

Work is also in progress to use ensemble techniques to provide a measure of landfall probability related to the spread of the ensemble envelope (Morrison et al. 1996).

4.4.5 Forecasting systems

The basic objective of TC forecasting systems is to provide a vehicle by which objective TC forecasts from dynamical or statistic models may be combined with the subjective interpretative and conceptualizing power of the human mind to produce the best possible official TC forecast. Such forecasting systems may employ an artificial intelligence or expert system approach, or a combination of both. In the artificial intelligence approach the machine "learns" to make better forecasts via programming that take into account past human knowledge of important parameters. In the expert system approach, the machine "assists" the human forecaster to expertly synthesize from objectively-generated TC track forecasts and numerical model field an improved official TC track forecast as well as a conceptual basis for the forecast (i.e., a meteorological picture).

In China significant progress is reported in the development and operational implementation of a TC track forecasting and warning system that uses artificial intelligence techniques (Wang et al. 1995). Elements of the system include use of trainable neural networks to incorporate the subjective experience of TC forecasters and other experts (Zhou et al. 1996), and automatic pattern recognition techniques to locate centers/axes of synoptic features in ECMWF global model fields (Jiang et al. 1996), which are then used as inputs into statistical TC track prediction models. Of the all the techniques used by the Chinese in 1994-1995, the artificial intelligence approach produced the lowest TC track forecast errors at 24 and 48 b.

A TC track forecasting expert system is being developed for forecasters at the Joint Typhoon Warning Center (JTWC), Guam (Peak et al. 1997) based on the "systematic approach" concept conceived by Carr and Elsberry (1994). The expert system assists forecasters in classifying the current meteorological scenario based on a "meteorological" knowledge base that explains TC motion in terms of environmental steering and changes thereto via normal environment evolution and various TC-environment interactions. The expert system also 4.4 Track Prediction Techniques- Les Carr

4.4. 1 Introduction

Not surprisingly, advances in dynamical (i.e., numerical) TC track prediction make up the majority of this report. However, interesting progress is also reported in statistical/ empirical forecast models, application of ensemble methods to TC track forecasting, and the development of man-machine interactive TC track forecasting systems. Because important work in TC prediction is not always documented in refereed journals with international distribution, the accomplishments presented below are identified by the representative country, so as to facilitate the reader contacting the appropriate member of the working group (listed above) if additional information or discussion is desired.

4.4.2 Dynamical track prediction models

In the Australian region, progress is reported in dynamical TC prediction using the Limited Area Prediction System (LAPS, see Puri et al. 1997 for overview). Significant improvements to LAPS include: (i) incorporation of synthetic TC data that includes beta-gyre asymmetries needed to insure proper initial motion using the same approach as Japan (see below); (ii) adding adiabatic initialization component that replaces the heating from moist processes by an imposed heating based on cloud patterns in satellite imagery (Davidson and Puri 1992, Davidson et al. 1998); and (iii) an ongoing effort to incorporate local read-out TOVS and hourly cloud drift and water vapor winds into the data assimilation process that is already beginning to show favorable results (LeMarshall et al. 1996).

After the installation of a new and more powerful computer system in March 1996, the Japan Meteorological Agency (JMA) reports significant improvement in the accuracy of TC track forecasts made by their Global Spectral Model (GSM), and modest improvement in the accuracy of the regional Typhoon Model (TYM). A strong poleward bias previously present in both models is now absent. Interestingly, the accuracy of the GSM exceeded the TYM during March to December 1996. The improvement of GSM is attributed to increased model resolution (now T213/L30), implementation of an Arakawa-Schubert-type cumulus parameterization scheme, and introduction of an asymmetric typhoon bogusing technique that retrieves asymmetries associated with TC motion from the GSM first guess fields (JMA, 1997).

The UK reports improvement on the order of 25% in the accuracy of TC tracks forecast made by their global model. The improvement is attributed mainly to the implementation of an improved TC bogus scheme that included changes to the radial dependence of the tangential wind field and TC asymmetries (Heming et al. 1995). Chan and Kwok (1997) investigated the possible reasons for the improvements and found that better initial specification of TC position, asymmetry-generating steering flow, and vortex structure all were contributing factors. Accurate specification of the steering flow was considered to be the most crucial factor.

A significant development in operational TC track prediction in the United States in recent years has been the implementation of the Geophysical Fluid Dynamics Laboratory (GFDL) region TC prediction system at the National Centers for Environmental Prediction (NCEP) for the Eastern North Pacific and North Atlantic regions, and at the Fleet Numerical Meteorology and Oceanography Center (FNMOC) for the western North Pacific, South Pacific, and Indian Oceans. Kurihara et al. (1997) documents the GFDL system used at NCEP and reports that GFDL TC track forecast errors were substantially lower than other models available to National Hurricane Center (NHC) forecasters for the 1995 hurricane season in both the Atlantic and Eastern North Pacific basins. Rennick (1998) found that the performance of the FMNOC implementation of the GFDL system (called GFDN) in the western North Pacific for all of 1996 was comparable the Navy Operational Global Atmospheric Predicti! on System (NOGAPS). However, significant differences in the skill of the two models were noted in individual cases, which agrees with the analysis of Goerss ( 1997) that identified sets of TCs for which GFDN made better track forecasts than NOGAPS, and vice versa. As a result, both Rennick and Goerss emphasize the need for more detailed analysis to determine if the models consistently perform well or poorly in particular synoptic situations.

Along with other progress in numerical TC prediction, a significant development in Korea has been the incorporation of the GFDL TC prediction system by Korean Meteorological Administration (KMA) .

. 4.4.3 Statistical/empirical track prediction models

Despite heavy reliance on numerical TC prediction models, all TC prediction centers continue to employ some statistical track prediction models ranging in complexity from basic climatology and persistence (CLIPER) to very sophisticated statistical-dynamic models that employ as predictors parameters obtained from numerical prediction model fields. Based on input from group members, most prediction centers are using models that were 4.3 .6 Bibliography

Callaghan, J. and R. K. Smith, 1998: The relationship between surface wind speeds and central pressure in tropical cyclones. Aust. Meteor. Mag., (to appear).

Carr, L. E., III, and R. L. Elsberry, 1997: Models of tropical cyclone wind distribution and beta-effect propagation for application to tropical cyclone track forecasting. Mon. Wea. Rev., 125, 3190-3209.

De Maria, M., S.D. Aberson, and K.V. Ooyama, 1992: A nested spectral model for hurricane track forecasting. Mon. Wea. Rev., 120, 1628-1640.

Depperman, R.C.E., 1947: Notes on the origin and structure of Philippine typhoons. Bull Amer. Met. Soc., 29, 399-404.

Holland, G.J., 1980: An analytic model of the wind and pressure profiles in hurricanes. Mon. Wea. Rev., 108, 1212-1218.

Kaplan, J. and M. De Maria, 1995: A simple empirical model for predicting the decay of tropical cyclone winds after landfall. Jnl. Appl. Met., 34, 2499-2512.

Lovell K.F., 1990: Review of tropical cyclone wind and pressure models. Master of Engineering Studies Thesis, Dept Civil Engineering, The University ofWestem Australia, Perth.

Merrill, R. T., 1984: A comparison oflarge and small tropical cyclone. Mon. Wea. Rev., 112, 1408- 1418

Schloemer, R.W., 1954: Analysis and synthesis ofhurricane wind patterns over Lake Okeechobee, Florida. US Weather Bureau, March.

Thompson, E.F. and V.J. Cardone, 1996: Practical modelling ofhurricane surface wind fields. Journal of Waterways, Port, Coastal and Ocean Engineering, ASCE, Vol122, No. 4, 195-205. where dpi is the pressure anomaly for the ith component, Ri and bi similarly, so that

with (pn-Pc) and a value of dp1 being needed to specify a value for a second dp2 etc. Encouraging results fitted against the Hurricane Gilbert flight-level wind profiles are presented for comparison.

Callaghan and Smith (1997) present a re-examination of the possible relationships between V max' storm size and translation speed using a number of case histories and a theoretical development based on an idealised profile. Their conclusion is that large, intense tropical cyclones appear to have lower central pressures than small cyclones of similar intensity, especially when in the midst of a concentric eye cycle. In association with this they further consider the possible form of a radial pressure profile where there may be a region of dead calm in the inner part of the vortex core.

Carr and Elsberry (1997) have recently developed a tangential wind formula that is based on the principle of conservation of absolute angular momentum and which takes account of frictional dissipation. Their study was concerned with basic numerical modeling of how the circulation of the tropical cyclone interacts with its environment so as to affect its motion. Initialization of their numerical model requires a reasonably realistic radial profile of the storm tangential winds. Their formula has the form: v (r) =(M/r"X) - 0.5 f r"2 (5) involving the relative angular momentum conservation effect, and a term to account for earth rotation via the Coriolis parameter. The first term (which by itself would give a Modified Rankine vortex tangential wind distribution) contains a parameter X (positive and less than one) that accounts for frictional dissipation. X is held constant for convenience, but in reality it probably decreases with increasing radius. The constant M is determined by using the boundary condition: v (r=Ro) = 0 (6) where Ro is the average radius at which the tangential winds are zero, being a measure of the radial extent or size of the storm. This cannot be determined directly for real storms, but could be obtained indirectly via knowledge of tangential wind at some other radius. Substituting (6) into (5) gives the following formula for M:

M= 0.5 f Ro"(l +X) (7)

The presence of the Coriolis parameter (f) implies that the distribution ofouter tangential winds should depend on latitude. They offer an observational study by Merrill (1984; see his Fig. 12; i.e., large hurricane north versus large hurricane south) provides provisional observational support for this aspect of the wind distribution model. It should be noted that the purpose of their formula is to represent the structure of the tropical cyclone in the free atmosphere, and adjustments would have to be made to adapt the formula for possible use in the boundary layer or to include asymmetric effects.

Kaplan and De Maria (1995) present a simple empirical model for estimating the filling rate of landfalling storms, arguably the first comprehensive update for more than 10 years. Their method predicts wind speed decay directly without inference from a pressure decay and also accounts for distance traveled over land. Results are presented for the US Gulf and Atlantic Coasts.

4.3.5 Summary

There appear to be many opportunities to gather together a more comprehensive parameterised description of tropical cyclones than has been attempted to date. A multi-faceted model which could address aspects of near-surface wind and pressure profiles both before, during and after landfall appears feasible and could be used to form a new standardised reference. This would provide the opportunity for a more consistent usage in many different applications and provide a framework to identify areas of weakness for future research effort. Accordingly a comprehensive model should conceivably address the following application-specific features list, recognising that several of the following items are intrinsically related:

(a) Open Ocean Conditions

• axisymmetric wind and pressure fields • frictional at the surface • wind speed versus central pressure relationships • size versus intensity • forward speed asymmetry • short term track variability • atmospheric boundary layer, mean and gust profiles • ocean surface responses • concentric eye formation, outer core structures • evolutionary developments (initiation, maturity, decay) • synoptic scale interactions • SST influences

(b) Landfall

• surface roughness changes • asymmetry changes • topographic influences • thermodynamic changes • atmospheric boundary layer, mean and gust profiles, decoupling etc. • convective elements (e.g. mesoscale vortices, tornadoes)

(c) Over Land

• decay mechanisms • potential reintensification mechanisms

In addition to the deterministic features list above, efforts should be directed towards definition of stochastic variable ranges applicable to specific ocean basins derived from climatological studies. Those essentially stochastic inputs should also be recognised in the representation of transient convective processes within the mean wind field reference.

4.3 4 Some Contemporary Developments

Future parametric model improvements will likely have their roots in a large number of existing observational and theoretical studies. Just such an example in terms of basic windfield structure comes from De Maria et al (1992) who recently proposed a model of the form:

V(r) =Vmax (r/R) exp[ (1-(r/R)"b)/b] (1) where b is a profile shape parameter, 0.2 < b < 0.8. This profile, like Holland (1980), provides a basic continuous wind speed function for which, because of cyclostrophic balance with a modified Schloemer pressure profile, there is no specific requirement to know the radius of maximum winds R. The profiles which can be derived generally fit within the same envelope as that of Holland, where V maxis again given by:

As outlined by Lovell (1990), a large number of modifications to basic parametric models has occurred in the ocean engineering field, driven by the demand to provide design criteria for offshore oil and structures, many in increasingly deeper water and more remote parts of the globe. Stripping away the various re-formulations and sometimes misquoted pedigrees of many of these models reveals a basic reliance still on the essential Schloemer formulation and, in later years, Holland. A recent contribution from this field by Thompson and Cardone (1996) extends the use of parametric descriptions of the surface pressure field based on Holland to incorporate a broad or double wind maxima as follows: 4.3 Parametric Wind Fields - Bruce Harper

4.3.1 Introduction

Parametric descriptions of the lower tropospheric wind profile in tropical cyclones have proven extremely valuable for a variety of applications since they were initially proposed some 50 years ago (e.g. Depperman (1947), Schloemer(1954)). Their relative simplicity has provided a significant utility in application and has prompted their extension to include a variety of additional dynamical effects.

Typical of the applications for such parametric models are:

• idealisation of structure and behaviour • forecasting • hindcasting • initialisation of storm steering models • numerical and statistical modeling generally • public education

Developments over time have resulted in such models furnishing simplified descriptions of the essential wind (and pressure) profiles to provide spatial and temporal surface forcing for incorporation into a variety of "downstream" engineering, numerical modeling and risk assessment studies such as

Parametric models are necessarily simplified and largely empirical in order to enable a broad description of the primary processes. They can but represent a consensus of the mean features of tropical cyclone behaviour based on accepted physics and observational data. However, their empiricism also provides a free

• offshore facilities wind, wave, continental shelf current and storm surge design criteria • onshore infrastructure design (e.g. ports, public housing, wind sensitive structures) • public evacuation and disaster planning (wind and storm surge) • insurance risk assessment

4 3.2 The Case for a Comprehensive Parametric Model

While various parametric model developments have occurred across a wide field (Holland (1980), Lovell (1990)), there appears no strongly formed consensus as to the single best representations which should be used. This is of course complicated by the many features which it might be expected that a comprehensive parametric niodel should address and often past efforts have typically concentrated on specific needs. It is proposed that there could be considerable benefit in working towards the identification of a "single" comprehensive and preferred parametric wind and pressure field model of tropical cyclones - acknowledging that such _a model is certain to initially be an amalgam of several existing models and theories. The benefits of such an approach are expected to include, over time:

• a common reference point for parametric modeling • a framework for adding additional features • a test bed for model sensitivities • a systematic facility for storm parameter identification and classification

A means of reducing potential confusion between various alt~mative approaches from within which hypothetical parameters can be explored, plausible dynamic ranges tested, new mechanisms developed etc. There is also no reason why differing scales of motion could not be acceptably combined in a parametric fashion. Clearly, many forecasting agencies already incorporate various elements of their preferred parameterisations in tool form. A consensus parametric model would represent a distillation of many current and evolving theories where overall progress could be measured.

4.3.3 Features of a Comprehensive Parametric Wind Field Model

As previously identified, parametric wind field models already find application in a wide range of areas where the approaches increasingly tend toward greater sophistication and search for new sensitivities to specific features. Examples include industrial aerodynamics and , spectral wave modeling, building fatigue resistance, insurance losses etc.

improving parameterisation of physical processes which are known to be crucial for improved handling of TCs in numerical models.

4.2.3. Summary

As the field of remote sensing, conceptual and numerical modeling has continued to broaden and mature, the technology to ingest, process and display these data grows in seemingly quantum jumps each year. Analogous to the medical profession, the explosive of information is leading to increased specialization and the need to rapidly reference multiple sources of data to arrive at an informed diagnosis. Thus, for the analyst, data management is a key element in selectively filtering data input to focus on what best in this particular situation. However, in contrast to doctors, meteorologists are confronted with a "hands off' diagnosis based almost entirely on estimation with little or no ground truth. This diagnosis, when coupled with an incomplete, and somewhat crude, understanding of the physical processes at work in the tropical atmosphere and within the TC itself, is a recipe for a less than perfect forecast. In comparison, the user of our warning products with high expectations and a tendency to trivialize the forecasting process- "because everyone can forecast, can't they?"­ is only one small step from concluding that the greatest threat is not from the TC itself, but from our inability to accurately portray the event that is unfolding. Remember, for him "the forecast is our only product." convection and boundary layer processes, and of course considerable increases in model resolution.

Since we are only concerned with analysis of tropical cyclones the following points need to be considered (note that since current NWP systems use data assimilation the analysis and model are closely connected as models provide the first guess)-

• 4.2.2.1. Bogus

The issue of whether or not to use bogus data is not totally resolved. Most Centres (BMRC, NCEP, UKMO, JMA) use TC bogus data while ECMWF do not. As I indicated above the 1996 season study shows that using bogus data does not give much advantage beyond 12h. Models with (JMA, UKMO) and without (ECMWF) bogus have very similar track errors. However I have cases for the Australian region with little conventional data where the use of bogus data is crucial (see for example Puri and Mills, 1997, J. Met. Soc. Japan, Vol 75, No lB, 395-413). I am sure other Centres also have examples.

Once a decision has been made to use bogus data, other issues arise such as removing a pre-existing vortex from the first guess, what profile (horizontal and vertical) to use in generating the bogus data. Considerable progress has however been made (see Davidson et al., 1992, J. Met. Soc. Japan, 71, 437-467 and references therein). The insertion of bogus data or vortex into a numerical model has to be done very carefully in order to maintain the required balances. The Kurihara model is a prime example of the care that is needed and the benefits of this. At BMRC we use dynamical nudging to maintain tropical cyclone structure and balances in the initial conditions (Davidson and Purl, 1992, Mon. Wea. Rev., 120, 2501-2522).

• 4.2.2.2. Objective analysis

Objective analysis at high resolutions poses special challenges such as the use of appropriate structure functions, maintaining appropriate mass-wind balance which is not so clear at small scales, preventing rejection of data because of quality control constraints coupled with errors in the first guess.

• 4.2.2.3. Data bases

Improvement in the data base include: 1) Availability of high resolution water vapor winds, 2) Availability of high resolution (time and space) visible winds - work at BMRC (Le Marshall) has shown that this data has a positive impact on TC track forecasts, 3) Availability of scatterometer winds which have the potential to improve the location of TCs

• 4.2.2.4. Analysis methods

Improvement in analysis methods: 1) Direct assimilation of TOVS radiances through the use of variational methods (so called 3DVAR) - this has for example led to improvements in the ECMWF system, 2) Development of methods to include diabatic heating inferred from satellite imagery, total precipitable water from SSM/1, or rainfall data, 3) Development of 4-dimensional variational assimilation (4DVAR) is probably the most important development in data assimilation for several years which has great potential as far as TC track forecasts are concerned. This method is very expensive to use but the current generation of supercomputers make it feasible for operational use. For example ECMWF plan to implement it operationally towards end-November or early December 1997.

• 4.2.2.5. Numerical models

As mentioned above the numerical model is intimately connected with analysis through data assimilation. Thus improved models will have an impact on analysis which could be significant for TCs because improved handling of cyclones by models will result in an improved first guess. Some of the ongoing model improvements include: 1) Increased model resolution - both horizontal and vertical, 2) Improvement in parameterisation physical processes particularly convection, treatment of clouds with many models now including prognostic cloud variables, 3) improved treatment of boundary layer processes, use of more detailed land-surface schemes which include vegetation types, root zones and stomatal resistance, 4) Improved (more accurate) numerics which can be important for retention and advection of cyclones.

• 4.2.2.6. Roadblocks

I would say that the major roadblock still is lack of observations (in spite of the progress made in remote sensing). This leads to problems in analyzing TCs and is a major stumbling block in increasing our understanding of various aspects of TCs. Moreover without appropriate data, we cannot make progress in 4.2 Analysis- Frank Wells

4.2.1 Conventional Analysis

Since Gary Foley penned Chapter 1, Observations and Analysis of tropical cyclones (TCs) in Global Perspectives on TCs (1995), synoptic understanding has benefited both from advances in the field of remote sensing -- water vapor winds for the mid and upper troposphere and visual cloud track winds in the low troposphere, multispectral imaging and scatterometer -- and, in the area of conceptual modeling -- the Systematic and Integrated Approach to TC tracking forecasting and introduction of general concepts of the reverse-oriented monsoon trough and monsoon gyre.

Roadblocks for analysis are: 1) a lack of conventional observations to 'ground truth' estimates derived from remote sensing platforms, aggravated by the fact that not all conventional data are disseminated, 2) an incomplete physical understanding and ability to conceptually differentiate between weather systems in the tropics and subtropics that have somewhat different structures than TCs, such as hybrids, monsoon gyres, monsoon depressions, subtropical cyclones, and cyclones transitioning from tropical to subtropical or extratropical, and 3) a related topic, a definition of TC genesis that operationally doesn't go much beyond the observation that, 11 a TC has appeared on the satellite imagery. 11

Although there are roadblocks, opportunities for increased understanding abound in: 1) applying pattern recognition techniques to satellite microwave and multispectral imagery, 2) tailoring TC track to the Systematic and Integrated Approach to TC Track Forecasting, 3) developing knowledge bases to gauge the sensitivities of numerical and objective guidance to data inputs, boguses of position, intensity, size, structure, and structure change, 4) determining the role of convective elements within the , such as (Stewart, et al, 1997) in eye intensification, rate of intensification and eye development, 5) improving the analysis of the wind distributions associated with hybrid-type (partially tropical) cyclones and TCs transitioning to extratropical cyclones where hurricane-force winds on the poleward flank of a category-! cyclones have in the past led to disastrous forecast failures (Callaghan, personal communication), 6) constructing algorithms that can be applied to Doppler radar data to convert radial velocities in the curvilinear flow associated with TCs to wind, 7) bettering remote sensing (radar and satellite) algorithms/techniques for positioning and intensity in areas where ground truth measurements continue to be available, such as in the North Atlantic basin, 8) working with the objectively determined digital Dvorak to improve the subjectively determined intensities, 9) reexarnining the Dvorak technique's intensity-to-wind-speed conversion and intensity-to-minimum-sea-level-pressure conversion as they relate to TC size, and seasonal variation/, 10) developing a Dvorak-type intensity estimation technique for use with microwave satellite imagery, 11) determining what intensity as applied to TCs applies to peripheral gale- and storm-force winds associated with monsoon gyres, depressions and hybrids, 12) applying excess along-track translation speed to the Dvorak technique, 13) improving intensity estimation from remotely sensed data when TCs are overland, 14) refining the relationship between maximum surface winds (intensity) and minimum sea-level pressure, 15) making more accurate scatterometer algorithms to eliminate 90°/180° directional ambiguities, adjusting for rain contamination and edge/land effects, 16) using multispectral data sets to prevent 'sunrise surprises' where the first visual image of morning detects a low-level circulation center that has moved out from under the resulting in a relocated warning, and 17) developing the use ofthe analysis contours poleward of the TC to help the forecast even though the numerical model has mishandled the cyclone in the low levels (Callaghan, personal communication).

Data assimilation and management also provides unique challenges as well as opportunities for improvement such as: 1) how much data is enough? 2) maintaining at least 24 hours of imagery to execute the Dvorak intensity estimation technique, 3) weighting imagery resolution against archive size, 4) accessing raw data to quality control the numerical fields, 5) obtaining web page stored imagery to supplement the real-time direct transmissions of remotely sensed data over the area of interest, and 6) combining the data, analysis, forecasting function under one individual or assigning them to specialists on the analysis/forecasting team.

4.2.2. Numerical Analysis (this section was kindly provided by K. Puri)

Very rapid progress has been made in numerical weather prediction in various aspects of tropical cyclone prediction. Global and high resolution limited area models are now easily outperforming the empirical methods such as CLIPER and statistical models beyond 12h. For example a detailed evaluation of all cyclones in the Pacific for 1996 season show that the global models from ECMWF, JMA and UKMO had distance errors of around 150km, 200km and 350km respectively at 24h, 48h and 72h. The use of the GFDL model operationally by NCEP (and the National Hurricane Centre) has been very successful. These improvements have been the result of considerable progress in analysis techniques and parameterization of physical processes particularly scheduled to be launched in 1998.

During 1995, the European Space Agency launched Earth Research Satellite2 (ERS-2) to join ERS-1. Both these polar orbiting satellites fly a scatterometer, which is capable of measuring near-surface wind speed and direction via capillary wave action on the ocean surface. The winds have been shown to be accurate between 3 and 25m/s with arms error of 1-2m/s R. Edson (1997).11 has proved ideal in mapping the environmental wind field, in particular radius of gale force winds. Scatterometer data are displayed in 500km swathes at a resolution of 50km. Unlike the SSMII, it is less sensitive to rainfall at the 14 GHz frequency. During 1996, the NASA scatterometer (NSCAT) was launched on-board the Japanese Advanced Earth Observing satellite (ADEOS) and is able to provide data over a swathe width of 1200 km (600 km on either side, with a 400 km blank in between) with a potential resolution of 25 km.

The NSCAT information became operational for a time during 1997 and was used operationally by the Joint Typhoon Warning Centre (JTWC) before a power problem on board the spacecraft put an end to this additional source of wind and rain data. According to Edson, the scatterometer has helped to improve JTWC's ability to depict the gale areas surrounding the tropical cyclone and the maximum intensity of systems below hurricane intensity. NASA is hoping to put up a replacement satellite for NSCAT during 1998. The demise ofNSCATwill cut in half the available scatterometer data and undoubtedly have a bearing on how well the NOGAPS model is able to depict the environmental wind field. Scatterometer data also produce incorrect wind directions (winds 180 degrees out ofphase)and these aliased winds will have to be dealt with in model data assimilation schemes before they can be successfully incorporated into model analyses and first guess fields (Hawkins and Helveston, 1997).

The NASA Tropical Rainfall Measurement Mission (TRMM) satellite has been launched and soon will be routinely providing multi-spectral measurements over the tropics for research interests. These TRMM sensors will include both passive and active measurements.

• 4.1.5.3 Radar Altimeters

Satellite radar altimeters are active microwave radars, which transmit and measure the return radar pulse from the ocean/land surface below. Information from the returned pulse can be used not only to determine the precise altitude of the satellite, but more importantly for the retrieval sea surface height and surface wind data. Satellite altimeters like U.S. Navy's , ERS-1, ERS-2 and Topex/Poseidon have been able to measure heights to within 3-8cm. The attenuation caused by heavy rain places limitations on the altimeter being able to measure sea surface height in a tropical cyclone.

Satellite altimeters can also measure the ocean surface wind speed. The range of wind speeds typically ranges from 3-20 m/s, with few verifications available for wind speeds in excess of20 m/s (Hawkins, 1997). Radar altimetry on board the NASA TOPEX satellite during hurricane Ernily was able to detect a sea surface depression called a barotropic trough (Shay and Chang, 1997), in the back of the eye, of about 12 to 16 cm. The same satellite radar altimeter was also able to detect a warm core anomaly over which hurricane Opal deepened from 965 hPa to 916 hPa in 14 hours.

4.1 6 Acknowledgments

I am indebted for the support and sharing of expertise by Chris Velden, Nick Shay, Jeff Hawkins, Kendall McGuffie, Lixion Avila and S. Kalsi, all members of the Working Group. Without it, this document would never have been possible. My thanks also go out to Kerry Emanuel, Greg Holland and Frank Andrews for their contributions, which have helped to fill in some of the gaps that would otherwise have been left. twelve channels is due for launching. The additional channels should open up opportunities for tropical cyclone applications as well as having access to imagery more frequently than every 30 minutes.

The availability of a 6.7mm WV channel on nearly all geostationary satellites has now made it possible for a global coverage of high density WV wind data (Velden et al. 1997). The extraction of useful wind vectors from sequential WV imagery is dependent on atmospheric moisture structure. The higher precision and resolution WV sensors combined with optimized data processing strategies has resulted in a major advance in the observation of winds in the upper-levels of the troposphere in non-cloudy regimes. The spatial coverage provided by the WV winds has dramatical1y increased the analyst's ability to define the TC's upper-level environment. In fact, the JTWC recently remarked that "the WV winds have revolutionized the analysis of the upper-level wind fields over the West Pacific". The WV winds complement the wind vector information provided by tracking clouds in IR and VIS channels. Promising research is exploring the use of additional water vapor channels (7.0-7.3mm on GOES) to obtain mid-tropospheric wind information (Velden et al. 1997).

The U.S. Navy has been assimilating near real-time, high-density GOES and GMS WV winds, IR and VIS cloud-tracked winds since July 1996 into the Navy Operational Global Assimilation and Prediction System (NOGAPS) model (Goerss, et. al., 1996a, b, 1998). These data have been developed and provided in a demonstration mode by the University of Wisconsin Madison- Cooperative Institute from Meteorological Satellite Studies (UW-CIMSS). The winds were assimilated operationally after sensitivity tests with NOGAPS revealed a positive impact with respect to 72 hour tropical cyclone track forecasts. The European Centre for Medium Range Weather Forecasting (ECMWF) and the U.S. National Centers for Environmental Prediction (NCEP) are currently evaluating the impact of these new high density wind observations. The European Space Agency Meteorological Satellite Agency (EUMETSAT) is also producing high density WV, IR and VIS wind vectors from . The ECMWF global assimilation system is currently ingesting the data (including WV wind vectors above 400hPa) while NCEP is planning to do so in the near future. The Japanese Meteorological Agency (JMA) is experimenting with WV froni GMS~5 at the time of writing this report (it wasn't clear whether these winds were being assimilated into the Japanese models). The extraction of useful WV wind vectors is very dependent on moisture availability and contrast, vertical shear and features like precision and resolution connected with the radiometer Veldon et al. (1997).

Fig 1 GLOBAL GEOSTATIONARY SATELLITE COVERAGE except for INSAT.

• 4.1.5.2 Polar Orbiting Satellites

The current U.S. TIROS series polar orbiting satellites, NOAA 12 and NOAA 14 have been around since May 1991 and December 1994 respectively. Both work in tandem such that no region is without data for more than 6 hours. The Russian Federation Meteor 2-21 and Meteor 3-5 provide WEFAX images in a similar way.

The Special Sensor Microwavellmager (SSMJI) is the instrument that measures reflected and emitted microwave radiation at frequencies 19.4, 22.2, 37.0 and 85.0 GHz on the U. S. Defense Meteorological Satellite Program (DMSP) satellites. It produces wind speed and rainfall data over a 1400 km swathe width and operates very effectively for tropical cyclone with maximum winds between 30 and 50 knots before the data is contaminated by heavy precipitation or very high water vapour content. The microwave radiation is able to penetrate through the non-raining clouds to reveal the convective structure, including double eyes (concentric eyewalls) associated with tropical cyclone intensity. Rainfall rates can be estimated from the brightness temperatures. M. Peng and S. Chang (1996) found that the assimilation of SSM/1 retrieved rainfall rates reduced the average 48-hour forecast distance error by more than 60% in the five cases that they studied. They incorporated the rainfall rates within 8 degree latitude of the cyclone into the initial fields by a reversed Kuo cumulus parameterization. E. Rodgers and H. Pierce (1995) studies for western North Pacific cyclones corroborated what had been found in the western North Atlantic that the more intense cyclones had higher rain rates close to the centre and a greater areal distribution of rain. Historical data from the SSMII 85GHz channel has also been used to develop algorithms for automating the satellite image analysis currently done manually by following the Dvorak procedures. The current state-of-the-art using automated algorithms to estimate TC intensity from SSMJI data is designed to operate with a root mean square error of 15 knots using computer vision techniques.

Merrill (1995) is fine tuning an algorithm to extract NOAA Microwave Sounding Unit (MSU) and DMSP SSM!T 550Hz data to observe the upper tropospheric warm anomaly (Velden 1989; Velden et al. 1991). There are difficulties with capturing the complete extent of the warm anomaly with this method due to limited instrument resolution and contaminated signature from scattering by liquid water and wet ice. The goal is to re-examine the utility of this technique by employing data from the advanced MSU, which is this key area might not get the attention it deserves (Shay, 1997). Insufficient data sets are also likely to hamper studies of the air-sea interface. Research opportunities to examine air-sea interactions have the greatest potential to succeed if they are linked to ongoing programs at the Hurricane Research Division of NOAA. Remotely sensing the surface currents from OTH radar, surface topography from the TOPEX satellite, and surface winds and stresses from scatterometers and SSM/1 (see under Satellite Observations) at the same time would greatly enhance researchers' ability to examine air-sea interactions on the large scale.

• 4.1.4.2 WSR-88D Doppler Weather Radars (NEXRAD)

NEXRAD is a system of state-of-the art 10cm weather surveillance radars(WSR-88D) which are now available outside the mainland USA and U.S. territories and are capable of producing high quality wind data when the eye falls within its 230km range. One of these radars monitored typhoon Herb (1996) as it was approaching northern Taiwan and its display showed a maximum Doppler velocity in the-eyewall of 82 rnfs which is the highest wind ever recorded by a Doppler radarWen-Chau Lee (1997).

The accumulated hourly rainfall images produce a swath of echoes which has helped to define the radius of curvature of the low level tropical cyclone circulation. This has provided invaluable information to the analyst when the centre of a weak or poorly defined tropical cyclone has proved troublesome to locate. According to (Willis et al. 1997), WSR-88D radars appear to underestimate convective rainfall at moderate to high rainfall rates.

Experience with Doppler radars has highlight the following -Doppler wind velocities at 2000 and 3000 feet are a better indicator of the gust potential at the surface than the winds above these levels where the relationship is more tenuous. Remember to double the velocity increment before having to measure winds in excess of64 knots, say, which enables wind speed observations in excess this figure.

Increasing Pulse Repetition Frequency (PRF) increases the maximum ambiguous doppler wind velocity but does lead to range folding which may obscure echoes of real interest. tune the precipitation algorithm for tropical rainfall rates. This is one of the objectives of the NASA Tropical Rainfall Measuring Mission (TRMM). Need to derive appropriate tropical cyclone algorithms for displaying wind vectors in place of radial wind velocities.

4 1.5 Satellite Observations

Satellite imagery (now multispectral) continues to provide important information to TC forecasters and researchers. For example, the Dvorak analysis technique is used globally and continues to be the primary tool for TC intensity and structure analysis. Recent studies have advanced the technique to a purely objective (computer-based) level with good results (Zehr, 1989; Velden et al. 1998). The current challenge is to convert information-wealthy imagery into quantitative data of a form that can be used by numerical models to improve their performance.

• 4.1.5.1 Geostationary satellites

.There are currently seven geostationary meteorological satellites in geosynchronous orbit viz Japanese Geostationary Meteorological Satellite (GMS-5), U.S. Geostationary Operational Environmental Satellites (GOES-8 or GOES-E and GOES-9 or GOES-W), the European Space Agency METEOSAT-6, the Indian INSAT, the Chinese Feng Yun-2 (FY-2) and the Russian Federation's Geostationary Operational Meteorological Satellite (GOMS). FY-2 was launched in June 1997 at lOSE to provide coverage of Asia and the Indian Ocean but was still not operational by December 1997. Its capabilities include Visible (VIS) imagery at 1.25km resolution, and both Infrared (IR) and water vapour (WV) at Skm. GOMS located at 76E provides VIS imagery at 1.5km and IR at 7km resolution but possesses no WV channel. INSAT-11 is scheduled for launch in 1998.

METEOSAT and GOES have been equipped with advanced WV sensors to improve the spatial resolution and reduce the signal-to-noise ratio. GMS-5 has acquired a WV (6.7mm) channel for the first time and the Next INSAT satellite will have one when it is finally launched in 1998. Once METEOSAT-6 becomes fully operational the European Space Agency plan to shift METEOSAT-5 to 60E for research duty in 1998. This is good for tropical cyclone monitoring in the South Indian Ocean as the region falls between METEOSAT-6 and GMS-5 views and INSAT, GOMS and FY-2 are not yet reliably available. The METEOSAT programme will continue until 2000 when METEOSAT Second Generation (MSG-1) with Table 1. Specifications for the Operational Phase I Aerosonde

Aerosonde Specifications

Weight 14 kg

Wing Span 2.9 m

Engine 20 cc, 750 w, normally-aspirated

Aerosonde Operation

Staff Single person operation

Ground Equipment Proprietary Staging Box Personal Computer GPS Antenna

Communications Equipment: To Aerosonde To Flight Service and other

Aircraft Telephone Access

Flight Autonomous, under Base Command

Takeoff 1. Car roof rack (requiring 2 people) 2. Catapult

Landing Autonomous

Navigation GPS

Communications UHF Radio (current), LEO Satellite (available mid-1998), and HF (under investigation).

Aerosonde Performance

Range 2500 km

Endurance 30 h

Altitude Range Service Maximum 100-4000 m 20-5000 m

Aerosonde Instrumentation

Temperature, Pressure, Humidity 3 Vaisala RSS901 Sondes ?O.loC, ?0.1 hPa,

?2%

Wind Proprietary ?0.5 ms-1

4.1 .4 Radar Observations

• 4.1.4.1 Over-the-Horizon (OTH) Radar and Ocean Surface Current Radar (OSCR)

The OSCR (Shay et al. 1995,1997) provides high quality surface current data over a 30 km x 50 km domain and is ideal for determining tidal variability for problems related to storm surge. The OTH radar(Georges et al.1993; Harlan et al. 1997) operates in a similar fashion except that it is able to monitor surface currents several hundred kilometres away by intercepting electromagnetic energy reflected off the ionosphere. Both these radars worked in tandem to produce observations about the coast and over the open ocean during hurricanes Claudette and Hortense. During Hortense, the US Navy OTH radar in Texas and Virginia constructed an unambiguous map of the wind direction and the surface current vectors at 15-km resolution.

The role of the upper ocean and the ensuing air-sea interactions with tropical cyclones may be greater than once thought but unless the wider research community is prepared to acknowledge it, Laboratory- Hurricane Research Division (AOML-HRD) through Black has used these buoys to help monitor SST conditions before and after a cyclone's passage in the ongoing effort to understand the air-sea interaction.

Automatic (AWS)'s are a good option for a continual data stream provided suitable sites can be found to facilitate periodic maintenance. They are being pursued as viable option for more and more Pacific Island Countries in the tropical South Pacific while a network of twenty Automated Meteorological Observing Stations (AMOS) have been deployed within the Micronesian Islands.

After the demise of the navigational system in September 1996, a number of developing countries have had to cut back the number of rawinsonde flights from two to one per day as the cost of launching the new Global Positioning System (GPS)-equipped sondes has increased by about 50%. Budgets might allow for two flights per day during the threat of a tropical cyclone.

4.1.3 Aircraft Reconnaissance

Routine tropical cyclone reconnaissance is still only undertaken in the Atlantic and Gulf of Mexico. The Reserve (USAFR) 53 rd Squadron operates WC-130 "Hercules" aircraft with a crew of six including a reconnaissance weather officer and a dropsonde system operator. The WC-130 penetrates the hurricane at 700 hPa and a dropsonde (a cylindrically shaped instrument 5m long and 1m in diameter) is released from the rear of the aircraft and dropped down into the eye. As the instrument descends to the sea surface with the helps of a parachute, it measures and relays to the aircraft, by high frequency radio a vertical profile of temperature, humidity, pressure and wind data. Once the data is collected is relayed directly , via satellite to the NHC, Miami every 15-30 minutes. The National Ocean and Atmospheric Administration (NOAA) uses WP-3D Orions and the Gulfstream (G)-IVSP, a state-of-the-art high altitude jet which became a "hurricane hunter" during 1996. Amongst its instrumentation, each WP-3D has two remote sensors for measuring the wind vectors in and around a tropical cyclone. One is a C-band Scatterometer (C-SCAT) which conically scans the ocean surface to obtain backscattered measurements from 20 to 50 degrees off- and the other is a Stepped Frequency Microwave Radiometer (SFMR) which retrieves nadir brightness temperatures at six frequencies between 4.6 and 7.2 GHz. The instrumentation on board the G-IVSP includes a C-Band nose radar, PRT-5 downward scanning radiometer, a Global Positioning System -Precision Position System (GPS-PPS), a 9600 bits/s satellite communication link with Inmarsat and a dropwindsonde system. Dropwindsondes updated with GPS wind finding technology are capable of producing atmospheric soundings with an accuracy at least comparable with that from rawinsonde Cogan et al. (1996).

Unmanned Aerial Vehicles (UAV)s carrying small sensors and dropsondes are tipped to provide a cost effective means of aerial reconnaissance in all tropical cyclone basins and to complement the large manned aircraft Programme that currently runs in the Atlantic and Gulf of Mexico. The NASA panel called ERAST (Environmental Research using Airborne Sensing Technology) is investigating the use of recently declassified U.S. military pilotless aircraft, as well as privately manufactured UAV's, for environmental measurements. Ultra-lightweight GPS dropsondes have been successfully deployed from UAV's. During 1999, a test mission is planned out of Guam to deploy sondes in NorthWest Pacific tropical cyclones from a platform known as Altus. Altus platforms have been involved in military missions for many years, but an updated version will enable it to fly at 60,000 ft for a day or so at a time. If this field test is successful, it will usher in a new era of relatively inexpensive TC reconnaissance. ERAST has many corporate members, including Flight Sciences, which manufactures Perseus. There have been setbacks with the Perseus development, which will prevent it from being used in the 1999 experiments in the NorthWest Pacific.

Besides Altus and Perseus, there are other platforms, including the Aerosonde. The aerosonde is highly flexible and economical observing platform for a wide range of meteorological purposes, from routine soundings to severe weather reconnaissance and local environmental monitoring. The goal of the Aerosonde program is to provide temperature, pressure and humidity information at affordable cost, while maintaining much of the flexibility available from larger aircraft. The aerosonde has been endorsed by both WMO CAS (Commission for Atmospheric Sciences) and IDNDR. The development program is approaching full Phase I design level. This will be a low altitude (up to 5 km) aircraft (refer to Table 1) capable of operation in a wide range of meteorological conditions, including severe weather. The range and duration have been set at 2500 km and 30 h. Eleven Phase I aerosondes will be fully tested in an operational trial for the Australian Bureau of Meteorology off the coast of Western Australia during January and February 1998. Following this trial, Aerosondes are scheduled to operate in a range of field experiments during 1998, including: the South China Sea Monsoon Experiment (April-May); coastal flights off Hawaii, Canada, Australia and Korea throughout the year; and a typhoon program in the western North Pacific (August-September). 4.1 Observational Issues- Steve Ready

4.1.1 Introduction

The information in the following sections is supposed to update what was contained in chapter 1 of "Global Perspectives of Tropical Cyclones". Given the constraints on the task involved, specialists in the various observational areas might be aggrieved about what they perceive as missing infonnation or inadequate discussion on key issues. Hopefully, what is missed or glossed over here will provide the basis for lively discussion at IWTC-IV and enable the matter to be addressed properly before "Global Perspectives" comes out in a new edition.

The continuing developments in technology have tried to address the problems caused by tropical cyclones spending long periods of their life history over very data sparse regions. Strategic placing of moored buoys and Automatic Weather Stations (AWS) sand the release of drifting buoys will all have their day at some time or another, should a tropical cyclone decide to pass over or close by, but they do pose difficulties for retrieval or maintenance in remote locations.

Big advances have been made since the addition of water vapour channels on the current generation of geostationary satellites. Wind extraction algorithms have been developed, tested or worked on to enable the global and regional models to acquire more and more information about the cyclone environment.

Maybe the question needs to be put and a response left for IWTC-IV attendees to make- has the increase in ·satellite based technologies compensated for the loss of conventional data?

Work has progressed for the last decade or so on the development of a low cost aerial reconnaissance programme involving Unmanned Aerial Vehicles (UAV)s. Over the next two years, the aerosonde and altus observing platforms will be tested in operational trials.

The availability of Over-The-Horizon (OTH) radars, Ocean Surface Current Radars (OSCR)s, Doppler Weather radars, passive microwave sensors on polar orbiting satellites and satellite radar altimeters have helped to pinpoint the location, size and intensity of the tropical cyclone much better. There again all these platforms have restrictions on spatial coverage and how frequently observations can be made.

There is no doubt that over the past two years, remote sensing techniques have improved and now provide quantitative estimates of various fields such as surface currents, surface topography, sea surface temperatures, and surface winds as well as other physical quantities.

Futures successes seem to lie in being able to combine as much of this remotely sensed information as possible in order to come up with the best estimate of tropical cyclone intensity and the radial wind distribution (Hawkins et al. 1997)

4.1.2 Conventional Surface and Upper Air Data

The Tropical Atmosphere Ocean (TAO) project in the equatorial Pacific involves an array of 70 moored buoys positioned from 135E to 95W which measure air temperature, relative humidity, surface winds, sea surfaces temp«,!ratures and subsurface temperatures down to a depth of 500 metres. The data are relayed via the Argos system with the help of polar orbiting satellites .to the various meteorological and global model centres. Besides their climatological function to monitor parameters associated with El Nino, the TAO buoys define the meteorological conditions at the equator and highlight any cross-equatorial flow. They supplement the information provided by just a few observation sites and the low-level cloud-drift winds derived from the geostationary satellites.

While the moored buoy concept is very expensive, drifting buoys are a proven and cost effective means of supplementing observations over the oceans. A feasibility study for the tropical SouthWest Pacific was undertaken by Quay le (1997) to see whether drifting buoys would be an answer to boasting observations through the busy cyclone traffic zones. Both the Joint Typhoon Warning Centre (JWTC), Guam and the National Hurricane Centre (NHC), Miami have access to important buoy data in their areas of interest. The western Pacific Compact Meteorological and Oceanographic (CMOD) buoys report surface pressure and temperature and a smaller number of Wind Speed and Direction (WSD) buoys also report the surface wind vector in addition to temperature/pressure. In the tropical Atlantic Ocean, up to six buoys are dropped in at the beginning of each hurricane season to map pressure, temperature, sea surface temperature and wind. They have proved successful in capturing data from tropical systems tracking nearby. The Atlantic Oceanographic and Meteorological

4. Tropical Cyclone Prediction - U. Col Mark Andrews

Foreword

As evidenced by recent trends in overall tropical cyclone forecast performance "metrics", gathered at both the U.S. National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC), the job of analyzing and predicting the track, intensity, and structure of tropical cyclones is gradually improving on a year to year basis. No single attribute of the entire "process" is responsible for the improvement; instead, the combined efforts of the research and operational community, which are attacking the various segments of the entire forecast problem, are paying the big dividends.

Clearly, the ability to provide accurate warning services to the end user is the desired end state. Forecasting future positions of the center of a tropical cyclone to the general community as a whole is the way we do business· today, and how we evaluate our success. But until we can accurately forecast the center position, central intensity, and wind structure to each and every individual being affected within the community we will not have the same individual satisfaction in the forecast and warning services being provided. Most end users are only going to evaluate the success or failure of the warning based on a "point" mentality, i.e., the warning verifies as strictly a binary yes/no verification, without any gray shades in between. Our ability to forecast individual effects of a tropical system to each and every "point" should beJhe ultimate focus of the tropical cyclone warning system.

Many of the sections contained within this topic can be applied to other topic sections as well. Tropical cyclone prediction encompasses the ability to correctly sense and analyze the tropical cyclone and the total environment within the TC resides. As such, one must be able to accurately describe the initial state of the atmosphere and the tropical cyclone in significant detail, especially with respect to scale.

We must also be cognizant of the fact that while research continues to advance the science of forecasting tropical cyclones, the "product" delivered by the research community must be understood and manageable by the operational forecasters. As the director of an operational forecast center, one of my biggest challenges is ensuring the center can accept and manipulate all of the various sources of observational and forecast data in a timely manner. If the operational end of the "process" cannot use the data developed by the research community, because human factor considerations are not integrated into the overall process, then the system as a whole has failed.

That notwithstanding, the research end of the "process" has taken great strides towards bringing new information to the operational forecaster. New and exciting forecast models are being developed throughout the international forecast community.

In these days offunding shortfalls and reduced operational and research budgets, it is important to be able to maximize the benefit gained from each and every monetary unit used to provide TC warning services. Nations can no longer afford to duplicate efforts. Collaborative efforts in satellite, manned and unmanned weather reconnaissance is showing great promise.

Topics 4.1, 4.2, and 4.3 will bring the reader up to date on the latest in observation and analysis techniques available. Topics 4.4 and 4.5 discuss the techniques used to forecast track and intensity, respectively. Longer-term seasonal prediction techniques are discussed in Topic 4.6. Rainfall prediction techniques, currently identified by the end user community as a serious shortfall in warning services, is discussed in topic 4.7. Finally, topic 4.8 updates the reader on the latest in the Global Forecasters Guide.

I must take the time to thank the numerous rapporteurs, who are individually listed below, along with the individuals who participated in providing information for each topic segment. They had the hard part in developing this outline of the current state-of-the-art of the TC warning system.

Table of Contents

TOPIC FOUR

TROPICAL CYCLONE PREDICTION

4.0 Report of the Topic Chairman M. Andrews (USA)

4.1 Observationallssues Rapporteur: S. Ready (New Zealand)

4.2 Analysis Rapporteur: F. Wells (USA)

4.3 Parametric Wind Fields Rapporteur: B. Harper (Australia)

4.4 Track Prediction Technigues Rapporteur: L. Carr (USA)

4.5 Intensity Prediction Technigues Rapporteur: C. Guard (USA)

4.6 Seasonal Prediction Technigues Rapporteur: W. M. Gray (USA)

4.7 Rainfall Prediction Technigues Rapporteur: X. Xiangde (China)

4.8 Upgrade to the Forecasters Guide to Tropical Cyclone Forecasting Rapporteur: - G. Holland (Australia)

155°E 135°E 20°N 20°N

5°N~------

c:CJ

Fig. 3 Schematic representation (based upon enhanced infrared GMS imagery at 1531 UTC 03 August, 1993) of the observed cold cloudiness associated with Robyn's central dense overcast and an MCS. Solid black cloud silhouettes represent areas of coldest convective cloud tops, outer contour shows boundary of dense cirrus overcast. 150°E --t- 40°N

.4..

. .

.,.., • 0 • e •• I • e •• I ••

TYPHOON MAC

20°N-+- 1300E

Fig. 2 Examples of the forecast degradation that can occur from incorrect interpretations of track meanders: two selected JTWC forecasts (dotted lines) for Typhoon Mac (small black circles- 6 h positions- connected by a solid line). 22/12 UTC V•

Fig. 1 An example of a tropical cyclone track containing small-scale cyclonic oscillations about its mean path. The track shown is that of Tropical (Australian region, 1974, dots indicate radar fix positions at irregular times). FIGURE CAPTIONS Fig. 1 An example of a tropical cyclone track containing small-scale cyclonic oscillations about its mean path. The track shown is that of Tropical Cyclone Tracy (Australian region, 1974, dots indicate radar fix positions at irregular times).

Fig. 2 Examples of the forecast degradation that can occur from incorrect interpretations of track meanders: two selected JTWC forecasts (dotted lines) for Typhoon Mac (small black circles- 6 h positions- connected by a solid li_ne).

Fig. 3 Schematic representation (based upon enhanced infrared GMS imagery at 1531 UTC 03 August, 1993) of the observed cold cloudiness associated with Robyn's central dense overcast and an MCS. Solid black cloud silhouettes represent areas of coldest convective cloud tops, outer contour shows boundary of dense cirrus overcast. Smith, R.K., W. Ulrich and G. Sneddon, 1997: The dynamics of hurricane-like vortices

in vertical shear flows. Quart. J. Roy. Met. Soc., (submitted).

Wang, Y., and G.J. Holland, 1996: Tropical cyclone motion and evolution in vertical

shear. J. Atmos. Sci., 53, 3313-3332. Weber, H.C., 1997a: A numerical study of tropical cyclone structure: Polygonal eye

walls. Quart. J. Roy. Met. Soc., (submitted).

Weber, H.C., 1997b: A numerical study of tropical cyclone structure: concentric

secondary wind maxima. Quart. J. Roy. Met. Soc., (submitted).

Willoughby , H.E., 1988: Linear motion of a shallow-water barotropic vortex.

J. Atmos. Sci., 4 5, 1906-1928. __,, 1990: Temporal changes of the primary circulation in tropical cyclones.

J. Atmos. Sci., 4 7, 242-264. __, 1992: Linear motion of a shallow-water barotropic vortex as an initial

value problem. J. Atmos. Sci., 4 9, 2015-2031. REFERENCES Flatau, M., W.H. Schubert, and D.E. Stevens, 1994: The role of baroclinic processes

in tropical cyclone motion: the role of vertical tilt. J. Atmos. Sci., 51, 2589-2601.

Frank, W.M., and E.A. Ritchie, 1997: Environmental effects on the structure of tropical

storms. Proceedings 22nd Conference on Hurricanes and Tropical Meteorology,

A mer. Meteor. Soc., Boston, MA 02108, 352-353. Harr, P.A., and R. L. Elsberry, 1996: Structure of a mesoscale convective system

embedded in Typhoon Robyn during TCM-93. Mon. Wea. Rev., 1 2 4, 634-652.

Holland, G.J., and M.A. Lander, 1993: The meandering nature of tropical cyclone

tracks. J. Atmos. Sci., 50, 1254-1266.

Jones, S.C., 1995: The evolution of vortices in vertical shear. Part 1: Initially

barotropic vortices. Quart. J. Roy. Meteor. Soc., 1 21, 821-851. Lander, M.A., and G. J. Holland, 1993: On the interaction of tropical cyclone scale

vortices. Part 1: Observations. Quart. J. Roy. Meteor. Soc., 11 9, 1347-1361. Meighen, P.J., 1987: Radar observations of tropical cyclones. M.S. thesis, Melbourne

University, 297 pp. [Available from the author at the Bureau of Meteorology, P.O. Box

1289K, Melbourne, Vie., 3001, Australia.]

Ritchie, E.A., and W.M. Frank, 1997: Effects of convective asymmetries on the structure

of tropical storms. Proceedings 22nd Conference on Hurricanes and Tropical

Meteorology, A mer. Meteor. Soc., Boston, MA 02108, 199-200.

Schubert, W.H., M.T. Montgomery, R.K. Taft, T.A. Guinn, S.R. Fulton, J.P. Kossin, and

J.P. Edwards, 1997: Polygonal eyewalls, asymmetric eye contraction and potential

vorticity mixing in hurricanes. J. Atmos. Sci., (submitted).

Smith, R.K., W. Ulrich, and G. Dietachmayer, 1990: A numerical study of tropical­

cyclone motion using a barotropic model. Part I. The role of vortex asymmetries.

Quart. J. Roy. Met. Soc., 11 6, 337-362. hypothesized impact on the track of a TC by an MCS that is either embedded in or adjacent to a TC is most likely sensitive to the compensating beneficial (e.g., saturated environment, low-level convergence) and detrimental (e.g., large midlevel shear) effects of the TC on the MCS. This study may have provided more questions than answers with regard to small­ scale interactions and TC track changes. For example, if the MCS had formed farther from the TC center, it might have been less impacted by shear and would have been able to better sustain a midlevel vortex. However, had the MCS been farther away, it might not have benefited from the strong low-level convergence of the high -e air that is found in the low levels of the TC environment. Also, it appears that the vorticity that is initially associated with the MCS and processes therein becomes filamented due to the shear in the TC. What happens to this vorticity? Does it go to increasing the strength or intensity of the TC, or can the shear vorticity also effect the TC track as it was hypothesized with regard to the midlevel vortex? These are open questions. In practice, the ability of the conventional observing network, and even of satellite imagery to detect many of these small-scale meanders is limited. A roadblock that is probably universal to all TC studies is the lack of data. lt is difficult to validate many

·hypotheses without supporting data. The potential use of unmanned aircraft may solve this problem somewhat. Increased use and analysis of new remote-sensing techniques should be exploited to provide additional observational evidence of small­ scale TC structure modifications and track change. c) The role of mesoscale convective systems Mesoscale convective systems (MCSs) may cause meanders in tropical cyclone tracks with amplitudes on the order of 100 km and periods of 1-2 days (Holland and Lander 1993; Lander and Holland 1993). In the foregoing studies, the tracks of a tropical cyclone and an MCS appear to have a mutual cyclonic orbit around a centroid. Lander and Holland (1993) assumed that the MCS had a substantial mesoscale vortex and used a barotropic model to demonstrate that such a combined MCS-TC system might cause the track deflections that were observed. Smith et al. (1990) showed how the presence of a weaker vortex near a stronger one produced track meanders of the latter, but that these diminished in amplitude with time as the smaller vortex was distorted by the horizontal shear of the circulation of the stronger one. Willoughby (1992), used a theoretical model to indicate that an MCS near the canter of a TC may cause track meanders on shorter time scales of of 4-12 hours. A mini-field program (TCM-93) was undertaken to obtain observations of MCS structure in a TC environment. Data were obtained (Harr and Elsberry 1996) documenting the structure and evolution of a mature MCS in the environment of Typhoon (TV) Robyn (Fig. 3), which was undergoing a track change from west to northwest. The change in motion of TY Robyn was assessed in terms of a possible interaction with the MCS and to the large-scale steering in which Robyn existed. Many of the structural features that previously had been associated with the generation and maintenance of an MCS and midlevel vortex are indeed found in the MCS embedded in TV Robyn. Because of the large differences between the strengths, sizes, and vertical extent of the MCS and Robyn circulations, it was concluded that it was unlikely that an interaction between the circulations could be responsible for the change in the speed and direction of Robyn. The shear effects of the strong TC circulation had a detrimental effect on the maintenance of the MCS and likely prevented the continued development of a well-defined vorticity structure in the MCS. Therefore, the verification of the correlation between looping motion and concentric secondary wind maxima from observations is called for. b) Environmental forcing The influence of the environment on generation of vortex asymmetries and associated track changes is a topic that is just now being addressed. lt has emerged that large-scale environmental shear may produce small- to mesoscale effects on the vortex track (e.g., Flatau, et al 1994; Jones 1995; Wang and Holland 1996). Jones (1995) explored the effects of vertical shear on a purely cyclonic, initially barotropic vortex. At first, the vortex tilts in the direction of the shear, but as time progresses, the plane of tilt rotates cyclonically and the vortex develops a "wobble". about a vertical axis. This wobble is attributed to an additional advective component over the vortex center near the surface and at upper levels. The advective component is associated with the circulation produced by the downward penetration of the potential vorticity (PV) in the upper part of the tilted vortex column and with the upward penetration of the PV in the lower part. Using a baroclinic vortex, Ritchie and Frank (1997) and Frank and Ritchie (1997) explored the effects of environmental forcing using both dry and moist dynamics. In the dry case, less of the rotational effect noted in Jones (1995) occurred because a weaker vortex was used. Nevertheless, a slight deflection of both the upper and lower vortex centers was observed which was attributed to the effect described in Jones (1995). The effect of tilt is much reduced when convective parameterization and explicit moist physics are included (Ritchie and Frank 1997).

Smith et al. (1997) have developed a simple analog model that helps to explain some of the behavior found in the numerical models; indeed it shows that wobbling should occur for relatively weak shear and/or a relatively strong vortex. However, for relatively strong shear, the vortex tilt increases monotonically. oscillations with scales around the eye diameter and periods of several hours. lt must be emphasized that there are significant ambiguities and potential for overlap in these classifications. We review here processes that lead to small-scale track meanders, keeping in mind that it may be possible for a small-scale mechanism(s) to act cumulatively (or collectively) in such a way so as to produce a medium-scale or synoptic track meander. 3.4.2 Properties of tropical cyclone meanders Holland and Lander (1993) studied 17 radar tracks of TCs provided by Meighen (1987) and showed that there was a strong tendency toward exclusively cyclonic rotation at shorter periods. Seven of these had no clearly discernible oscillation; the remainder contained 23 small-scale meanders, all of which were cyclonic. Only 50% of the longer-period meanders and virtually none of those at high frequency are anticyclonic. There is no relationship between the oscillation and inertial frequencies. The amplitudes of all scales of meanders does not vary with latitude. 3.4.3 Potential mechanisms for small- (-1 0-50 km) and medium- (50-1 00 km) scale meanders a) Eye-wall convection asymmetries

Perhaps the small- and mesoscale features that have been most examined in TCs are convective eye-wall cycles, or asymmetries in the i,nner-core convection that are associated with short-term track changes. These have been studied most extensively by Willoughby (e.g., Willoughby 1988, 1990). Further studies (e.g., Schubert et al. 1997; Weber 1997a) emphasize the great importance of barotropic instability and/or of neutral modes to solid rotation in the vortex core/eye wall in connection with track oscillations of amplitudes up to 50 km. Pseudo-regular looping or erratic tracks with amplitudes between 50-150 km were found by Weber (1997b) as a result of the barotropic instability of vortices with concentric secondary wind maxima. Because of problems involved with operational forecasting of looping or erratically moving TCs, Topic 3.4 Small-scale interactions (motion)

Rapporteur: M. A. Lander Water and Energy Research Institute University of Guam U.O.G. Station Mangilao, Guam 96911 (USA)

Email: [email protected] or [email protected] FAX: (USA) 671-349-6106 or 671-734-8890 Working Group: M. Montgomery, R. Smith, E. Ritchie, P. Harr 3.4.1 Introduction lt is well known that tropical cyclones (TCs) tend to meander or oscillate about a mean path. As illustrated in Fig. 1, these meanders cover a wide range of scales and take on several forms, including small-amplitude and short-period trochoidal oscillations around an otherwise relatively smooth track, larger-scale and longer period meanders, more erratic and nonperiodic meanders, occasionally including stalling, or small loops, or highly erratic wandering with no well-defined track. A wide range of scales are involved as the meanders vary in period from a few days to less than an hour and have amplitudes up to a few hundred kilometers. Such meanders are often poorly predicted (Fig. 2) since short-period perturbations can be confused with longer-term track changes, and vice versa. The result is a strong limitation on the accuracy of track forecasting at both short and extended time scales.

Similar problems exist when interpretations have to be made between short-period meanders and the commencement of substantial track changes. Despite this potential impact on forecast accuracy, these meanders have received little research attention. Following Holland and Lander (1993), track meanders are separated into three categories: 1) synoptic track, 2) medium-scale meanders with period greater than one day and amplitude of several tens to hundreds of kilometers, and 3) small-scale

Schubert, W. H., M. T. Montgomery, R. K. Taft, T. A. Guinn, S. R. Fulton, J. P. Kossin and J. P. Edwards, 1997: Polygonal eye walls, asymmetric eye contraction and potential vorticity mixing in hurricanes. Submitted to J. Atmos. Sci. Shapiro, L. J. and M. T. Montgomery, 1993: A three-dimensional balance theory for rapidly rotating vortices. J. Atmos. Sci, 50, 3322-3335. Smith, G. B. and M. T. Montgomery, 1995: Vortex axisymmetrization and its dependence on azimuthal wavenumber or asymmetric radial structure changes. Quart. J. Roy. Meteor. Soc., 121, 1615-1650. Smith R.K., 1991: An analytic theory of tropical cyclone motion in a barotropic shear flow. Quart. J. Roy. Met. Soc., 117,685-714. Smith R.K. and W. Ulrich, 1990: An analytical study of tropical cyclone motion using a barotropic model. J. Atmos. Sci., 47, 1973-1986. Smith R.K. and H. C. Weber, 1993: An extended analytic theory of tropical cyclone motion. Quart. J. Roy. Met. Soc., 119, 1149-1166. Smith R.K. and A. Glatz, 1997: The detection ofhurricane asymmetries from aircraft reconnaissance flight data: some simulation experiments. Submitted to Quart. J. Roy. Met. Soc. Wang. B. and X. Li, 1995: Propagation of a tropical cyclone in a meridionally varying zonal flow: An energetic analysis. J. Atmos. Sci., 52, 1421-1433. Wang. B. and X. Li, and L. Wu, 1997: drift direction in horizontally sheared flows. J. Atmos. Sci., 54, 1462-14 71. Weber, H. C., 1997a: A numerical study of tropical cyclone structure: polygonal eye walls. To be submitted to Quart. J. Roy. Meteor. Soc. Weber, H. C., 1997b: A numerical study of tropical cyclone structure: quasi-stationary spiral bands. Submitted to Quart. J. Roy. Meteor. Soc. Weber, H. C., 1997c: A numerical study of tropical cyclone structure: concentric secondary wind maxima. To be submitted to Quart. J. Roy. Meteor. Soc. Weber, H. C. and R. K. Smith, 1993: The stability of barotropic vortices: Implications for tropical cyclone motion. Geophys. Astrophys. Fluid Dyn., 70, 1-30. Willoughby, H. E., 1992: Linear motion of a shallow-water barotropic vortex as an initial-value problem. J. Atmos. Sci., 49, 2015-2031. Willoughby, H. E., 1994: Nonlinear motion of a shallow water barotropic vortex. J. Atmos. Sci., 51, 3722-3744. Willoughby, H. E., 1995: Normal-mode initialization of barotropic vortex motion models. J. Atmos. Sci., 52, 4501-4514. Zehnder, J. A., 1993: The influence of large-scale topography on barotropic vortex motion. J. Atmos. Sci. 50, 2519-2532. Zehnder, J. A. and D. M. Powell, 1997: The interaction of easterly waves, orography and the ITCZ in the generation of eastern Pacific tropical cyclones. Preprints 22nd Conference on Hurricanes and Tropical Meteorology, 547-548. Zehnder J. A. and M. J. Reeder. 1997. Barotropic vortex motion near large-scale orography: sensitivity to orographic and environmental parameters. Meteorol. Atmos. Phys., 64, 1-19. Chan, J. C. L., 1997: Barotropic dynamics of tropical-cyclone motion as inferred from the TCM-90 fmal analysis data set. Preprints 22nd Conference on Hurricanes and Tropical Meteorology, 625-626. Chan, J. C. L. and A. C. K. Law, 1995: The interaction of binary vortices in a barotropic model. Meteorol. Atmos. Phys., 56, 135-155. Cheung, K. K. W. and J. C. L. Chan, 1997: Ensemble forecasting of tropical cyclone motion using a barotropic model. Part I: perturbations of the environment. Submitted to Mon. Wea. Rev. Elsberry, R. L., 1999: Chapter 4 of Global perspectives on tropical cyclones. Report No. TCP-38, World Meteorological Organization, Geneva, Switzerland, 289 pp. Franklin, J. L., S. E. Feuer, J. Caplan and S. D. Aberson, 1996: Tropical cyclone motion and surrounding flow relationships: Searching for the beta gyres in omega dropwindsonde data sets. Mon. Wea. Rev., 124, 64-84. Glatz A. and R. K. Smith, 1996: Vorticity asymmetries in Hurricane Josephine (1984). Quart. J. Roy. Met. Soc., 122, 391-413. Guinn, T. A., and W. H. Schubert, 1993: Hurricane spiral bands. J. Atmos. Sci. 50, 3380-3403. Hart, R. E., 1997: Shallow fluid simulations of vortex interaction within environmental shear. Preprints, 22nd Conf. Hurr. Trop. Meteor., Amer. Meteor. Soc., Boston, MA 02108,537-538. Horsfall, F., M. DeMaria and J. M. Gross, 1997: Optimal use of large-scale boundary and initial fields for limited-area hurricane forecast models. Preprints 22nd Conference on Hurricanes and Tropical Meteorology, 571-572. Jones, R. W. and H. E. Willoughby, 1997: Sensitivity of a spectral shallow-water barotropic vortex to variations of domain size and spectral truncation. Preprints, 22nd Conf. Hurr. Trop. Meteor., Amer. Meteor. Soc., Boston, MA 02108, 577-578. Kraus A., R. K. Smith and W. Ulrich, 1995: The barotropic dynamics oftropical-cyc1one motion in a large-scale deformation field. Beitr. Phys. Atmos., 68, 249-261. Kwon, H. J., and S. W. Park, 1997: Barotropic adaptive grid typhoon simulation model. Preprints 22nd Conference on Hurricanes and Tropical Meteorology, 471-472. Lander, M. A., 1994: Description of a monsoon gyre and its effects on the tropical cyclones in the western North Pacific during August 1991. Wea. Forecasting., 9, 1994. Li, X., and B. Wang, 1994: Barotropic dynamics of beta gyres and beta drift. J. Atmos. Sci., 51, 746-756. Li, X., and B. Wang, 1996: Acceleration of the hurricane beta drift by shear strain rate of an. environmental flow. J. Atmos. Sci., 53, 327-334. Llewellyn-Sm.ith, S. G., 1997: The motion of a non-isolated vortex on the beta-plane. J. Fluid Dyn., 346, 149- 179. Marks, F. D. Jr., and P. P. Dodge, 1997: Hurricane concentric eyewall characteristics as revealed by airborne Doppler radar analyses. Preprints 22nd Conference on Hurricanes and Tropical Meteorology, 102-103. Moller, J. D. and M. T. Montgomery, 1997: Nonlinear vortex motion using the Asymmetric Balance theory. Preprints, 22nd Conf. Hurr. Trop. Meteor., Amer. Meteor. Soc., Boston, MA 02108, 149-150. Montgomery, M. T. and R. J. Kallenbach, 1997: A theory for vortex Rossby waves and its application to spiral bands and intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc. 123, 435-465. Montgomery, M. T., and C. Lu, 1997: Free waves on barotropic vortices. Part 1: Eigenmode structure. J. Atmos. Sci. (in press). Montgomery, M. T., D. P. Stepaniak and W. H. Schubert, 1997: Free waves on barotropic vortices. Part 11: the non-axisymmetric Rossby adjustment problem. J. Atmos. Sci. (to be submitted). Montgomery, M. T., J. D. M oiler and C. T. Nicklas, 1997: Linear and nonlinear vortex motion in an asymmetric balance shallow water model. Submitted to J. Atmos. Sci. Morison, R. P., L. M. Leslie and G. D. Hess, 1997: Combining data assimilation and ensemble techniques for tropical cyclone forecasting. Preprints 22nd Conference on Hurricanes and Tropical Meteorology, 304-305. Nicklas, C. T. and M. T. Montgomery, 1997: Linear beta drift in an asymmetric balance model. Preprints, 22nd Conf. Hurr. Trop. Meteor., Amer. Meteor. Soc., Boston, MA 02108, 148-148. Peng, M. S. and R. T. Williams, 1991: Stability of barotropic vortices. Geophys. Astrophys. Fluid Dyn., 58, 263- 283. Re~ik, G. M. and W. K. Dewar, 1994: An analytical theory of distributed axisymmetric barotropic vortices on the [3-plane. J. Fluid Mech., 269, 301-321. vorticity gradient from the data. Smith and Glatz (1997) have used barotropic model simulations to investigate the data requirements for determining the flow asymmetry of a hurricane using omega dropwindsonde soundings from reconnaissance aircraft. 3.3.5 Barotropic Forecasting Systems

Barotropic models continue to be developed for use in tropical cyclone track forecasting. The Korean Meteorological Administration has implemented a model with an adaptive grid which has shown some skill in track forecasts in comparison with CLIP ER (Kwon and Park, 1997). At the Hurricane Research Division, assessments of forecasts with the VICBAR model have been carried using a recently developed data assimilation technique (Jones et al., 1995), including a new initialization procedure and modified boundary conditions (Horsfall et al., 1997). These changes led to some improvements in the accuracy of track forecasts in some cases, but not all. The use of a barotropic model for carrying out ensemble forecasts has been investigated by Aberson et al. (1995) and Morison et al. (1997). The former study has shown inter alia that the ensemble mean produces better track forecasts than individual members of the ensemble. The study by Morison et al. (1997) indicates the potential of ensemble forecasts using a barotropic model in combination with newly developed data assimilation techniques. Different techniques to generate better initial conditions for ensemble forecasts with barotropic models have been investigated also by Cheung and Chan (1997). However, the above studies show also that, at present, the quality of the ensemble forecasts is case­ dependent and further research is required to improve these methods. I. Roadblocks A difficulty in verifying theoretical predictions from observational data is the partitioning problem explained above. II. Promising research areas There is a continued need for studies of barotropic processes in vortex dynamics. An understanding of these must go hand-in-hand with investigations of fully three-dimensional processes in identifying the fundamental mechanisms involved in tropical-cyclone movement and development. Theoretical studies of vortex Rossby waves, vortex-induced planetary waves and their interaction with environmental flows, and the growth and saturation of vortex instabilities all seem profitable and tractable avenues for further research. Barotropic models have a role in the construction of synthetic vortices for initializing tropical cyclones in forecast models in data-sparse regions, and as a basis for ensemble forecast systems, where their simplicity and computational cheapness are advantageous. References

Aberson, S. D., S. J. Lord, M. DeMaria and M. S. Tracton, 1995: Short-range ensemble forecasting ofhurricane tracks. Preprints 21st Conference on Hurricanes and Tropical Meteorology, 494-496. Carr, L. E. Ill and R. L. Elsberry, 1995: Monsoonal interactions leading to sudden tropical cyclone track changes. Mon. Wea. Rev., 123, 265-289. Carr, L. E. Ill and R. L. Elsberry, 1997: Models of tropical cyclone wind distribution and beta-effect propagation for application to tropical cyclone track forecasting. Mon. Wea. Rev., 125, 3190-3209. Carr, L. E. III M. A. Boothe, and R. L. Elsberry, 1997: Observational evidence for alternate modes of track­ altering binary tropical cyclone scenarios. Mon. Wea. Rev., 125, 2094-2111. vortices with concentric secondary wind maxima are barotropically unstable. The development and temporal modification of wave-number one modes may have dramatic effects on vortex motion, with looping or erratic tracks ranging between 50 and 150 km. 3.3.3.3 Long term motion

Willoughby (1992, 1995) showed that during a long term(~ 10 days) numerical model integration, the drift speed of a given vortex increases with uniform acceleration. In contrast, using the asymmetric balance formulation of Shapiro and Montgomery (1993), Moller and Montgomery (1997) and Montgomery et al. (1997) found that for a range of symmetric vortices, the drift speed always asymptotes to a finite value. Furthermore, in contrast to the findings of Peng and Williams (1991), Weber and Smith (1993) and Montgomery et al. (1997), Willoughby (1992, 1995) attributes the uniform vortex . acceleration to a normal mode of near-zero frequency that occurs in a storm-relative coordinate system. Finally, Moll er and Montgomery (1997) showed also, in contrast to Willoughby (1994 ), that during long-term numerical model integrations, the vortex track is sensitive to azimuthal wavenumber truncation and the size of the integration domain. A too small domain would contaminate the results of the numerical integration by boundary effects. ' 3.3.3.4 Binary vortices and monsoon-gyre interactions Because of the intrinsic complexity of the problem, barotropic models have an important role to play in unravelling the dynamics of interacting binary vortices. Recent contributions to this problem include papers by Chan and Law (1995) and Hart(1997). Carr and Elsberry (1995) show that a class of tropical-cyclone track changes can be explained to a first approximation as a barotropic interaction between a tropical cyclone and a larger scale monsoon gyre (Lander, 1994). The interaction involves a Fujiwhara­ like merger, following by poleward motion of the merged vortices that arises from advection by a strong anticyclone that develops to the east and usually equatorward of the vortices. 3.3.3.5 Interaction with orography The interaction of tropical cyclones with orography has been investigated by Zehnder (1993), Zehnder and Reeder (1997)and Zehnder and Powell (1997), using a shallow­ water model on an equatorial beta-plane. They find that the vortex tracks in such models can be explained in terms of modification of the beta-gyres through vortex stretching associated with horizontal convergence and divergence as the air flows over the topography. The studies were motivated by a desire to understand the development and motion of tropical cyclones in the eastern Pacific and the western Gulf of Mexico 3.3.4 Verification studies, Data simulation experiments Recent attempts to identify the beta-gyres in observational data are reported by Franklin et al. (1996) and Glatz and Smith (1996). The former study was based on a composite analysis of field data from synoptic field experiments conducted in the Atlantic and Carribean areas by the Hurricane Research Division. The latter authors carried out a case study of an individual storm: Hurricane Josephine (1984). Both these studies highlight certain intrinsic problems in interpreting hurricane motion in terms of current (barotropic) theories. This is partly due to the non-uniqueness of the partition required to separate the storm environment from the vortex asymmetries and partly because of the difficulty of determining a representative model-equivalent value for the environmental absolute Topic 3.3 Barotropic Processes (TC Motion) / Rapporteur: Roger K. Smith Working Group: Les Carr, Dominique Moller, Bin Wang, Harry Weber, Zhu Yongti

3.3 .1 Introduction

Studies of barotropic processes have contributed much to our understanding of tropical cyclone motion, even prior to the IWTC-III Workshop in 1994, and barotropic models have been a part of the suite of numerical forecast products available in some forecast centres for many years. Since the last workshop, interest in barotropic processes has not abated as is evident- from the list of references below; indeed barotropic forecast models continue to have a role and are still being developed an refined. In the space available here, we can do little rriore than scratch the surface of the research that has occurred since IWTC-111. In writing this update, I assume that the reader some degree of familiarity of results reviewed by Elsberry (1995).

3.3.2 Analytic studies Further development of an analytic theory for vortex motion on a beta-plane generalizing that worked out by Smith and Ulrich (1990), Smith (1991) and Smith and Weber (1993) has been carried out by Reznick and Dewar (1994) and Llewellyn-Smith (1997). 3.3.3 Theoretical and numerical studies

3.3.3.1 The beta-gyres The azimuthal wavenumber-one relative vorticity asymmetry is known to play a central role in vortex motion as it leads to a flow across the vortex centre. For a moving vortex on a beta-plane, the advection of planetary vorticity by the vortex makes an important contribution to this asymmetry and the associated flow asymmetries are often referred to as beta-gyres. The contribution of this flow to vortex motion is sometimes referred to as beta-drift. Recent research on the structure of the beta-gyres and their consequences for tropical-cyclone motion is described by Li and Wang (1995), Carr and Elsberry (1997) and Carr et al. (1997). Wang arid Li (1995) have examined the energy transfers between the primary (axi-symmetric) vortex and the beta-gyres. The effects of large-scale flow deformation on the structure of the vortex asymmetry has been investigated by Kraus et al. (1995), Li and Wang (1996) and Wang et al. (1996). · 3.3.3.2 Vortex dynamics To date, polygonal eye walls of convection are still seen as curiosities of hurricanes. However, barotropic studies as e.g. Schubert et al. (1997) and Weber (1997a) have shown the great importance of polygonal vorticity patterns in the vortex core, possibly related to polygonal eye walls of convection, for vortex modification and vortex motion. Complementary to the study of Guinn and Schubert (1993), Weber (1997b) has investigated the evolution of vorticity spiral bands, possibly related to outer spiral bands of convection, to a vortex moving on a beta-plane. It was found that the structure, orientation and development of outer spiral bands and vortex motion are mutually dependent. Observations of e.g. Marks and Dodge (1997) suggest vortex Rossby waves as one possible mechanism for the generation of concentric secondary eye walls (Montgomery and Kallenbach, 1997). In another study, Weber (1997c) found that Wu, C.-C., and K. A. Emmanuel, 1995b: Potential vorticity diagnosis of hurricane movement. Part 11: Tropical Storm Ana (1991) and Hurricane Andrew (1 992). Mon. Wea. Rev., 123, 93-109.

Wu, C.-C., and Y. Kurihara, 1996: A numerical study of the feedback mechanisms of hurricane-environment interaction on hurricane movement from the potential vorticity perspective. J. Atmos. Sci., 53, 2264-2282.

3.2.6 Moeller, J. D., 1995: Tropical cyclones: a potential vorticity perspective. Doctoral dissertation, University of Munchen, 11 Opp.

Moeller, J.D. and S.C. Jones, 1998: Potential vorticity inversion for tropical cyclones using the asymmetric balance theory. J. Atmos. Sci. (in press).

Shapiro, L. J. 1992: Hurricane vortex motion and evolution in a three-layer model. J. Atmos. Sci. 49, 140-153.

Shapiro, L. J., 1996: The motion of Hurricane Gloria: A potential vorticity diagnosis. Mon. Wea. Rev., 124, 2497--2508.

Shapiro, L. J., and J. L. Franklin, 1995: Potential vorticity in Hurricane Gloria. Mon. Wea. Rev., 123, 1465-1475.

Shapiro, L. J., and J. L. Franklin, 1997: Potential vorticityand hurricane motion. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Boston, MA 12108, 507-508.

Shapiro, L. J., and J. L. Franklin, 1998: Potential vorticity asymmetries and tropical cyclone motion. To be submitted to Mon. Wea. Rev.

Shapiro, L. J., and M. T. Montgomery, 1993: A three-dimensional balance theory for rapidly rotating vortices. J. Atmos. Sci., 50, 3322-3335.

Smith R. K., W. Ulrich, and G. Sneddon, 1998: The dynamics of hurricane-like vortices in vertical shear flows. To be submitted.

Ulrich, W., K. Dengler and S.C. Jones, 1998: Vortex motion in a two-layer fluid with . uniform shear: the prediction of different numerical models. In preparation.

· Wang, 8., and X. Li, 1992: The beta-drift of three-dimensional vertices: A numerical study. Mon. Wea. Rev., 120, 579-593. -

Wang, Y. and G. J. Holland, 1996a: The beta drift of baroclinic vortices. Part 1: Adiabatic vortices. J. Atmos. Sci., 53,411-427.

Wang, V. and G. J. Holland, 1996b: The beta drift of baroclinic vortices. Part 11: Diabatic vortices. J. Atmos. Sci., 53, 3737-3756.

Wang, Y. and G. J. Holland, 1996c: Tropical cyclone motion and evolution in vertical shear. J. Atmos. Sci. 53, 3313-3332.

Wu, C.-C., and K. A. Emmanuel, 1993: Interaction of a baroclinic vortex with background shear: Application to hurricane motion. J. Atmos. Sci., 50, 62-76.

Wu, C.-C., and K. A. Emmanuel, 1995a: Potential vorticity diagnosis of hurricane movement. Part 1: A case study of (1991). Mon. Wea. Rev., 123,69-92.

3.2.5