Fourth International Workshop on Extratropical Transition (IWET4) Mont Gabriel Lodge St.-Adèle, Québec Canada May 20-25, 2012
Schedule of presentations and abstracts: Updated May 21, 2012
Monday, May 21: 0815: Welcome and Introduction
0830 Session 1 (ET Climatology) Chair: Shawn Milrad
Kimberly M. Wood and Elizabeth A. Ritchie The University of Arizona Title: A 40-year climatology of extratropical transition in the eastern North Pacific. Part I: General characteristics.
Extratropical transition has been frequently observed in many tropical basins around the world, including the western North Pacific, the northern Atlantic, and the southwestern Pacific. Conversely, only rare cases have been documented in the eastern North Pacific. This presentation will showcase a climatology of extratropical transition in this basin from 1970 to 2010, including cases which complete ET over open ocean and cases which begin ET but then make landfall before completing the process.
This study utilizes 6-hourly reanalysis data from the European Centre for Medium-Range Weather Forecasts at a nominal resolution of 0.703 degrees from 1979 onward (ERA-Interim) and a nominal resolution of 1.125 degrees before 1979 (ERA-40) to produce cyclone phase space plots as well as examine the large-scale features present during extratropical transition in the eastern North Pacific. This presentation will also discuss the structure of ET in the eastern North Pacific. It will cover the average structural changes that occur during ET in this basin as well as explore how these changes differ from those seen elsewhere in the tropics.
0900 Elizabeth A. Ritchie and Kimberly M. Wood The University of Arizona Title: A 40-year climatology of extratropical transition in the eastern North Pacific. Part II: Structure and common large-scale circulation patterns.
1 The extratropical transition of tropical cyclones has been documented in many basins worldwide, including the western North Pacific, the north Atlantic, and the southwest Pacific. However, very few cases have been documented in the eastern North Pacific, and certainly no climatology has been presented thus far. This presentation will continue to explore extratropical transition in this basin from 1970 to 2010. In part II of this study, the structural changes associated with ET in the eastern North Pacific are explored. The data used in the analysis are the 6-hourly reanalysis data from the European Centre for Medium-Range Weather Forecasts at a nominal resolution of 0.703 degrees from 1979 onward (ERA-Interim) and a nominal resolution of 1.125 degrees before 1979 (ERA-40). Cyclone phase space is used to both define a common “ET time” around which to composite the analyses, and also to describe the passage of the ET-ing system as it transitions into an extratropical cyclone. This presentation will discuss the composite large-scale circulation features and structural changes that occur during ET in this basin and also explore how these changes differ from those seen elsewhere in the tropics.
0930 Chris Fogarty Canadian Hurricane Centre Title: A summary of recent notable ET events over Eastern Canada
A number of significant ET events have impacted the region over the past number of years – almost twice as many as the longer-term average. This is partly a result of heightened tropical cyclone activity in the Atlantic Basin. Recent events will be presented including particular forecast challenges and successes associated with each.
1000 Health Break
1030 Discussion and Summary of Session 1
1045 Session 2 (Ensemble prediction, and data assimilation) Chair: Elizabeth R. Sanabia
Sim D. Aberson NOAA/AOML/Hurricane Research Division Altug Aksoy University of Miami/CIMAS and NOAA/AOML/Hurricane Research Division
2 Kathryn Sellwood University of Miami/CIMAS and NOAA/AOML/Hurricane Research Division Title: The Transition of Hurricane Earl (2010) from a Category Four Hurricane to a Large Extratropical Cyclone
The Transition of Hurricane Earl (2010) from a Category Four Hurricane to a Large Extratropical Cyclone The National Oceanic and Atmospheric Administration is in the midst of a multi- year field program, the Intensity Forecasting Experiment to improve the understanding and prediction of hurricane intensity change (Rogers et al. 2006). The experiment is conducted by the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory, the Aircraft Operations Center, the Environmental Modeling Center and the National Hurricane Center. The goal is to collect observations to aid in the improvement and development of operational models during all stages of the tropical cyclone lifecycle from genesis to decay. One particular emphasis is to gather observations during the extratropical transition (ET) of tropical cyclones that are not being impacted by land. As part of the effort to improve the operational numerical guidance, an ensemble- based data assimilation system was developed (Aksoy et al. 2012). The system, named the Hurricane Ensemble Data Assimilation System (HEDAS), is based on a serial implementation of the square root ensemble Kalman filter. It is configured with a horizontal grid spacing of 9/3 km on the outer/inner domains, with the latter about 1000 km square centered near the storm center. The largest data volume assimilated are wind radials from the Airborne Doppler radar aboard the NOAA P-3 aircraft (Aberson et al. 2006). Additional sources are dynamic and thermodynamic dropwindsonde and high-level flightlevel and surface wind-speed data from NOAA and Air Force research and reconnaissance aircraft. HEDAS was run for all cases in which Airborne Doppler Radar data are available from 2008-2011. One set of cases is from Hurricane Earl (2010) as it moved northward just off the United States east coast and began its ET. A series of six missions were conducted every 12 h from about 0800 UTC 01 September to 0400 UTC 04 September in the core of Earl as it strengthened into a Category 4 Hurricane and then began its ET. During this period, the NOAA G-IV aircraft obtained environmental dropwindsonde data once daily, and six operational reconnaissance missions were conducted by the United States Air Force C-130 aircraft. This series of 15 flights represents an unprecedented data set with which to explore the ET of a major hurricane over open water. An example of the type of analyses available are shown in Fig. 1, a series of north-south cross sections of HEDAS analyses through the center of Earl. During the first mission (A), a low- to midlevel double wind-speed maximum is seen on the west, north, and east sides (only north shown), suggesting an eyewall replacement cycle. The radius of maximum wind speed contracts (B), and then the wind field broadens (C), and another secondary wind-speed maximum develops
3 (D). As Earl begins moving over cool water north of the Gulf Stream (E), the wind field becomes relatively shallow and broadens, though some vestiges of the double-wind-speed maximum remain, and the asymmetry shifts so that the wind- speed maximum is to the south rather than the north. By the final mission before land interaction (F), the wind field is relatively shallow and very broad, and the highest wind speeds are now near 600 hPa. Concurrent with this process is a cooling of the mid- to upper-levels by more than 10K (not shown), signaling that transition to an extratropical cyclone is nearly complete. This analyses represent the first time the inner-core structure of a major hurricane undergoing ET away from land interaction has been sampled and studied. The presentation will include further research on the HEDAS analyses, both in terms of dynamic and thermodynamic quantities, and also of the high-resolution model runs initialized with these analyses. Figure 1: North-south cross sections in Hurricane Earl of tangential (shaded) and radial (contoured) wind speeds (ms-1) from HEDAS analyses of aircraft data. Analyses are valid every 12 h from 1200 UTC 01 September (A) to 0000 UTC 04 September (F) 2010. The thick vertical line represents the location of the 850 hPa wind center, and the thin vertical lines represent one degree longitude north and south of that center.
References Aberson, S. D., M. L. Black, R. A. Black, J. J. Cione, C. W. Landsea, F. D. Marks, Jr., and R. W. Burpee, 2006: Thirty years of tropical cyclone research with the NOAA P-3 aircraft. Bull. Amer. Met. Soc., 87, 1039-1055. Aksoy, A., S. Lorsolo, T. Vukicevic, K. J. Sellwood, S. D. Aberson, and F. Zhang, 2012: The HWRF Hurricane Ensemble Data Assimilation System (HEDAS) for high- resolution data: The impact of airborne Doppler radar observations in an OSSE. Mon. Wea. Rev., in press. Rogers, R., S. Aberson, M. Black, P. Black, J. Cione, P. Dodge, J. Gamache, J. Kaplan, M. Powell, J. Dunion, E. Uhlhorn, N. Shay, and N. Surgi, 2006: The Intensity Forecasting Experiment: A NOAA multiyear field program for improving tropical cyclone intensity forecasts. Bull. Amer. Met. Soc.,
4 87, 1523-1537.
1115 Heather M. Archambault, Patrick A. Harr, Richard W. Moore DEPARTMENT OF METEOROLOGY NAVAL POSTGRADUATE SCHOOL MONTEREY, CA 93943 Title: The Interaction Between Recurving Western North Pacific TC Chaba (2010) and the Extratropical Flow: A Diagnostic and Modeling Study
In late October 2010, a complex interaction between recurving western North Pacific tropical cyclone (TC) Chaba and the extratropical flow helped to initiate a Rossby wave train that dispersed rapidly downstream to North America. The subsequent establishment of a high- amplitude ridge–trough pattern over North America provided favorable synoptic conditions for record-breaking heat over western North America and heavy rain over eastern North America. The objective of this study is to identify the physical and dynamical mechanisms linking the interaction between TC Chaba and the extratropical flow to the large-scale flow reconfiguration and high-impact weather that occurred downstream. Diagnostic analyses and high-resolution numerical simulations produced by the WRF- ARW will be used to examine the relative roles of precipitation within the core of TC Chaba versus frontogenetically forced precipitation along a developing baroclinic zone just poleward of Chaba in driving divergent outflow that interacts with the extratropical flow. Of particular interest with respect to frontogenetically forced precipitation associated with the recurvature of TC Chaba is the role of a diabatic Rossby vortex-like disturbance originating along the baroclinic zone poleward of Chaba in (i) forming a jet streak via its divergent outflow and (ii) serving as a precursor to subsequent explosive extratropical cyclogenesis over the eastern North Pacific.
1145 Richard W. Moore,Heather M. Archambault and Patrick A. Harr Department of Meteorology, Naval Postgraduate School Monterey, CA 93943 Title: The Probabilistic Prediction of Diabatic Rossby Vortex Genesis Associated with Recurving Western North Pacific TC Chaba (2010)
A companion IWET4 paper by Archambault et al. presents diagnostic analyses and high-resolution numerical simulations to elucidate the interaction between recurving western North Pacific tropical cyclone Chaba (2010) and the extratropical flow. An intriguing aspect of the event, which is the focus of this work, is the observed genesis of a diabatic Rossby vortex (DRV). It forms in response to frontogenetically-forced precipitation along a midlatitude baroclinic
5 zone that is, at least in part, interconnected with the recurving tropical cyclone. The DRV is observed to play an integral role in determining downstream impacts, which include a number of high-impact weather events over the North American continent. DRV genesis has proven to be a particularly challenging forecast problem, likely due to the predominance of diabatic processes and their proclivity to form in regions with limited in situ measurements (i.e. ocean basins). We will investigate this particular DRV genesis event from a probabilistic viewpoint via the examination of ensemble prediction system data obtained from the THORPEX Interactive Grand Global Ensemble. The goals are to: i) assess the general performance of the ensemble prediction systems with regard to the formation of the DRV, and ii) utilize the ensemble spread to provide dynamical insight into the processes that were ultimately responsible for the diabatic generation of potential vorticity in an environment lacking any discernable upper-level forcing.
1215 – 1315 Lunch
1315 Doris Anwender1, Nadia Fourrie2, Florence Rabier2, Philippe Arbogast2 1Karlsruher Institut f¨ur Technologie, Karlsruhe, Germany, 2CNRM-GAME, Meteo France and CNRS, Toulouse Title: Data impact experiments using IASI observations during the life cycle of Hurricanes Gustav, Hanna and Ike (2008)
1345 Discussion and Summary of Session 2
1400 Session 3a (T-Parc; Part I) Chair: Heather Archambeault
Patrick A. Harr Naval Postgraduate School Monterey, CA 93943, USA Title: T-Parc: An Overview
6 The THORPEX Pacific Asian Regional Campaign (T-PARC) was a multi-national program with objectives that address the shorter-range dynamics and forecast skill of tropical cyclones over the western North Pacific and East Asia and their impact on a downstream region over the eastern North Pacific and North America. With the collaboration of the Tropical Cyclone Structure-2008 (TCS-08) program, the scope of the combined experiments expanded to examine the entire life cycle of a tropical cyclone over the western North Pacific. For the first time over the western North Pacific, systematic implementation of a variety of observation platforms examined factors related to tropical cyclone formation, intensification, and structure change. Additionally, detailed observation strategies were employed to examine the process of extratropical transition. Specific objectives were designed to investigate the mechanisms and structures directly related to forecast variability downstream of the extratropical transition event. A third observation strategy was aimed primarily at reducing errors associated with forecasts of tropical cyclone tracks over the western North Pacific. Systematic examination of multiple products designed to identify regions of initial condition sensitivity lead to placement of targeted observations.
During the two-month field program, 76 aircraft missions were flown into 11 tropical weather systems that included four typhoons, one tropical depression, one ex-tropical cyclone and five non-developing tropical circulation systems. The multi-national collaboration required during the combined field program, the extensive geographic area over which tropical cyclones occur, their downstream impacts, and the wide variety of observing platforms and instrumentation provided unique logistical challenges that lead to an innovative experiment design. A complex communications network and utilization of internet technology provided unprecedented access to field program assets through a virtual operations center. The design, collaborations, and implementation of the observing strategies are described to highlight the coordination of resources assigned to specific objectives and a consensus decision-making process.
During the field campaign, the entire life cycle of Typhoon Sinlaku was observed. The combination of in situ aircraft data and remotely-sensed observations have provided opportunities to observe factors related to the rapid intensification, eyewall replacement cycles, track forecast sensitivities, and extratropical transition. The combined field program produced a variety of unique data sets that are being carefully analyzed to address the science objectives. The integrated nature of the unique observing capabilities combined aircraft, radar, lidar, dropwindsonde, and satellite observations to analyze the complete life cycle of a tropical cyclone over the western North Pacific, and its influence on the predictability of downstream synoptic-scale features.
1430 Patrick A. Harr and Andrew B. Penny Naval Postgraduate School Monterey, CA 93943, USA Title: Typhoon Sinlaku during T-PARC: Variability in Forecasts of the Extratropical
7 Transition and Downstream Development
An accurate forecast of many tropical and extratropical factors associated with the extratropical transition (ET) of tropical cyclones remains a challenging problem and was a principal focus area for the THORPEX-Pacific Asian Regional Campaign (T-PARC). In September 2008, Typhoon Sinlaku became an intense tropical cyclone that recurved over Taiwan and moved poleward to pass just south of the primary islands of Japan. Typhoon Sinlaku presented an excellent opportunity to examine predictability issues associated with ET, as there was a large uncertainty in the track at recurvature and there was large variability in predicted downstream development following the ET. The ET of Typhoon Sinlaku was observed by multiple satellite and airborne reconnaissance platforms in support of the T-PARC field program, which provides for a unique opportunity to compare and contrast forecast, analyzed, and observed conditions associated with the character of ET and impacts on the midlatitude circulation into which the decaying tropical cyclone is moving. Following recurvature over the northern tip of Taiwan, deep convection in TY Sinlaku was severely reduced under the influence of strong vertical wind shear due to the midlatitude westerly winds. However, significant convection developed downshear of the low-level center of the decaying tropical cyclone. The resurgence of deep convection contributed to the re-intensification of TY Sinlaku such that it regained typhoon intensity south of Japan. The re- intensification altered the structure of the typhoon, enabled the onset of ET, and contributed to reduced predictability of the impact of the tropical cyclone on downstream midlatitude circulations. Forecasts initiated from the point of recurvature through the re-intensification stage exhibited a large amount of variability in the character of ET of Sinlaku and the impact of Sinlaku on the midlatitude circulation into which it was moving. The variability is examined with respect to the forcing of downstream development as defined by a local eddy kinetic energy analysis. Numerical experiments were conducted at the European Center for Medium-range Weather Forecasts (ECMWF) that produced forecast fields from model initial conditions that did and did not contain the observations gathered during T-PARC operations in Sinlaku. The impact of the observations on the numerical forecasts of the ET and downstream impact of Sinlaku is examined in terms of the local eddy kinetic energy analysis. The variability in forecasts associated with the inclusion of the observation data is compared with the forecast spread contained in the operational ensemble prediction system of the ECMWF.
1500 Health Break
1530 Christopher A. Davis1, Sarah C. Jones2, Doris Anwender2, Janina Badey2 and Leonhard Scheck2 1National Center for Atmospheric Research, Boulder, USA
8 2Karlsruhe Institute of Technology, Karlsruhe, Germany Title: Mesoscale Cyclogenesis over the Western North Pacific Ocean during T-PARC
Three cases of mesoscale subtropical marine cyclogenesis originating from tropical waves are investigated. Each case occurred during the TPARC and TCS08 field phase in 2008. Two of the cyclones produced gale-force surface winds, and one resulted in a significant coastal storm over eastern Japan. Each cyclone developed from a primary burst of organized mesoscale convection that consolidated and intensified the surface cyclonic circulation in a period of 12-24 h. Upper-tropospheric potential vorticity anomalies modulated the vertical wind shear, which determined the periods of cyclone intensification and weakening. Weak baroclinicity associated with vertical shear was also deemed important in organizing mesoscale ascent and the convection outbreaks. The remnant tropical disturbances contributed exceptional water vapor content to higher latitudes that led to strong diabatic heating, and contributed vorticity that was the seed of the subtropical development. Predictability of these events more than three days in advance appears to be minimal.
1600 J. F. Quinting, S. C. Jones, P. A. Harr, M. M. Bell Karlsruhe Institute of Technology, Karlsruhe, Germany Title: Structural characteristics of Typhoon Sinlaku (2008) during its extratropical transition: an observational study
Structural changes of a tropical cyclone (TC) during extratropical transition (ET) cause high impact weather. One of the primary goals of the THORPEX-Pacific Asian Regional Campaign (T-PARC) in 2008 was to obtain insights into the various physical processes that account for these structural changes. During this campaign one of the major typhoons in the West Pacific in 2008 -Typhoon Sinlaku- was investigated from tropical cyclogenesis until ET. As Sinlaku approached the midlatitude baroclinic zone a strong convective system developed in the vicinity of the transitioning typhoon. Research flights with the NRL-P3 and the USAF-C130 aircrafts captured unique observations of the detailed structure of this convective event during ET using the Dual-Doppler- Radar ELDORA and dropsondes. The observational data are assimilated with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software tool at 4 km horizontal resolution. The obtained SAMURAI analysis enables us to identify deep convection, a stratiform region, warm- and cold frontal structures, and a dry intrusion in the vicinity of the transitioning TC. Q-vector diagnostics indicate that forced ascent in a potentially unstable environment triggers the deep convection. Strong potential vorticity production by latent heat release within the deep convection leads to a
9 deformation of Sinlaku's wind field and presumably favors the cyclone's decay. Finally, a validation of the ECMWF operational analysis against the SAMURAI analysis identifies remarkable differences with respect to the representation of the convective event. This study, based on the T-PARC airborne observations, presents a unique documentation of various physical processes involved in ET.
1630 Discussion and Summary of Session 3a
1800-2000 Dinner
Tuesday, May 22: 0830 Session 3b (T-Parc; Part II) Chair: Richard Moore
Christian M. Grams (1,a), Sarah C. Jones (1), Christopher A. Davis (2), Patrick A. Harr (3) (1) Institute for Meteorology and Climate Research Karlsruhe Institute of Technology, Karlsruhe, Germany (2) Mesoscale & Microscale Meteorology, National Center for Atmospheric Research, Boulder, CO, USA (3) Department of Meteorology, Naval Postgraduate School, Monterey, CA, USA (a) current affiliation: Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland Title: Quantification of the impact of Typhoon Jangmi (2008) on the midlatitude flow
Extratropical transition (ET) of tropical cyclones (TCs) has the potential to cause high impact weather in the vicinity of the transforming cyclone as well as in downstream regions such as Europe or North America. Thereby ET is often accompanied by reduced predictability of the midlatitude flow. In this study we aim to identify the physical processes governing the direct and downstream impact of the extratropical transition of Typhoon Jangmi during the THORPEX Pacific Asian Regional Campaign (T-PARC) in 2008. The main tool are numerical simulations with the mesoscale COSMO model. T- PARC observations are used to validate the model analysis and to investigate the interaction of Jangmi with the midlatitude flow. A potential vorticity (PV) surgery technique based on PV inversion is developed to further quantify the contribution of the TC to the modification of the midlatitude flow during ET. The detailed case study of Typhoon Jangmi (2008) revealed the diabatically enhanced net transport of low PV air from the lower troposphere to jet level as the crucial physical process governing the direct impact of ET. This results in a pronounced ridgebuilding directly downstream of the ET system accompanied by
10 a strong acceleration of the midlatitude jet. A set of Jangmi scenarios with relocated initial storm position revealed that a critical bifurcation point for the track of Jangmi exists south of the approaching midlatitude trough. If the TC moves beyond a critical line, the storm will move poleward and reintensify. Otherwise it will move back to the tropics and decay. It is shown that this critical line sharply separates neighbouring scenarios into reintensifiers and decayers. In the reintensification scenarios Jangmi further triggers a pronounced Rossby wave train and thus has a strong downstream impact. These results highlight the crucial role that ET has on the modification of the midlatitude flow. The strong sensitivity of the downstream flow evolution on the relative position of the ET system and the approaching midlatitude trough gives an explanation for the reduced predictability during ET. Also shown for a particular case study these results were confirmed in four other cases of Western North Pacific and North Atlantic ET.
0900 Elizabeth R. Sanabia Oceanography Department, United States Naval Academy, Annapolis, MD, USA Title: The Re-intensification of Typhoon Sinlaku (2008) In September 2008, Typhoon Sinlaku re-intensified from a sheared, asymmetric, weak tropical storm to a typhoon southwest of Japan. The evolution of the tropical cyclone (TC) structure was observed by multiple aircraft as part of the TCS-08 and T- PARC field programs. Airborne dual-Doppler radar, dropwindsondes, and flight-level observations reveal critical interactions among the decaying TC and three mesoscale vortices that initiated the re-intensification of Sinlaku. The structural characteristics of the three vortices, which include a vigorously growing convective tower, a mesoscale convective vortex, and a low-level hybrid vortex, are defined with respect to representative precipitation processes. Following interaction with the mesoscale vortices, re-intensification proceeded via processes consistent with axisymmetrization as multiple convective bursts rotated cyclonically and inward while a region of older convection propagated outward to become a principal band. Subsequent aircraft observations revealed a warm-core, near- symmetric typhoon. The overall re-intensification scenario is examined with respect to interactions among a variety of processes that vary from synoptic scale to convective scale. Synoptic- scale southwest monsoon flow over elevated sea-surface temperatures and high ocean heat content preconditioned the region where the critical convective episodes began. Mesoscale processes then acted to produce and re-distribute positive vorticity that defined the re-intensification of TY Sinlaku.
0930 Annette Foerster1, Michael Bell2, Patrick Harr2, and Sarah Jones1 1Karlsruhe Institute of Technology
11 Karlsruhe, Germany 2Naval Postgraduate School Monterey, CA USA Title: The Structure of Typhoon Sinlaku in a Vertically Sheared Environment
During the initial stages of extratropical transition (ET), a tropical cyclone (TC) undergoes significant structural changes. During the THORPEX-Pacific Asian Regional Campaign (T-PARC) in 2008 a unique observational data set for the entire life cycle of Typhoon Sinlaku was collected. In this study, we focus on the late tropical stage of Sinlaku when the TC weakens in an increasingly complex midlatitude environment. Research flights with the NRL P-3 and the USAF C-130 aircraft were conducted in the core region of Sinlaku. The P-3 aircraft carrying the Dual Doppler Radar ELDORA penetrated through the eyewall into the TC's center and thus allowed to capture a high resolution data set for the entire eyewall. These radar data, complemented by dropsonde and flight level data, were assimilated with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software at 1 km horizontal resolution. In both radar reflectivity and wind field the SAMURAI analysis shows marked asymmetries in the structure of the core region during the late tropical stage of Sinlaku. The maximum radar reflectivities are located to the north of the center. To the west and south pronounced gaps are apparent in the eyewall. The eastern part of the eyewall is covered by broad ascent and descent occurs mainly to the west of the center. Since for this case the vertical shear of the environmental flow is assumed to be the dominant environmental factor influencing the structure of the typhoon, the SAMURAI analysis domain is split up into quadrants according to the west- southwesterly shear vector. The characteristics of the quadrants are analyzed using Contoured Frequency by Altitude Diagrams (CFADs) and reveal that the main precipitation areas are located left of the shear vector and the main ascent region to the downshear side. These results are consistent with existing modeling and observational studies and highlight the importance of the vertical shear of the environmental flow on the dynamics and thus the structure of the core region of a tropical cyclone. This study enables an unprecedented, very detailed view of the eyewall structure of a typhoon near the end of its tropical stage.
1000 Health Break
1030 Discussion and Summary of Session 3b
1045 Session 4 (ET Structure/diagnosis Part A) Chair: Kimberly M. Wood
John L. Beven II Tropical Prediction Center/National Hurricane Center
12 Miami, FL, USA Title: RGB Airmass Imagery: A New Tool to Diagnose Extratropical Transition of Tropical Cyclones
The process by which a tropical cyclone transforms into a frontal cyclone is known as extratropical transition. During this process, the cyclone entrains significant amounts of cooler and drier air accompanied by major changes in structure and energetics. A specialized satellite product called the RGB Airmass Imagery employs multiple frequencies that are sensitive to water vapor and ozone, atmospheric constituents that undergo notable spatial and temporal changes near a transitioning cyclone. Thus, this imagery has the potential to aid real-time diagnosis of extratropical transition. RGB Airmass imagery is currently available from the SEVIRI imager on the Meteosat Second Generation satellite, the MODIS imager on the NASA Aqua and Terra satellites, and the sounders on the GOES I- P satellites. The paper shows examples of RGB Airmass imagery from cyclones that underwent extratropical transition during the 2011 Atlantic hurricane season, highlighting the observed features.
1115 Clark Evans* and Robert E. Hart^ *Atmospheric Science Group, Dept. of Mathematical Sciences, Univ. of Wisconsin- Milwaukee ^Department of Earth, Ocean, and Atmospheric Science, Florida State University Title: The Thermodynamic Evolution of an Extratropically Transitioning Tropical Cyclone
In this study, the numerically-simulated four-dimensional thermal structure evolution associated with a representative warm-to-cold core extratropical transition event, North Atlantic Tropical Cyclone Bonnie (1998), is detailed. The vertically-integrated, area-averaged temperature near the center of the simulated cyclone cools by approximately 5 K during extratropical transition. This cooling is maximized in the boundary layer and upper troposphere. The simulated cooling is highly asymmetric with approximately 8-10 K (2-4 K) of cooling noted north (south) of the cyclone throughout the troposphere. This cooling asymmetry is of similar magnitude to the environmental meridional temperature gradient and both are in approximate thermal wind balance with the vertically-sheared flow that impacts the cyclone during extratropical transition. Using back trajectory and thermodynamic budget analyses, the thermal structure evolution of the transitioning cyclone is shown to be intricately tied to the interaction and superposition of the cyclone with the synoptic-scale baroclinic zone initially found to its north. This interaction leads to the development of asymmetric ascending and descending air streams commonly observed with extratropical cyclones. These air streams – particularly the cold conveyor belt and
13 dry intrusion – erode the recirculating tropical air stream that characterizes air flow around the cyclone at the start of extratropical transition and transport colder environmental air toward the cyclone. Locally-enhanced compressional warming leads to the development of relative middle-upper tropospheric warmth immediately south of the cyclone during extratropical transition. The findings are placed in the context of previous hypotheses relating to the thermal structure evolution and have implications toward the wind field evolution of transitioning cyclones. Parallels to future work are drawn, encouraging further research into the evolution of material conservation and thermal wind balance during extratropical transition and the nature of vertical wind shear-driven transverse circulations in complex baroclinic environments.
1145 Kyle S. Griffin and Lance F. Bosart Department of Atmospheric and Environmental Sciences University at Albany/SUNY, 1400 Washington Avenue, Albany, NY 12222 Title: Mid-latitude influences on two cases of extratropical transition in the southwest Indian Ocean
Relatively little published literature currently exists on tropical cyclones (TCs) in the Southwest Indian Ocean (SWIO). Long-term annual means show the region averages around 9-10 TCs per season, with about 4 undergoing extratropical transition (ET). This ratio (~43%) is similar to the North Atlantic. To highlight some of the most impressive cases of ET in the SWIO, case studies of TC Edisoana (1990) and TC Jade (2009) are presented. TC Edisoana forms in early March 1990 and reaches a peak intensity of 100 kt on 5 March east of Madagascar. By 6 March, Edisoana begins to accelerate poleward ahead of an approaching shortwave trough and is declared extratropical early on 7 March. Both the shortwave trough and ex-TC Edisoana begin to fall under the influence of an amplifying longwave trough approaching from the southwest on 8 March. Between 1200 UTC 8 March – 1200 UTC 9 March Edisoana deepened by 40 hPa, reaching a minimum pressure of 938 hPa. This rapid deepening occurs in conjunction with the aforementioned longwave trough that contained air of near- Antarctic origin (a source of higher potential vorticity [PV] and strong baroclinicity). Furthermore, rapid amplification of a subtropical ridge immediately downstream of Edisoana due to the combination of strong warm air advection and diabatic heating aids in the poleward amplification of this subtropical ridge. Continued amplification of this subtropical ridge into the polar jet stream results in the creation of a phased jet/ridge downstream of Edisoana. This strong phased jet/ridge becomes quasi-stationary and blocks the progress of Edisoana to the east, while the aforementioned longwave trough continues to approach from the west. The convergence of these two features allows the downstream half-wavelength (measured from trough axis to downstream ridge
14 axis) of the mid-latitude flow pattern surrounding Edisoana to contract. As a result, the greater implied flow imbalance leads to a stronger ageostrophic circulation, further supporting the rapid intensification of Edisoana. A similar case of rapid deepening occurred during and after the ET of TC Jade in mid-April 2009. While ET is induced by a weak shortwave in the subtropical jet stream, the post-ET intensification of Jade occurs in conjunction with a highly anomalous meridionally-elongated PV streamer that approaches from the west on 11 April. An unusually broad, flat subtropical ridge (~6000 km east-west) situated poleward and eastward of Jade prevents the storm from progressing eastward. Subsequently, Jade accelerates poleward and undergoes rapid deepening similar to Edisoana and reaches a minimum central pressure 948 hPa by 0000 UTC 12 April. These cases of rapid intensification after ET highlight the complex interactions between TCs in the SWIO and their surrounding mid-latitude environments.
1215-1315 Lunch
1315 Naoko KITABATAKE Typhoon Research Department, Meteorological Research Institute Japan Meteorological Agency 1-1, Nagamine, Tsukuba, 305-0052 Japan Title: Climatology of Extratropical Transition of Tropical Cyclones in the Western North Pacific
Tropical cyclones (TCs) in the western North Pacific are examined to present climatology of their extratropical transition (ET) during 1979-2004. Two parameters of the cyclone phase space (CPS) are calculated using the Japanese 25-year Reanalysis (JRA-25) dataset as indices of ET for the TCs in the best track data of the Japan Meteorological Agency. Out of 687 TCs in 26 years, 274 (40%) TCs completed ET. The mean transition period from the ET onset until the ET completion is estimated to be about 18 hr, although 16.8% of all ET storms are transformed into the cold-core structure before the increase in thermal asymmetry. Meridional fluctuation of the location of ET completion by season attains its peak in August, while the peak of the ET events occurs in September-October in respect of the ratio of the ET frequency to all TCs, the transition period, and intensity at the time of ET completion. The background environments of ET events are also examined using the monthly mean of the JRA-25 and sea surface temperature of the centennial in situ observation-based estimates (COBE). Both air-sea thermal contrast and the tropospheric vertical shear are greater in autumn than in the Baiu season and summer. The results suggest that, whereas a TC can translate into higher latitude with a warm-core structure in an environment of weak vertical shear and the small air-sea contrast in summer, it tends to complete ET in lower latitude in an
15 environment of strong vertical shear and the large air-sea contrast in autumn- winter. These characteristics of ET affect the seasonal variation in the structure of landfalling TCs in the main islands of Japan.
1345 Zhenxia Long and Will Perrie Fisheries & Oceans Canada, Bedford Institute of Oceanography, Dartmouth NS, Canada Title: Air-sea interactions during an Arctic storm
The impacts of increased open water in the Beaufort Sea were investigated for a summer Arctic storm in 2008 using a coupled atmosphere-ice-ocean model. The storm originated in northern Siberia and slowly moved into the Beaufort Sea along the ice edge in late July. The maximum wind associated with the storm occurred when it was located over the open water near the Beaufort Sea coast, after it had moved over the Chukchi and Beaufort Seas. The coupled model system is shown to simulate the storm track, intensity, maximum wind speed and the ice cover well. The model simulations suggest that compared to conditions of past decades, the decrease of ice cover in the Beaufort Sea results in increased local surface wind, surface air temperature, surface specific humidity and enhanced storm radius. In addition, the surface latent and sensible heat fluxes into atmosphere significantly increased due to the increase of open water. The increase in storm intensity continued during the weakening phase of its life cycle, but there were no significant impacts on the storm track. The expanded open water results in increases in the surface air temperature by as much as 8oC due to the change in the surface albedo, but the atmospheric warming is mainly limited to the boundary layer. As a result, there is increased atmospheric boundary turbulence and downward kinetic energy transport. These changes result in enhanced storm- generated surface winds, by as much as ~ 4 m/s compared to higher ice concentration conditions of past decades. The dominant sea surface temperature response to the storm occurs over open water, and storm-generated mixing in the upper ocean results in sea surface cooling of up to 2oC along the southern Beaufort Sea coastal waters. The Ekman divergence associated with the storm caused a decrease in the fresh water content in the central Beaufort Sea by about 7 cm.
1415 Discussion and Summary of Session 4
1430 Session 5 (High-impact weather of ET/Rainfall) Chair: Eyad Atallah
Shawn M. Milrad1, Eyad H. Atallah2, John R. Gyakum2 1Department of Geography, University of Kansas, Lawrence, KS
16 2 Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec Title: Precipitation modulation by the Saint Lawrence River Valley in association with transitioning tropical cyclones
The St. Lawrence River Valley (SLRV) is an important orographic feature in eastern Canada that has previously been shown to contribute to pressure-driven wind channeling, and locally enhanced precipitation owing to the mechanism of valley-enhanced low-level frontogenesis. The objective of this study is to qualitatively assess the impact of the SLRV on named tropical cyclones that have undergone or are undergoing extratropical transition and track near the SLRV. Such cases can result in heavy precipitation events during the warm season, e.g. Katrina (2005), which in one day produced 55% of the total August rainfall at Montreal, Quebec. From National Hurricane Center best track data, we find 39 cases of former tropical cyclones to have tracked within 500 km of the SLRV from 1979-2010. Using the NCEP North American Regional Reanalysis (NARR), we find that 28 (Group A) of the 39 cases had large differences between near-surface total frontogenesis and 1000 hPa geostrophic frontogenesis, and that these differences were oriented parallel to the SLRV. A qualitative comparison for a subset of 10 cases from 2004-2008 is performed utilizing high-resolution (15 km) gridded Canadian precipitation data. Comparisons with NARR frontogenesis differences show that enhanced regions of precipitation were co-located with the largest frontogenesis differences and were also oriented parallel to the SLRV. The suggested physical pathway to enhanced ascent and precipitation within the SLRV during Group A cases is rooted in 1) enhanced low-level frontogenesis and 2) weak atmospheric stability within the SLRV. The enhanced low-level frontogenesis owes to an along-valley pressure gradient established by the approaching named tropical cyclone, which subsequently induces pressure-driven wind channeling within the valley. Regions of weak convective (potential) stability are often co-located with areas of valley-enhanced low-level frontogenesis
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1530 Michael Svatek, Heather Archambault, and Patrick Harr*; and Naoko Kitabatake> *Naval Postgraduate School, Monterey, CA USA >Japan Meteorological Agency/Meteorological Research Institute, Tsukuba, Japan Title: Precipitation Patterns Associated with the Recurvatures of Hurricane Irene and Typhoon Roke in September 2011
Hurricane Irene recurved and moved poleward along the east coast of North
17 America during the first week of September 2011. A few weeks later, Typhoon Roke recurved over the western North Pacific and moved poleward over the east coast of Japan. In both cases, extreme amounts of precipitation caused inland flooding and damage. The distribution of rainfall is examined to define temporal evolution of rainfall relative to each tropical cyclone. In the case of Typhoon Roke, the temporal distribution of rainfall over Northern Japan exhibited a bimodal character with the initial period of heavy rain occurring while the tropical cyclone was nearly 1000 km to the south. The distribution of rainfall was representative of the occurrence of a predecessor rain event (PRE). This first period of heavy rainfall is examined relative to the forcing of a PRE-like episode. A second period of heavy rainfall occurred 24 h after the end of the PRE-like rainfall episode, and was related to the passage of Typhoon Roke. The temporal distribution of rainfall over Japan during Roke is compared with that over New England during the passage of Hurricane Irene. The distributions of precipitation patterns over Japan during Typhoon Roke, and New England during Hurricane Irene, are related to the environment in which each storm is moving using analyses from the National Centers for Environmental Prediction/Global Forecast System (NCEP/GFS) and simulations performed with the Advanced Weather Research (ARW) forecast model. Specific factors are examined with respect to the interaction between the midlatitude jet moving across eastern Asia and the outflow from TY Roke, the lower-tropospheric temperature advection patterns associated with the tropical cyclone, and poleward transport of moisture by the tropical cyclone. Differences in the spatial and temporal variations in these various forcing mechanisms are related to the variations in precipitation that exist as each tropical cyclone moved poleward. For example, whereas two precipitation events occurred over northern Japan as defined above, locations over southern Japan primarily experienced one precipitation maximum that was associated with the landfall of Typhoon Roke.
1600 Discussion and Summary of Session 5
1800-2000 Dinner
Wednesday, May 23: 0830 Session 6 (ET Structure/diagnosis Part B) Chair: John L. Beven II
Florian Pantillon (1), Jean-Pierre Chaboureau (1), Christine Lac (2) and Patrick Mascart (1) (1) Laboratoire d'Aérologie, University of Toulouse and CNRS, Toulouse, France (2) CNRM/GAME, Météo-France and CNRS, Toulouse, France
18 Title: What does a cloud-resolving model bring during an extratropical transition?
Deterministic forecasts initialised during the extratropical transition of Hurricane Helene, in September 2006, delivered a poor description of an upper-level trough downstream on Europe. As a consequence, they missed the explosive development of a surface cyclone into a tropical-like storm over Italy. A Rossby wave train played a major role on the skill of forecasts during this chain of events: it propagated model uncertainties downstream, both from the outflow of Helene towards Europe, and from North America towards the environment of Helene. Known as a source of uncertainty in global models, the diabatic effets that occurred during this extratropical transition are investigated here. In this purpose, the Meso-NH model was run over a semi-hemispheric domain, in a regional (dx=24km) and in a cloud-resolving mode (dx=4km).
Helene showed a complex behaviour during its extratropical transition. Three intensifications of Helene in three days were forced by three filaments of potential vorticity, formed from the Rossby wave train and which wrapped around Helene. In response to these dynamically forced ascents, strong diabatic effects brought heavy precipitation and contributed to the building of a downstream ridge. In order to show its impact on the Rossby wave train, Helene was filtered out from the initial state. A quick cyclonic wave breaking occurred without Helene, and the building of a downstream ridge was inhibited. This modification of the Rossby wave train propagated downstream, eventually leading to the absence of an upper- level trough on Europe, crucial for the development of the tropical-like storm over Italy.
The model resolution had a dramatic impact on the track of Helene. After its extratropical transition, Helene was correctly steered towards the British Isles in the cloud-resolving run, whereas it was wrongly steered equatorwards in the regional run. During the two first intensifications, no striking difference appeared between the runs in the dynamics of Helene. The bifurcation of tracks was caused later, by a different relative position of Helene with the third filament of potential vorticity. A filament closer to Helene in the regional run, compared to the cloud- resolving run, resulted in a stronger intensification and a southward track. The improvement of the cloud-resolving run was therefore mainly related to the very sensitive phasing of Helene with the Rossby wave train.
0900 J. F. Quinting, D. Anwender, S. C. Jones, L. Scheck Karlsruhe Institute of Technology Title: Description of the lifecycle of Super Typhoon Choiwan based on the YOTC-dataset
The knowledge of the physical processes in tropical convection and their representation in global forecast systems are a grand challenge in weather
19 forecasts. To address this challenge the World Climate Research Programme and the World Weather Research Programme/THORPEX initiated in 2008 the Year of Tropical Convection (YOTC). An outcome was the YOTC-dataset available at the European Centre for Medium-Range Weather Forecasts. This dataset contains amongst others the three-hourly model tendencies of specific humidity and temperature for each day of the YOTC period. For the first time these tendencies are provided as a set of 3-dimensional physical process fields containing the individual parts of the absolute tendencies like the dynamics, the convection, clouds, and turbulent diffusion. One of the major typhoons in the YOTC period was Super Typhoon Choiwan (2009). The lifecycle of Choiwan from its genesis to its extratopical transition (ET) was investigated in students in a seminar on the base of the model tendencies. The tendencies afford on the one hand insights into the various physical processes that determine each period of the cyclone's lifecycle and on the other hand give information about the parameterization of physical processes by the ECMWF model. The assessment of the importance of the individual tendencies on the cyclone's lifecycle is the main result of the presented study.
0930 Emilie Scherer Risk Management Solutions, Peninsular House, 30 Monument Street, London EC3R 8NB, United Kingdom, [email protected] Title: Duration of extratropical transition in the North Western Pacific: WRF-ARW simulations
The tropical or extratropical nature of a cyclone is crucial to assess the wind and rain hazard occuring during the lifecycle of a typhoon. In particular, Japan is situatedat such latitudes where the extratropical transition occurs: during the period 1979-2004 about half of the landfalling cyclones over Japan were purely tropical cylcone, the other half being in the process of transitioning or fully transitioned (Kitabatake 2011). The cyclone phase space methodology developped by Hart (2003) enables one to quantify the start and end time of the extratropical transition with simple criteria. Computing cyclone phase spaces for typhoons simulated with WRF-ARW provides a quantification of the start time and duration of the transitionning. Using historical initial and environmental conditions, we simulate typhoons with a 4km resolution that are close to historical events as well as additional new scenarios, and therefore create a distribution of transitioning start time and duration for the North Western Pacific. Such distribution can be compared to and complement the ones created from reanalysis data with lower resolution (Kitabatake 2011 in the North Western Pacific - 1:25 resolution , Hart and Evans 2003 in the Northern Atlantic - 2:5 resolution).
References: . Robert E. Hart and Jenni L. Evans, A climatology of the extratropical transition of
20 Atlantic tropical cyclones, Journal of Climate, 2001, 14, 546-564. . Robert E. Hart, A cyclone phase space derived from thermal wind and thermal asymmetry, Monthly Weather Review, 2003, 131, 585-616. . Naoko Kitabatake, Climatology of extratropical transition of tropical cyclones in the Western North Pacific defined by using cyclone phase space, Journal of the Meteorological Society of Japan, 2011, 89, 309-325.
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1030 TANG Jie Shanghai Typhoon Institute, Laboratory of Typhoon Forecast Technique/CMA, Shanghai, China Title: Moist Frontogenesis during the Extratropical Transition (ET) process of the typhoon Toraji(0108) landfall in China Mainland
1. Introduction (more chapters were sent in the abstract) Extratropical transition of TCs have been discussed from different angles in previous sutdiess (e.g.Jones et al 2003). Harr et al. (2000) treated the development of frontal characteristics as a manifestation of the physical processes that taken place during ET firstly, since the change of frontal features signifies the evolution undergoing ET from a nearly symmetric TC structures to the typical asymmetric extratropical cyclone structure. Cylonegenesis is always highly related with frontogenesis (Bluestein,1993). ET, which is defined by the process from a tropical cyclone to an extratropical cyclone, can be considered as a frontogenesis process in some content. The difference between ET and classical cylonegenesis maybe just lied in some tropical characters during ET process. This difference may make the evolution of the front during ET more complicate and distinct. If this is true then, detail diagnosis based on the front function is reasonable to understanding ET. Petterssen (1956) defines frontogenesis as the tendency toward formation of a discontinuity in the density field or the intensification of an existing sloping transition surface. This frontogenesis also can be described by the discontinuity of potential temperature more often nowadays. And vector frontogenesis is defined as the Lagrangian temporal derivative of the horizontal potential temperature gradient (Keyser et al 1988). The vector frontogenetical function is shown in eq(1) and eq(2) (1) is the scalar frontogenetical function as shown and is the rotational frontogenetical function. Harr and Elsberry(2000) and Kitabatake(2000) diagnosed some ET cases frontogenetical function shown above respectively. And rotational frontogenesis induced by background circulation was found to be important to the
21 reintensification of ET(Harr and ELSBERRY,2000). Up to now, there is few research discuss the role of frontogenesis during the reintensification process of ET happened in China mainland. Unlike other cases, ET process happened in China mainland always are affected by southwest low level jet with large humidity derive from monsoon. So the contribution of the moist to ET needed to be discussed in detail here. Some typhoon cases that landfall at China mainland will be transient to have extratropical cyclone characters and be intensified again which is called as ET under the influence of southward and westward air stream. These ET cases with complicate structure always brought severe rainfall disaster. The 0108 typhoon named Toraji landfalled at Fujian on 31 Jul 2001 and finished ET at 00Z/02 02 Aug is such a typical ET case. This paper is to discuss the front evolution of the ET process of Toraji to study the detail story of ET cases happened in China mainland. A brief model description and verification is introduced in section 2. And then the ET evolution is presented by the TBB figures and model result. The Last section is a preliminary summary. 4.Summary A numerical resolution is employed to simulate the frontal evolution of typhoon Toraji(0108).By preliminary front diagnosis ,it is found there is a warm front developed in the north part of Toraji. And this low level warm front is related with the low lever jet stream originated in monsoon. During the frontogensis,the divergence and adiabatic in low to middle level is the main role. Moist process is important to ET process of the ET casesin China mainland where is much closer to the monsoon region. More verify and descript work is necessary to judge the importance of different factor of vector Frontgenesis equation in (2) Acknowledgements This work was supported by Supported by Major State Basic Research Development Programof China(No.2009CB421504) ,National Natural Science Foundation of China(No.40905029).
1100 Session 7 (Application of remote sensing for ET analysis and forecasting) Chair: Chris Fogarty
William Perrie2, ,Biao Zhang1, 2 1School of Oceanic Science, Nanjing University of Information Sciences & Technology Nanjing, Jiangsu, China 2Fisheries and Oceans Canada, Bedford Institute of Oceanography Dartmouth, Nova Scotia, Canada Title: Cross-Polarized Synthetic Aperture Radar: A New Potential Measurement Technique for Hurricanes
The focus of this work is the determination of surface wind fields from satellite
22 SAR (synthetic aperture radar) observations. The high resolution and ground- breaking new imaging properties of spaceborne SAR systems have made SAR imagery a crucial new source of information for various marine applications, independent of sunlight and cloud conditions, and largely independent of precipitation. Moreover, as we enter the second decade of the 21st century, the rapid expansion in the availability of spaceborne SAR sensors has significantly increased the temporal and spatial observations over the sea. New satellites that have recently been launched or are intended to be launched in the near future include the European Space Agency Sentinel series, German Aerospace Center’s TerraSAR and TanDEM series, Canadian RADARSAT-2 and RADARSAT Constellation, and the Italian COSMO Skymed series. Worldwide, several research and operations groups are poised to develop new analyses and methodologies to develop and enhance SAR applications in marine areas, by deriving innovative new marine products, from C-/X-band SAR sensors with multi-polarisations. Performance of the novel SAR oceanographic products will go significantly beyond the present state-of-the-art. In particular, the new SAR sea surface wind products will be suitable for new investigations related to extreme winds in hurricanes. The results presented here represent a contribution to significantly enhanced SAR applications in observations of marine winds in hurricanes. We present an empirical C-band Cross-Polarization Ocean backscatter (C-2PO) model for wind retrievals, from synthetic aperture radar (SAR) data collected by the RADARSAT-2 satellite. The C-2PO model relates normalized radar cross section (NRCS) in cross-polarization (VH) to the wind speed at 10-m height. This wind retrieval model has the characteristic that it is independent of wind direction and radar incidence angle but is quite linear with respect to wind speed, over the range of available wind speed data. To evaluate the accuracy of the proposed model, winds with resolution on the scale of 1 km were retrieved from a dual- polarization SAR image of Hurricane Earl on September 2, 2010, using the C- 2PO model and compared with CMOD5.N, the newest available C-band geophysical model function (GMF), and validated with collocated airborne Stepped Frequency Microwave Radiometer (SFMR) measurements and National Data Buoy Center (NDBC) data. Results suggest that for winds up to 38 m/s, C- 2PO has a bias of -0.89 m/s and a root mean square (RMS) error of 3.23 m/s, compared to CMOD5.N, with bias of -4.14 m/s and RMS difference of 6.24 m/s. Similar results are obtained from Hurricane Ike from a comparison of wind retrievals from C-2PO and CMOD5.N with H*Wind data. The advantage of C- 2PO over CMOD5.N and other GMFs is that it does not need any external wind direction and radar incidence angle inputs. Moreover, in the presently available quad-polarization dataset, C-2PO has the feature that the cross-polarized NRCS linearly increases even for wind speeds up to 26 m/s and it reproduces the hurricane eye structure well, thereby providing a potential technique for hurricane observations from space.
1130 Discussion and Summary of Sessions 6 and 7
23
1200-1330 Lunch
1330 Session 8 (Operational ET topics) Chair: Sim D. Aberson
John L. Beven II Tropical Prediction Center/National Hurricane Center Miami, FL, USA Title: Verification of National Hurricane Center Forecasts of Extratropical Transition 1991-2011
The process of a tropical cyclone becoming an extratropical cyclone – extratropical transition (ET) – is an important one in tropical cyclone forecasting and impact assessment. However, no tropical cyclone warning center currently performs an official rigorous verification of ET forecasts. This paper presents results from a continuing experimental verification of ET forecasts at the National Hurricane Center, with the record now covering the period from 1991-2011. The verification has two parts. The first is a version of the 2 x 2 contingency diagram verification used by the National Weather Service for severe weather warning verification – was the event forecast versus did it actually occur. This includes the various skill statistics such as critical success index, false alarm ratio, and probability of detection. The second is an examination of the timing errors of forecasts of the cyclone being extratropical. To maintain a homogeneous data set, the verification is split into a 5-day forecast verification for 2001-2011 and a 3- day verification for the full verification period. The results show that the NHC forecasts have skill according the contingency diagram metrics, and that the level of skill has generally increased during the 1991-2011 period. However, even in the current era of improving analysis and forecasting tools there is considerable year-to-year variability in the statistics, and certain individual storms have been very problematic. The paper highlights recent cyclones that have been very problematic in terms of large forecast timing errors, incorrect transition forecasts, and missed transitions, along with their implications from a warning viewpoint.
1400 Darin Borgel, Chris Fogarty Canadian Hurricane Centre Halifax, Nova Scotia, Canada Title: CHC Hurricane forecasting with NinJo and future plans
24 The Canadian Hurricane Centre (CHC) has been providing Tropical Cyclone warnings and information specific to Canada for nearly 25 years. Co-located with the Atlantic Storm Prediction Centre (ASPC) in Dartmouth, Nova Scotia, the CHC has gained prominence in recent years due to several high-impact hurricanes affecting Canada, such as Hurricane Juan (2003), Hurricane Igor (2010) and Hurricane Irene (2011).
In 2010 the CHC began the initiative to port its production software into NinJo, and the first operational version of the Hurricane Layer will be included in version 1.6. It is anticipated the CHC will use the Hurricane Layer, together with the Product WorkBench (PWB), to produce its operational graphic and text products for the 2012 tropical season. In this presentation, a brief history of the CHC and hurricane climatology in the North Atlantic will be provided as a background to hurricane forecasting in Canada. The requirements for the initial functionality of the Hurricane Layer, based on the capabilities of existing CHC software, will be discussed; especially in the context of how the Met- Object/Event-based approach is ideal for handling Tropical Cyclones. The features, functionality and output capabilities of the current version of the Hurricane Layer will be demonstrated. Changes and additional functionality for the next operational version will be discussed, along with a wish list for future CHC and NinJo Hurricane Layer capabilities.
1430 Chris Fogarty Canadian Hurricane Centre Halifax, Nova Scotia, Canada Title: Forecast products tailored for effective communication of ET impacts
Explaining how hurricanes undergo changes in structure as they move into the mid- latitudes is a challenge, and the Canadian Hurricane Centre has been focusing on different ways to educate users and package their forecasts for effective use. A presentation on this topic showing examples of products will hopefully lead to some fruitful discussions and new ideas.
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1530 Chris Fogarty Canadian Hurricane Centre Halifax, Nova Scotia, Canada Title: Revisitation of IWTC7 2010 – topic 2.5 on ET Forecasting
The IWET4 will be a good forum to revisit topic 2.5 from IWTC7 which focussed on ET prediction in various basins around the world. A review of the material will hopefully lead to discussions on sharing of methods and practices among various forecast agencies
25 where ET occurs.
1600 Jim Abraham Canadian Hurricane Centre Halifax, Nova Scotia, Canada Title: ET in Canada: a partnership between research and operations
Canada has been impacted significantly by tropical cyclones undergoing transition in the middle latitudes. Some notable examples are the Saxby Gale in 1869 and "Hurricane" Hazel in 1954. It was not until significant advances in measurement and modeling technology that Canada undertook the leadership responsibility associated with its own Canadian Hurricane Centre in the early 1990's
Subsequent capacity and knowledge building was achieved over the next (and last) 20 years to enable Canada to be recognized internationally for its expertise in the extratropical transition understanding and prediction challenge. This advancement was successful largely due to close relationships developed between research and operations that are currently enabled by a "National Marine Laboratory", that facilitates effective interaction between government and academic research, as well as between research and operations. This has been reflected in the forecast and warning performance of the Canadian Hurricane Centre, as demonstrated by the impressive prediction of Hurricane Igor in 2010.
1630 Discussion and Summary of Session 8
1800-2000 Dinner
Thursday, May 24: 0830 Session 9 (Impacts of ET on downstream predictability) Chair: Doris Anwender
Kristen L. Corbosiero1, Ron McTaggart-Cowan2, Lance Bosart 3 1University of California, Los Angles, Los Angeles, California 2 Environment Canada, Dorval, Québec, Canada 3 University at Albany, State University of New York, Albany, New York Title: The Impact of Recurving Eastern North Pacific Tropical Cyclones on the
26 Downstream Flow Pattern over North America
Most eastern North Pacific tropical cyclones (TCs) form off the Mexican coast and track due west into the central Pacific bothering nothing but the ocean flora and fauna. However, a few TCs that form anomalously close to the Mexican coastline in the later part of the hurricane season track towards the north and east around the western edge of the southward retreating subtropical high and/or in association with a digging mid-latitude trough off the west coast of the United States. Thirty-five such storms have been identified between 1948 and 2003, accounting for 25-100% of the warm season precipitation over the southwest United States. Furthermore, about half of these systems maintain their tropical moisture and low-level vorticity signatures, and can be tracked thousands of kilometers downstream over the continental United States. In this presentation, a certain subset of these long-lived systems, those that were associated with significant amplification of the downstream flow pattern, will be analyzed using a dynamic tropopause (DT) perspective. Specifically, the role of eastern North Pacific TC-induced diabatic heating on downstream ridge development and potential vorticity non-conservation will be explored in composites and through individual case studies of Hurricanes Nora (1997) and Isis (1998). While Nora recurved in response to a mobile mid-latitude trough and underwent extratropical transition (ET) and Isis weakened rapidly while recurving around the western periphery of a large, stationary disturbance on the DT, both storms resulted in significant ridge eruption over the southwest United States and Rocky Mountains. Subsequent evolution led to a contraction of the half wavelength between the ridge and downstream trough couplet, and significant cyclonic activity (including the ET of Hurricane Earl (1998)) along the Gulf coast states.
0900 Christian M. Grams (1,2), Heini Wernli (2), Maxi Böttcher (2), Jana Campa (3), Ulrich Corsmeier (1), Sarah C. Jones (1), Julia H. Keller (1), Claus-Jürgen Lenz (1), Lars Wiegand (4) (1) Institute for Meteorology and Climate Research (IMK-TRO), Karlsruhe Institute of Technology (KIT), Germany (2) Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland (3) Institut für Physik der Atmosphäre, Universität Mainz, Germany (4) Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK Title: The key role of diabatic processes in modifying the upper-tropospheric wave guide: a North Atlantic case-study
This study highlights the importance of diabatic processes for the complex interaction of weather systems in the North Atlantic-European sector during the week of 7-14 September 2008. A chain of events occurred including the
27 extratropical transition (ET) of Hurricane Hanna, a subsequently developing extratropical cyclone, the formation of an upper-level PV streamer that protruded towards Europe and triggered intense rainfall, and the genesis of a Mediterranean cyclone. A potential vorticity (PV) perspective is adopted along with trajectory calculations to elucidate the diabatic modification of the midlatitude flow. Important diabatic PV modifications occurred at upper levels, associated with the cross-isentropic transport of low PV air within warm conveyor belts (WCBs). These were diagnosed during the ET of Hanna and the development of the extratropical cyclone near Newfoundland. The WCBs contributed to the amplification of ridges downstream of each cyclone and to the subsequent elongation of Hanna's upstream trough into a PV streamer. This streamer eventually triggered the Mediterranean cyclogenesis. The second major effect of the diabatic processes occurred on smaller scales, in the low and middle troposphere. The remnants of Hanna's tropical PV core advected moist air towards the baroclinic zone leading to condensational PV production in the lower troposphere. In contrast, in the case of the extratropical cyclone diabatic PV production occurred within its WCB at mid levels. These diagnostic analyses corroborate the potential of diabatic processes associated with extratropical flow systems for the modification of both the low-level vortices and the upper-level Rossby wave guide.
0930 Julia H. Keller and Sarah C. Jones, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany Patrick A. Harr, Naval Postgraduate School, Monterey, CA Title: The Eddy Kinetic Energy Budget for Several Forecast Scenarios of the Extratropical Transition of Typhoon Choi-Wan
The extratropical transition (ET) of a tropical cyclone may strongly influence the mid-latitude circulation pattern by amplifying or even triggering a Rossby wave. This modification of the the mid-latitude flow impacts even regions far downstream of the ET event, as the amplification of the mid-latitude wave pattern coincides with the potential for strong cyclogenesis. The interaction between the tropical cyclone and the mid-latitude circulation involves complex interactions of physical processes, whose mis-representation in NWP often leads to a reduction of predictability around and downstream of an ET event. Hence, a deeper understanding of the physical and dynamical aspects of the interaction between the tropical cyclone and the mid-latitude flow is crucial to improve numerical forecasts during ET events. A possibility to examine the processes during the downstream impact of ET events is provided by an analysis of the eddy kinetic energy budget. Thereby, the kinetic energy distribution of the actual atmospheric circulation is examined with respect to a monthly mean distribution. In this framework of downstream baroclinic development, a downstream dispersion of eddy kinetic energy, emanating from the transitioning storm may support the amplification of the downstream wave pattern.
28 To examine the role of a transitioning tropical cyclone in different forecast scenarios, the analysis of the eddy kinetic energy budget in the context of downstream baroclinic development is employed on ensemble forecasts for the extratropical transition of Typhoon Choi-Wan (2009) for the first time. Using an EOF- and subsequent cluster analysis the dominant possible development scenarios for the ET of TY Choi-Wan are extracted from an ECMWF ensemble forecast. The eddy kinetic energy budget is then computed for representative ensemble members of interesting and opponent forecast scenarios for the ET event. The budget terms, involving the baroclinic conversion of eddy available potential into eddy kinetic energy and the advection and dispersion of eddy kinetic energy enable to identify distinct physical processes during the ET. The investigated forecast scenarios range from a strong reintensification of the transitioning tropical cyclone, together with a strong amplification of the mid- latitude wave pattern, to a nearly decaying cyclone during its transition. Hence, the influence of the TC itself as well as the role of the distinct mid-latitude flow pattern that surrounded the transitioning storm can be examined in detail. In this presentation the important processes during the ET of TY Choi-Wan causing the distinct forecast scenarios will be highlighted and discussed, after a brief introduction of the underlying analysis method.
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1030 Carolyn A. Reynolds and James D. Doyle Naval Research Laboratory, Monterey, CA Title: Adjoint-based Predictability Studies of Tropical Cyclones and Extra-tropical Transitions
Adjoint-based tools can provide valuable insight into the mechanisms that influence the evolution and predictability of atmospheric phenomena. An adjoint model can be used for the efficient and rigorous computation of forecast sensitivity to changes in the initial state, as well as for the calculation of leading singular vectors, which are the fastest growing perturbations to a given forecast trajectory. We apply adjoint-based tools from the non-hydrostatic Coupled Atmosphere/Ocean Mesoscale Prediction System (COAMPS) and the Navy Operational Global Atmospheric Prediction System (NOGAPS) to explore the sensitivity and predictability characteristics associated with tropical cyclones, with a focus on the recurvature and extra-tropical transition (ET) stages. The adjoint-based calculations illustrate complex influences on tropical cyclone evolution that occur over a wide range of scales from convective clusters within the storm itself to larger-scale weather systems that are remote from the storm. We focus primarily on the Northwestern Pacific Super Typhoon Lupit, which threatened landfall in the northern Philippines before recurving into the mid- latitudes and eventually undergoing ET during October 2009. A series of
29 COAMPS moist adjoint calculations reveals that 24-h forecast sensitivities to initial perturbations in temperature and moisture peak during the recurvature and ET phases. During recurvature, maximum sensitivities occur in the mid- troposphere and extend to the north and north-west of the storm, consistent with the results from previous studies. During ET, maximum sensitivities occur in the mid- to lower-troposphere, but in contrast to the recurvature period, sensitivities extend eastward of the storm into a region of anticyclonic flow. While the maximum sensitivity values are comparable during recurvature and ET, total sensitivity integrated over the domain is significantly larger during ET. Implications of these findings for the predictability of tropical cyclones will be discussed.
1100 Leonhard Scheck, Sarah C. Jones, Vincent Heuveline Karlsruhe Institute of Technology Title: Perturbation Growth Mechanisms for Barotropic Vortices in Horizontal Shear
Tropical cyclones that approach the midlatitudes and recurve experience an environment characterized by strong horizontal and vertical shear. These TCs may undergo extratropical transition and reintensify or dissipate. The predictability of the track and the intensity evolution of recurving cyclones is often reduced. Small changes in the initial state can have a large impact on the extratropical transition and the subsequent evolution. The influence of uncertainties in the initial state on the forecast error can be investigated using singular vectors (SVs). SVs reveal the structure and location of the perturbations that experience optimal growth during the forecast time interval. However, the complex structure of SVs in three-dimensional full-physics models impedes their interpretation and the identification of the perturbation growth mechanisms they are based on. Idealised studies with reduced complexity allow for a clearer view on selected growth processes. In this study we focus on the influence of the horizontal shear on barotropic perturbation growth. For this purpose we use a nondivergent barotropic model. Our results demonstrate that horizontal shear has a significant influence on the structure and growth rates of the SVs. For typical values of midlatitude shear we find strongly increased singular values. The sensitive regions indicated by initial SVs extend considerably further away from the vortex than without shear and are aligned with streamlines in the frame comoving with the vortex. The evolved SVs are characterized by a dipole mode in the vorticity that can be interpreted as a displacement of the vortex. A detailed analysis of the vortex displacement caused by the leading SVs reveals that the increased growth rates are related to two factors. Firstly, a vortex displaced from its original position in a horizontal shear background experiences a modified steering flow and is thus displaced further. And secondly, anticyclonic shear slows down the advection of perturbations around the vortex core and thus facilitates the displacement of the vortex by the circulation associated with the
30 perturbations. We discuss the influence of shear strength, grid resolution, viscosity and different vortex profiles on the SVs. In particular, we show that the growth rates for barotropic unstable vortices do not profit from a background shear flow. Finally, we use the results for single vortices to interpret SVs for the interaction of a tropical cyclone and a tropopause front.
1130 Discussion and Summary of Session 9
1200-1330 Lunch
1330 Session 10 (Planetary-scale implications) Chair: Clark Evans
David Small, Eyad Atallah, John Gyakum Department of Atmospheric and Oceanic Sciences, McGill University Title: The triggering of warm-season blocking by extratropical transition
While there are numerous definitions of blocking in the literature, most of them involve defining some sort of height anomaly relative to a mean state. Consequently, most blocking indices are particularly suited to capturing events that occur in the cold season when height gradients and anomalies are particularly pronounced. However, recent events ranging from the extended Canadian Prairies drought of 2001-2003 to the heat wave and subsequent wild fires in Russia and extensive flooding in Pakistan this past summer suggest a need to investigate warm season blocking episodes more deeply. As such, the definition of blocking recently developed by Pelly and Hoskins (2003), which is roughly based on a reversal of the potential temperature gradient on the dynamic tropopause is adopted to investigate the frequency, location, and triggering of warm season blocking events. Preliminary results indicate that there is a significant temporal peak in blocking episodes, most notably in the North Pacific that appears coincident to the time of peak extratropical transition (ET) activity in the Pacific Basin. While we are not suggesting that ET is either a sufficient or necessary condition for the triggering of a blocking event, we do suggest that an ET in the presence of a significant zonal jet can lead to the development of mid and high latitude blocking as evidenced by the ET of super-typhoon Bart (1999). The resulting block was associated with a collapse of the Hemispheric available potential energy (APE) and led to a high wind and storm-surge event along the Arctic coastline of Canada. Therefore, the goal of this study is to systematically investigate the relationship between ET and warm season blocking, and to hopefully diagnose
31 and illustrate the pertinent dynamics and thermodynamics involved.
1400 Eyad Atallah and John Gyakum Department of Atmospheric and Oceanic Sciences, McGill University Title: The impact of tropical cyclone activity in the Northern Hemisphere on the Available Potential Energy
Traditionally, the increase of Available Potential Energy (APE) over synoptic time scales has been thought of as a radiatively driven process, controlled by net radiation deficits and surpluses at low and high latitudes respectively. The inherent implication of this mechanism is that increases in APE generally occur slowly, on the order one to a several weeks. In contrast depletions of APE are most often related to the development of large storms which act to dissipate the mean temperature gradient by displacing cold air to low-latitudes and warm air to high latitudes. The inherent implication of this mechanism is that depletions of APE can happen over relatively short time scales, ranging from a few days to about two weeks. Consequently, one can reasonably expect quick depletions of APE followed by a relatively slow recovery, as the atmosphere slowly “regains its energy reserve.” However, an examination of the intraseasonal variation of APE does not really show a mismatch between the preferential time scales of APE increase and depletion. This suggests that synoptic-scale features such as cyclones, including tropical and transitioning cyclones, may also be playing a role in the increase of APE, not only in its depletion. Consequently an investigation of the impact of tropical cyclones and extratropical transition on the Hemispheric APE since 1979 is performed. Preliminary results suggest that there are two mechanisms by which tropical cyclone activity (including extratropical transition) can act to contribute to rapid increases in APE. These mechanisms include 1) latent heat release at low latitudes associated with non-recurving tropical cyclones and 2) extratropical transition occurring in a strong deformation zone. The former mechanism is associated with the preferential heating of tropical and subtropical latitudes due to large amounts of condensation, while the latter mechanism is associated with in an increase in the temperature gradient by juxtaposing tropical and polar airmasses in a confluent flow regime.
1430 Lance F. Bosart and Kyle S. Griffin Department of Atmospheric and Environmental Sciences The University at Albany 1400 Washington Avenue Albany, NY 12222 Title: Five Linked September (2011) Northern Hemisphere Tropical Cyclones: Noru,
32 Talas, Lee, Nate, and Katia
The purpose of this presentation is to use potential vorticity (PV) thinking to document the structure and evolution of the large-scale flow from eastern Asia to the western Atlantic from 1-10 September 2011. This period featured five tropical cyclones (TCs), two over the western Pacific (Noru and Talas) and three over the Gulf of Mexico and western Atlantic (Lee, Nate, and Katia), that could be linked to one another through mutual interactions and downstream development. Slow-moving, large TC Talas produced widespread heavy rainfall over western and central Japan in early September 2011. An apparent predecessor rain event that occurred northeast of TC Talas culminated in the formation of a diabatic Rossby vortex (DRV) that raced northeastward toward the Aleutians and transitioned to a prominent extratropical cyclone (EC) over the North Pacific. Anticyclonic wave breaking (AWB) northeast of TC Talas enabled a potential vorticity (PV) streamer to form to the south over the subtropical western Pacific. TC Noru formed southeast of TC Talas by the tropical transition process along this PV streamer. A second AWB event occurred over the northwestern Pacific as TC Noru engaged in a binary interaction with TC Talas and both storms subsequently underwent extratropical transition along the northeastern coast of Asia. The second AWB culminated in the formation of a deep downstream trough over the Gulf of Alaska. This deep trough was associated with an intense EC that originated from the aforementioned DRV. Farther downstream, TC Lee formed over the southeastern Gulf of Mexico on 1 Sep, drifted northeastward, and made landfall in eastern Louisiana on 5 Sep at which time it was experiencing ET ahead of a meridionally elongated trough over the central U.S. This trough became meridionally elongated in response to the aforementioned downstream trough development over the Gulf of Alaska and subsequent ridge amplification over western North America. TC Nate formed over the Bay of Campeche at the southern end of a wind shift line that marked a remnant cold front that stretched southward from TC Lee and was associated with the meridionally elongated trough. TC Nate produced very heavy rains over eastern and southeastern Mexico and subsequently made landfall over eastern Mexico. The meridionally elongated trough west of TC Lee collapsed to a PV tail as the southern part fractured from the main PV reservoir while the northern part of the PV tail continued eastward toward New England. The remnants of LC Lee moved northeastward along a low-level frontal boundary ahead of the fractured trough and produced very heavy rainfall. Diabatic outflow-induced upper-level ridging over the western Atlantic ahead of Lee helped steer TC Katia, an Atlantic storm that was active at the time that TC Lee made landfall in Louisiana, toward extreme southeastern New England. Deep tropical moisture ahead of TC Katia flowed northward and north-northwestward around the western side of the western Atlantic ridge. This moisture, in conjunction with moisture flowing north-northeastward from the remnants of TC Lee, contributed to a brief PRE over parts of New England and Atlantic Canada.
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1500 Health Break
1530 Andrea Lang1, Jason Cordeira2, Lance Bosart1, Daniel Keyser1 1 University at Albany, SUNY, Albany, NY 2 NOAA/ESRL, Physical Sciences Division, Boulder, CO Title: The impact of the extratropical transition of TC Dale (1996) on the early 1996-97 wintertime stratospheric circulation
It has become increasingly accepted that recurving tropical cyclones (TCs) can have substantial impacts on the hemispheric general circulation as well as downstream predictability. A recurving TC that transitions into an extropical cyclone can excite a Rossby wave train that is associated with an enhancement of meridional fluxes of heat and momentum. In some cases, meridional fluxes of heat and momentum extend well into the stratosphere, where such fluxes are associated with an upward EP flux from the troposphere into the stratosphere. An environment characterized by EP flux convergence experiences an increase in wave activity and consequently a decrease in the westerly momentum of mean zonal wind. When an extratropical transitioning TC occurs during the spinup of the northern hemisphere wintertime circulation, the impact of the EP flux convergence in the stratosphere can be to slow the establishment of the wintertime stratospheric polar vortex and the associated stratospheric polar night jet. This talk will explore the hypothesis that the processes involved in the recurvature of TC Dale were responsible for the weakening of the stratospheric polar vortex. After TC Dale recurved and became extratropical, there was a surge in EP flux from the troposphere to the stratosphere. The EP flux convergence (associated with a decrease in westerly momentum) was located along the flank of the stratospheric polar vortex and was associated with stratospheric ridge amplification over Alaska. The ridge amplification occurred at a point in the season when the stratospheric polar night jet was climatologically spinning up for the winter; however, the flux of wave activity from the troposphere to the stratosphere from recurving TC Dale resulted a weakened stratospheric polar vortex and substantial departure from climatology
1600 Discussion and Summary of Session 10
Dinner 1800-2000
Friday, May 25:
34 0830 Breakout sessions
1000 Health Break
1030 Summary reports
1130 Wrap-up and Final Synthesis
1200-1330 Lunch
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