The Pre-Depression Investigation of Cloud Systems in the Tropics
Total Page：16
File Type：pdf, Size：1020Kb
THE PRE-DEPRESSION INVESTIGATION OF CLOUD-SYSTEMS IN THE TROPICS (PREDICT) EXPERIMENT Scientific Basis, New Analysis Tools, and Some First Results BY MICHAEL T. MON T GO M ERY , CHRIS T OPHER DAVIS , TI M O T HY DUNKER T ON , ZHUO WANG , CHRIS T OPHER VEL D EN , RYAN TORN , SHARANYA J. MA J U md AR , FUQING ZHANG , ROGER K. SM I T H , LANCE BOSAR T , MICHAEL M. BELL , JENNI F ER S. HAASE , AN D RE W HEY M S F IEL D , JORGEN JENSEN , TERESA CA M POS , AN D MARK A. BOO T HE A field study involving 25 flights into Atlantic tropical disturbances tested the principal hypotheses of a new model of tropical cyclogenesis, known as the marsupial paradigm. longstanding challenge for hurricane forecasters, at understanding the science of tropical cyclone theoreticians, and numerical weather forecast formation. These include the National Aeronautics and A systems is to distinguish tropical waves that will Space Administration (NASA) Tropical Cloud Systems develop into hurricanes from tropical waves that will and Processes (TCSP) experiment in 2005 (Halverson not. While tropical easterly waves occur frequently et al. 2007), the NASA African Multi-Disciplinary over the Atlantic and east Pacific, only a small frac- Monsoon Analyses (NAMMA) project in 2006 (Zipser tion of these waves (~20%; e.g., Frank 1970) evolve et al. 2009), and the Tropical Cyclone Structure ex- into tropical storms when averaged over the hurricane periment in 2008 (TCS-08; Elsberry and Harr 2008). season. The problem was insightfully summarized Adding the results of earlier efforts such as the Tropical by Gray (1998): “It seems unlikely that the formation Experiment in Mexico (TEXMEX; Bister and Emanuel of tropical cyclones will be adequately understood 1997; Raymond et al. 1998) and even serendipitous until we more thoroughly document the physical observations of the early intensification of Hurricane differences between those systems which develop Ophelia in the Hurricane Rainband and Intensity into tropical cyclones from those prominent tropical Change Experiment (RAINEX; Houze et al. 2006), and disturbances which have a favorable climatological occasional observations from reconnaissance aircraft and synoptic environment, look very much like they (Reasor et al. 2005), we have a collection of studies that will develop but still do not.” have sampled pieces of a large and complex scientific The formation of tropical cyclones (TCs) is one of puzzle. However, with the exception of the TCS08 the remaining mysteries of the atmosphere (Emanuel experiment, the greatest shortcoming of previous cam- 2005). As for why it remains unsolved, after decades paigns has been the limited in situ observational sam- of research, it is unfortunately true that in situ ob- pling, both in space and in time. It has been difficult servations are lacking over remote tropical oceans. to piece together snapshots of tropical disturbances Recent years have seen several field campaigns aimed taken at different times. “Genesis” (defined here as AMERICAN METEOROLOGICAL SOCIETY FEBRUARY 2012 | 153 the formation of a tropical depression1 at subsynoptic also that the upper-level environmental conditions scales) often occurs between sampling times, or after over a developing low-level disturbance may play an disturbances move out of range. important modulating role (Dvorak 1975; Sadler 1976; There are very few in situ observations of both McBride and Zehr 1981). Yet, all of these conditions in precursors to genesis and the ensuing tropical cyclone conjunction are well known to be insufficient. formation process. The paucity of observations is In late summer 2010 a trio of field campaigns aggravated by a lack of a priori knowledge of where to was conducted by NASA, the National Oceanic sample a candidate disturbance. For a typical African/ and Atmospheric Administration (NOAA), and the Atlantic easterly wave, the trough axis extends meridi- National Science Foundation (NSF) to investigate onally over a thousand kilometers or more. The dimen- tropical cyclogenesis in the Caribbean and west sion of a tropical storm in this sector is much smaller, a Atlantic and the subsequent intensification of named few hundred kilometers. Convective clouds and cloud storms in these regions. While two of the campaigns2 systems are smaller still. Tropical cyclone formation included intensification in their portfolio of objec- is intrinsically a multiscale process (e.g., Gray 1998). tives, the Pre-Depression Investigation of Cloud- The fate of a tropical easterly wave depends on the Systems in the Tropics (PREDICT) campaign was large-scale environment in which it is embedded, the designed exclusively to study genesis. Priority was wave’s synoptic structure (e.g., low-level vorticity and given to developing storms prior to their classifica- middle-level moisture), and convective-scale processes tion as tropical depressions even when mature storms associated with the wave’s evolution. The large-scale were present nearby. The primary measurement necessary conditions for tropical cyclone formation platform of PREDICT was the NSF–National Center have been known for over four decades (e.g., Gray for Atmospheric Research (NCAR) Gulfstream V 1968; DeMaria et al. 2001): nonzero cyclonic vorticity (GV), equipped with dropsondes and onboard sen- in the lower troposphere, sufficiently warm sea surface sors for meteorological variables and ice microphys- temperatures (≥26°C), a moist middle troposphere, ics (see Table 1). The range and speed of the GV, and and at most a moderate vertical shear between upper the high altitude (~12–13 km) from which it could and lower troposphere (no larger than approximately release dropsondes, were exploited to sample storm 12 m s−1). Some observational studies have suggested formation from Central America to the mid-Atlantic 1 The glossary on NOAA’s Hurricane Research Division’s website uses “tropical cyclone as the generic term for a nonfrontal synoptic-scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activ- ity) and a definite cyclonic surface wind circulation.” Notably, this definition does not invoke any wind threshold. The same glossary defines a “tropical depression” as a tropical cyclone with maximum sustained surface winds of less than 17 m s−1 (34 kt, 39 mph, and, in the Atlantic and eastern Pacific basins, a “tropical storm” as a tropical cyclone with surface winds between 17 and 33 m s−1. In this study we will define genesis as the formation of a tropical depression and we impose no formal threshold on wind speed. 2 The Genesis and Rapid Intensification Processes (GRIP) project of the National Aeronautics and Space Administration and the Intensify Forecasting Experiment (IFEX) of the National Oceanic and Atmospheric Administration. AFFILIATIONS: MON T GO M ERY , BELL , AN D BOO T HE —Depart- HAASE —Department of Earth and Atmospheric Sciences, Purdue ment of Meteorology, Naval Postgraduate School, Monterey, University, West Lafayette, Indiana California; DUNKER T ON — Northwest Research Associates, ADDITIONAL AFFILIATIONS: MON T GO M ERY AN D BELL —NOAA’s Bellevue, Washington; DAVIS , HEY M S F IEL D , JENSEN , AN D CA M POS — Hurricane Research Division, Miami, Florida; DUNKER T ON — Naval National Center for Atmospheric Research, Boulder, Colorado; Postgraduate School, Monterey, California WANG —Department of Atmospheric Sciences, University CORRESPONDING AUTHOR: Michael Montgomery, Naval of Illinois at Urbana—Champaign, Urbana, Illinois; VEL D EN — Postgraduate School Department of Meteorology, 1 University University of Wisconsin/CIMSS, Madison, Wisconsin; TORN Circle, Monterey, CA 93943 AN D BOSAR T —Department of Atmospheric and Environmental E-mail: [email protected] Sciences, University at Albany, State University of New York, The abstract for this article can be found in this issue, following the Albany, New York; MA J U md AR —Rosenstiel School of Marine table of contents. and Atmospheric Science, University of Miami, Miami, Florida; DOI:10.1175 / BAMS - D -11- 0 0 046.1 ZHANG —Department of Meteorology, Pennsylvania State Uni- versity, University Park, Pennsylvania; SM I T H —Meteorological In final form 22 July 2011 Institute, Ludwig-Maximilians University, Munich, Germany; ©2012 American Meteorological Society 154 | FEBRUARY 2012 (roughly 40°W) operating TA B L E 1. Instrumentation payload on the NSF/NCAR GV research out of St. Croix in the U.S. aircraft. Virgin Islands (see Fig. 1). The purpose of this State parameters Pressure article is to present an over- Temperature (3x) view of the scientific basis Dewpoint Buck Research for PREDICT, a summary Water vapor density VCSEL and CARI TDL of the scientific products Position Honeywell IRS used during the experi- ment, and some notewor- Novatel GPS thy first results that have 3-D wind Dynamic pressure (2x) Honeywell emerged since the comple- Attack/sideslip Honeywell in radome tion of the field phase on 30 Attack/sideslip Honeywell in gust pod September 2010. An out- Microphysics Icing rate Rosemount line of the remaining paper is as follows. A summary of Liquid water content DMT King probe the scientific basis of the Cloud droplets/ice DMT CDP probe experiment and practical Ice particles PMS/DMT 2D-C impacts is presented in Ice particles SPEC 3V-CPI section 2. Section 3 pres- Condensation nuclei TSI water CN ents a glimpse of the daily weather briefings and re- Aerosol particles DMT UHSAS search tools employed to Aerosol particles CVI assess candidate tropical Remote sensing Temperature profile JPL Microwave Temperature Profiler disturbances for possible Water vapor profile Purdue GISMOS flight operations. Section True air speed EOL LAMS 4 presents a summary of all of the disturbances flown Dropsondes Pressure, temperature, EOL/Vaisala GPS sonde dewpoint, wind during the field program and provides some first clues as to the development versus nondevelopment issue. Section 5 provides our conclusions about the field phase of this work and sketches a path forward. SCIENTIFIC BASIS AND PRACTICAL IMPACTS. In the experi- ment’s conception it was recognized that tropical cyclogenesis is inherently a multiscale process (Gray 1998) as noted above.