542 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 70 An Investigation of Composite Dropsonde Profiles for Developing and Nondeveloping Tropical Waves during the 2010 PREDICT Field Campaign WILLIAM A. KOMAROMI Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida (Manuscript received 15 February 2012, in final form 20 August 2012) ABSTRACT Composite dropsonde profiles are analyzed for developing and nondeveloping tropical waves in an attempt to improve the understanding of tropical cyclogenesis. These tropical waves were sampled by 25 re- connaissance missions during the 2010 Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) field campaign. Comparisons are made between mean profiles of temperature, mixing ratio, relative humidity, radial and tangential winds, relative vorticity, and virtual convective available potential energy (CAPE) for genesis and nongenesis cases. Genesis soundings are further analyzed in temporal pro- gression to investigate whether significant changes in the thermodynamic or wind fields occur during the transition from tropical wave to tropical cyclone. Significant results include the development of positive temperature anomalies from 500 to 200 hPa 2 days prior to genesis in developing waves. This is not observed in the nongenesis mean. Progressive mesoscale moistening of the column is observed within 150 km of the center of circulation prior to genesis. The genesis composite is found to be significantly moister than the nongenesis composite at the middle levels, while comparatively drier at low levels, suggesting that dry air is more detrimental to genesis when located at the middle levels. Time-varying tangential wind profiles reveal an initial delay in intensification, followed by an increase in organization 24 h pregenesis. The vertical evolution of relative vorticity, in addition to a warm- over-cold thermal structure, is more consistent with a top-down than a bottom-up genesis mechanism. Last, CAPE values are much greater for nongenesis than genesis profiles, indicating that greater instability does not necessarily favor genesis. 1. Introduction surface temperatures greater than or equal to 268C (Palmen 1948), an unstable or conditionally unstable Predicting tropical cyclogenesis remains one of the environment, and relatively high moisture content from great forecasting challenges to today’s meteorological the surface through 5 km (Gray 1979). However, despite community (Emanuel 2005). Much of our limited un- these well-known criteria, the exact sequence of events derstanding can likely be attributed to our inability to culminating in tropical cyclogenesis remains unknown. differentiate the often subtle physical differences be- Two differing views of tropical cyclone formation are tween developing and nondeveloping tropical cyclones the top-down and the bottom-up hypotheses. Ritchie (TCs), and any such differences, when observed, have and Holland (1997) and Simpson et al. (1997) describe been insufficiently documented (Dunkerton et al. 2009). a top-down mechanism for genesis by which successive Among the well-known necessary dynamic conditions mergers of mesoscale convective systems (MCSs) in- for tropical cyclogenesis are background cyclonic vor- crease the size and/or strength of the midlevel vortex, ticity, 850–200-hPa tropospheric wind shear of less than 2 2 which induces a surface circulation through vertical 15 m s 1 and preferably below 10 m s 1, and a suffi- penetration and vortex stretching. Similarly, Bister and ciently high Coriolis parameter (Gray 1968). Ther- Emanuel (1997) propose that a stratiform rain region modynamic prerequisites exist as well, including sea associated with an existing MCS acts to moisten and cool the mid- to lower levels. The level of peak cooling descends within the stratiform rain region, thereby Corresponding author address: William Komaromi, RSMAS, Di- vision of Meteorology and Physical Oceanography, 4600 Rickenbacker lowering the level of maximum potential vorticity (PV) Causeway, Miami, FL 33149. production, while moistening acts to limit the occur- E-mail: [email protected] rence of dry downdrafts. Along with the necessity of DOI: 10.1175/JAS-D-12-052.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 09/25/21 07:15 AM UTC FEBRUARY 2013 K O M A R O M I 543 a strengthening midlevel circulation, Nolan (2007) also disturbance within the pouch is repeatedly moistened by found humidification of the inner core due to moist deep moist convection within the critical layer while detrainment and precipitation from deep convective remaining somewhat protected from lateral intrusion of towers preceding genesis. However, Nolan (2007) does dry air and deformation by horizontal and vertical shear. not necessitate a top-down genesis process. Lastly, a re- This protovortex, collocated with the critical latitude, is cent study by Raymond et al. (2011) of five tropical cy- then able to keep pace with the parent wave until it has clogenesis events in the northwestern Pacific suggests strengthened into a self-maintaining entity. Hypotheti- that tropical cyclogenesis is facilitated by a preexisting cally, the marsupial paradigm could be used in conjunc- midlevel vortex. This midlevel vortex creates a cold core tion with either the top-down or bottom-up genesis at low levels, which alters deep convection as to facilitate hypotheses. Dunkerton et al. (2009) assume a bottom- spinup. up progression of genesis. A slightly differing sequence, known as bottom-up As already alluded to, much of the difficulty in iden- genesis, is proposed by Hendricks et al. (2004) and tifying the exact order of processes that occur during Montgomery et al. (2006), in which individual deep genesis, or whether top-down or bottom-up sequences moist convective updrafts or vortical hot towers (VHTs) both occur under different conditions, can be attributed develop within the tropical wave, amplify preexisting to a lack of in situ data prior to genesis. In an attempt to cyclonic vorticity, and gradually consolidate to form expand upon the limited dataset, several field campaigns a low-level center of circulation. Latent heat released have sampled tropical cyclones during and shortly after within these VHTs aids in the development of the mid- the genesis stage, including the Tropical Experiment in level warm core, and surface convergence and upper- Mexico (TEXMEX; Bister and Emanuel 1997; Raymond level divergence commence. Observational evidence et al. 1998), the Tropical Cloud Systems and Processes supporting a top-down mechanism for genesis is pre- (TCSP) experiment in 2005 (Halverson et al. 2007), the sented by Ritchie and Holland (1997) and Mapes and National Aeronautics and Space Administration (NASA) Houze (1995), while Houze et al. (2009) find evidence component of the African Monsoon Multidisciplinary that support the VHT argument, all for individual case Analyses (AMMA) project in 2006 (Zipser et al. 2009), studies. the Tropical Cyclone Structure experiment in 2008 Regardless of the exact order of processes by which (TCS-08; Elsberry and Harr 2008), as well as a handful genesis occurs, the dependence upon some initial MCS of observations from the Hurricane Rainband and In- or VHTs assumes sufficient tropospheric instability tensity Change Experiment (RAINEX) of 2005 (Houze to allow deep convection. Using in situ data, Molinari et al. 2006). Case studies using data from these experi- and Vollaro (2010) find that highly sheared, generally ments, such as Zipser et al. (2009), emphasize the diffi- weaker tropical cyclones tend to be associated with culty of achieving genesis in excessively dry air masses. higher convective available potential energy (CAPE) Ritchie and Holland (1997), Davis et al. (2008), Houze than their nonsheared, generally stronger counterparts. et al. (2009), and Braun et al. (2010) have shown that the Similarly, Braun (2010) found higher CAPE in envi- progressive strengthening of a midlevel vortex, a grad- ronments for weakening TCs compared to strengthening ual moistening of the column in a region of deep con- TCs in the days following genesis. In idealized numerical vection, and the development of a warm core are all simulations, Nolan et al. (2007) found that greater maxi- evident in observations of various tropical cyclones mum potential intensity (MPI) resulted in greater likeli- during and shortly following genesis. While these studies hood of genesis, while greater CAPE did not. Nonetheless, allude to the development of a warm core, the altitude of the question of whether genesis becomes increasingly the warm-core maxima and the timing of the devel- favored with increasing instability, or whether there is opment of the warm core are generally neglected. Ear- some threshold beyond which decreasing stability is lier observational studies such as La Seur and Hawkins detrimental to genesis, has not been conclusively an- (1963) and Hawkins and Rubsam (1968) have found swered via observational evidence. maximum warm anomalies at around 250 hPa in mature A recent endeavor to better understand tropical TCs, while Hawkins and Imbembo (1976) and Stern and cyclogenesis from a wave-relative framework is under- Nolan (2012) suggest that the primary warm core is lo- taken by Dunkerton et al. (2009). Known as the mar- cated from 500 to as low as 650 hPa. The level of max- supial paradigm, tropical depression formation from imum warm anomalies for pregenesis disturbances a predepression wave trough in the lower troposphere is remains