Tropical cyclogenesis observations from the NASA Tropical Cloud Systems and Processes Experiment

Our understanding of the physical processes involved in the genesis of tropical cyclones (TCs) has long remained elusive in large part due to a lack of observations. Utilizing flight level and dropsonde observations obtained from NOAA-P3 aircraft, we investigate the mesoscale structure and evolution of two tropical disturbances observed during the TCSP experiment of July 2005. The unprecedented temporal continuity of this genesis dataset was made possible by the tasking of two flight missions per day for both cases. Although the large-scale environment appeared to provide favorable conditions for genesis in both cases, the Eastern Pacific disturbance did not transform to a tropical cyclone while the Carribean disturbance near the Yucatan peninsula eventually became Tropical Storm Gert in the Bay of Campeche. The latter case is of particular interest since it was the first time that a tropical disturbance was declared a tropical depression by NHC during a NOAA-P3 flight mission.

Central to the tropical cyclogenesis problem, as recent studies have suggested, is the establishment of a cyclonic surface circulation that is able to initiate the air-sea interaction processes (WISHE) necessary to intensify a vortex of tropical cyclone strength. Observations suggest that mesoscale convective vortices (MCVs) generated in the stratiform precipitation region of mesoscale convective systems (MCSs) and embedded within the broader scale midlevel circulation associated with tropical disturbances often play an important role in the surface vortex development. However, a debate exists as to whether the surface vortex is developed from the downward penetration of the midlevel vortices (Bister and Emanuel 1997) or built via the merger of surface vortices generated by the deep cumulonimbus towers existing in a vorticity rich environment (Montgomery et al. 2005). Our analysis of the available kinematic and thermodynamic data for the two contrasting cases is intended to clarify the important physical processes responsible for tropical cyclogenesis.