An observational analysis of the genesis tropical storm Gert (2005)

Central to the long-unsolved problem of tropical cyclogenesis, as recent studies have suggested, is the establishment of a cyclonic mesoscale surface vortex of sufficient strength to initiate the self-sustaining air-sea interaction feedback processes (WISHE) necessary for intensification of the incipient vortex to a mature hurricane. Although it is well-known that mesoscale convective systems (MCSs) play an essential role in the formation of tropical cyclones (TCs), the dynamical pathway to the creation of the TC vortex has been the subject of scientific controversy for the past decade.

Early conceptual models, based on observations of midlevel mesoscale convective vortices (MCVs) associated with MCS stratiform cloud regions in pre-genesis tropical disturbances, proposed that multiple MCV mergers (Ritchie and Holland 1997, Simpson et al. 1997) or vorticity advection by stratiform downdrafts (Bister and Emanuel 1997) results in the eventual surface vortex formation. Recent high-resolution numerical modeling studies (Hendricks et al. 2004, Montgomery et al. 2006) have advanced the contrasting “bottom-up” conceptual model of Montgomery and Enagonio (1998) by demonstrating the construction of a warm-core surface-based TC vortex from multiple diabatic mergers of intense small-scale low-level cyclonic vortices generated by individual cumulonimbus clouds (vortical hot towers, or VHTs). In essence, these “top-down” and “bottom-up” genesis pathways involve contrasting vorticity enhancement mechanisms associated with either the stratiform or deep convective modes of MCS convection.

The genesis debate still remains open largely due to a lack of conclusive supporting observational evidence for the existing theories. We attempt to further our understanding in an observational case study of the genesis of tropical storm Gert, which emerged from the NASA/TCSP field program of July 2005. Utilizing flight level, dropsonde and radar data obtained from four NOAA WP-3D aircraft missions during July 23-24, in conjunction with satellite imagery, NCEP-GFS analyses, Quikscat and TRMM data, we investigate the kinematic and thermodynamic structure of a Carribean easterly wave during its transformation to a Gulf of Mexico tropical storm.

Gert's primary development occurred during two periods of intense convective activity: 1) on July 22, before the system traversed the Yucatan peninsula, and 2) on July 24, after moving well into the southern Gulf of Mexico following the convectively inactive period of July 23. A preliminary data analysis revealed that particularly intense convective bursts during each of the active periods resulted first in a tropical depression-like vortex and then in a marginal tropical storm vortex, possessing a kinematic vertical structure suggestive of deep-convective bottom-up type development. Prior to formation of the tropical storm, however, the lower troposphere was characterized by a cool boundary layer and near-saturation, consistent with thermodynamics aspects of the Bister and Emanuel (1997) “top-down” conceptual model. A complete analysis of the key findings will be presented.