Although the importance of deep convective and stratiform processes during Gert's genesis (Braun et al. 2009) is recognized, our primary focus here is not on the final genesis stage (July 24) when persistent and centralized organized convection led to the formation of a tropical storm vortex. Instead, we emphasize the earliest and intermediate stages of the genesis process (July 21-23), beginning with an outbreak of organized deep convection within the trough region of a western Carribean tropical wave. We observe that the initial low-level mesoscale cyclonic circulation did not develop from the direct effects of an impressive MCS, but rather surprisingly formed within an adjacent region of relatively shallow convective cells. A similar theme characterized the remainder of this period where the pre-depression vortex development was accompanied by relatively weak and scattered small-scale convection, while organized deep convection associated with peripheral MCSs was evidently secondary in importance.
Our observational analysis suggests that unspectacular convection characterizing much of the early and intermediate stages may have been responsible for a significant fraction of the low-level circulation development during Gert's lifecycle, thereby increasing the likelihood of genesis occurrence by providing an environment of enhanced low-level cyclonic vorticity. The validity of our results depends to some degree on the accuracy of the GFS initial analyses, which are utilized to interpret the gross features of Gert's evolution. The assimilation of abundant aircraft data obtained during the TCSP field program, however, greatly enhances our confidence in these results. In addition, independent land and ship surface data provides surprising GFS validation during the earliest development stages, prior to the NOAA flight investigations. Time permitting, results from a diagnostic analysis of high-resolution numerical simulations will also be presented.
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