Pre-depression vortex development from “unspectacular” convection: A genesis case study of tropical storm Gert (2005)

A central focus of the tropical cyclogenesis problem in recent years has centered on the relative importance of vorticity enhancement mechanisms associated with the deep convective and stratiform modes of organized convection. This combined observational and numerical study of Gert (2005) additionally considers what may perhaps be an overlooked aspect of the core formation process: the contribution of relatively weak convection that often accompanies Mesoscale Convective Systems (MCSs) and sometimes persists between these more spectacular episodic events.

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.