Roles of upper-level processes in tropical cyclogenesis

Previous studies have focused mostly on the impact of lower-level vorticity growth and other lower-level processes on tropical cyclogenesis (TCG). In this study, the importance of upper-level processes in TCG is studied in terms of the minimum sea-level pressure (MSLP) changes with two cases exhibiting different warm-core heights and vorticity structures due to their developments in the respective weak- and strong-sheared environment. Results show that the upper-level warming could account for more than 75% the MSLP changes in both cases. Widespread deep convection during the early TCG stage tends to warm the upper troposphere and induce meso-α-scale surface pressure falls. Upper-level flow and vertical wind shear (VWS) will suppress the formation of a warm core due to the presence of weak inertial stability, whereas the development of upper-level divergent outflows favors its formation. Results also show that TCG is triggered when the upper-level warming amplitude and depth increase as a result of weak or significantly reduced ventilation and VWS aloft. Results suggest that both the upper- and low-level processes be considered in the understanding and prediction of TCG.