Multiscale Shear Impacts During The Genesis Of Hagupit (2008)

The impact of vertical wind shear (VWS) on tropical cyclogenesis is examined from the synoptic to mesoscales using airborne Doppler radar observations of predepression Hagupit during the Tropical Cyclone Structure 2008 (TCS08)/THORPEX Pacific Area Regional Campaign (T-PARC) field campaigns. The high temporal and spatial resolution observations reveal complex localized convective and vortical characteristics of a predepression in a sheared environment. Predepression Hagupit interacted with an upper-tropospheric trough during the observation period. The strong deep-layer VWS (>20 m s(-1)) had a negative impact on the development through misalignment of the low- and midlevel circulations and dry air intrusion. However, the low-level circulation persisted, and the system ultimately formed into a tropical cyclone after it left the high-shear zone. Here we propose that a key process that enabled the predepression to survive through the upper-tropospheric trough interaction was persistent vorticity amplification on the meso-gamma scale that was aggregated on the meso-alpha scale within the wave pouch. Multi-Doppler wind analysis indicates that cumulus congestus tilted the low-level horizontal vorticity into the vertical in the early stage of convective life cycle, followed by stretching from maturing deep convection. Variations in low-level VWS on the meso-beta scale affect convective organization and horizontal vorticity generation. The results provide new insights into multiscale processes during TC genesis and the interactions of a predepression with VWS at various spatial scales.