Multi-Scale Shear Impacts During the Genesis of Typhoon Hagupit (2008)

The impact of vertical wind shear on cyclogenesis is examined from the synoptic to the meso-alpha and meso-gamma scales using airborne Doppler radar observations of pre-depression Hagupit during Tropical Cyclone Structure 2008 (TCS08) field campaign. The disturbance that eventually developed into Typhoon Hagupit contained a closed circulation, identified as a wave pouch, up to ten days prior to its transition into a tropical depression. The pre-depression vortex continued to develop until the system was affected by an upper-tropospheric trough around 13th September. The midlevel vorticity in the disturbance started to decrease under the influence of strong vertical wind shear, delaying cyclogenesis. However, low-level circulation persisted and the system ultimately formed into a tropical cyclone as soon as it left the high-shear zone. The analysis of NCAR Electra Doppler Radar (ELDORA) around ∼0000 UTC 14 September 2008 aircraft mission revealed there was continuous deep convection, enhancing the low level circulation through vortex stretching under the strong vertical wind shear (> 20 m s-1).

The negative impact of strong vertical wind shear on the development of pre-depression Hagupit was evident in the synoptic and meso-alpha scale dynamics and thermodynamics through mis-alignment of low and mid-level circulation and dry air intrusion. On the other hand, it is hypothesized that the local shear actually helped organize the meso-gamma scale convective elements, enabling the pre-depression Hagupit to survive through the hostile high-shear environment. The high-resolution ELDORA data is used to examine the role of local and environmental shear in organizing the meso-gamma scale convection at this stage of pre-depression Hagupit, while suppressing the development of the meso-alpha scale system. These results reaffirm that TC genesis intrinsically involves multi-scale processes including the interactions with vertical wind shear in various spatial scales.