Nam, C. C., M. M. Bell, : Multi-scale shear impacts during the genesis of Hagupit (2008). Monthly Weather Review, In review
The impact of vertical wind shear (VWS) on cyclogenesis is examined from the synoptic to meso scales using airborne Doppler radar observations of pre-depression Hagupit during 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 pre-depression in a sheared environment. Pre-depression 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 mid-level 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 mesoalpha scale within the 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 multi-scale processes during TC genesis and the interactions of a pre-depression with VWS at various spatial scales.
FIG. 13. Schematics of the multi-scale interactions in Hagupit’s cyclogenesis. a) Synoptic scale’s active features are upper-level trough and easterly waves, which carry the marsupial wave pouch shown as red circulation. b) Inside the meso-a scale wave pouch, shown are the low-level cyclonic stream lines with its center marked with black cross and mid-level center as yellow cross. Over the clouds shifted to southwest from the low-level center is vorticity (positive - red, negative - blue), showing vorticity dipoles from tilting, enhanced bigger vortex from stretching, and mid-level MCV over the stratiform area. c) In the meso-b scale S box (Fig. 7b), the squall-like NW-SE convection can be explained by the local wind shear profile consist of winds from cyclonic circulation inside pouch, marsupial wave propagation, and the trough. Thick black lines in the lowerleft corner of the circle indicates convective line; L indicates ENE low-level shear; M indicates WNW mid-level shear. d) Meso-g scale convective cells produce vorticity that serves as building block for the wave pouch intensification through tilting the horizontal shear (S cell) and stretching the existent vorticity (N cell).
Research was supported by National Science Foundation award AGS-1701225 and 412 Office of Naval Research award N000141613033.