Multi-scale shear impacts during the genesis of Hagupit (2008)

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  • 1 Colorado State University, Fort Collins, Colorado
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Abstract

The impact of vertical wind shear (VWS) on tropical cyclogenesis is examined from the synoptic to meso scales using airborne Doppler radar observations of pre-depression 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 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 pre-depression to survive through the upper-tropospheric trough interaction was persistent vorticity amplification on the meso-γ scale that was aggregated on the meso-α 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-β 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.

Corresponding author address: Chaehyeon Chelsea Nam, Colorado State University, 3915 W. Laporte Ave. Fort Collins, CO 80526. E-mail: c.chelsea.nam@colostate.edu

Abstract

The impact of vertical wind shear (VWS) on tropical cyclogenesis is examined from the synoptic to meso scales using airborne Doppler radar observations of pre-depression 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 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 pre-depression to survive through the upper-tropospheric trough interaction was persistent vorticity amplification on the meso-γ scale that was aggregated on the meso-α 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-β 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.

Corresponding author address: Chaehyeon Chelsea Nam, Colorado State University, 3915 W. Laporte Ave. Fort Collins, CO 80526. E-mail: c.chelsea.nam@colostate.edu
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