Analysis of Deep Convective Towers in a Southwest-Vortex Rainstorm Event

Abstract

The structure and organization of the extreme-rain-producing deep convection towers and their roles in the formation of a southwest vortex (SWV) event are studied using the intensified surface rainfall observations, weather radar data and numerical simulations from a high-resolution convection-allowing model. The deep convection towers occurred prior to the emergence of SWV and throughout its onset and development stages. They largely resemble the vortical hot tower (VHT) commonly seen in typhoons or hurricanes and are thus considered as a special type of VHT (sVHT). Each sVHT presented a vorticity dipole structure, with the upward motion not superpose the positive vorticity. A positive feedback process in the SWV helped the organization of sVHTs, which in turn strengthened the initial disturbance and development of SWV. The meso-γ-scale large-value areas of positive relative vorticity in the mid-toupper troposphere were largely induced by the diabatic heating and tilting. The strong mid-level convergence was attributed to the mid-level vortex enhancement. The low-level vortex intensification was mainly due to low-level convergence and the stretching of upward flow. The meso-α-scale large-value areas of positive relative vorticity in the low-level could expand up to about 400 hPa, and gradually weakened with time and height due to the decaying low-level convergence and vertical stretching in the matured SWV. As the SWV matured, two secondary circulations were formed, with a weaker mean radial inflow than the outflow and elevated to 300-400 hPa.