Formation mechanism and structural characteristics analysis of meso-γ-scale vortex in comma-shaped mesoscale convective system

This study analyzes the formation mechanisms and structural characteristics of meso-γ-scale vortices in a comma-shaped mesoscale convective system. Using the data from automatic weather stations, ERA5 reanalysis data, Doppler radars, and other sources, the observational and numerical simulations analyses were conducted on a heavy rainstorm event on the west coast of the Bohai Sea on the night of July 5, 2022. The results show that the heavy rainfall occurred under the synoptic conditions of an eastward-moving upper-level trough and a northward-shifting low-pressure trough following the weakening of Typhoon "Chaba". The event was driven by the continuous influence of the low-centroid vortex echo in the head of the comma-shaped mesoscale convective system on the west coast of the Bohai Sea. The meso-γ-scale vortex formed at the top of the low-pressure inverted trough, where frontogenesis occurred due to the interaction between northwest airflow formed by the early precipitation and warm, humid southeast airflow. The formation of γ-scale mesoscale vortices appeared successively at 925 hPa and 850 hPa, with the precipitation predominantly occurring on the right side of the vortices. The budget diagnostic of the vorticity equation indicated that the vortices were primarily initiated by horizontal convergence-divergence terms at the lower levels. These vortices were then transported upward through vertical vorticity. Ultimately, a deep γ-scale vortex with a closed cyclonic circulation formed in the lower half of the troposphere. The mesoscale vortices exhibited a warm-core structure between 700 hPa and 850 hPa, while the infiltration of weak cold air below 925 hPa provided thermodynamic instability and dynamic forcing for the development of convective rainbands. The vortices displayed an asymmetric structure, with higher ascent rates on the east side, resulting in significantly stronger precipitation on the east side than on the west side. Furthermore, the water vapor transport associated with the vortices exhibited a distinct asymmetric structure, with moisture convergence primarily concentrated on the east side of the vortices, specifically on the west coast of Bohai Bay. This asymmetry was identified as a crucial factor contributing to the stronger precipitation on the east side of the vortices.