Meso-γ-scale dynamic and thermodynamic mechanisms in an extreme rainfall event in Zhengzhou, China

Abstract The meso-γ-scale dynamic and thermodynamic mechanisms in an extreme rainfall event that occurred in Zhengzhou, China, on 20 July 2021 were investigated through a Weather Research and Forecasting (WRF) simulation and decomposing the anelastic vertical momentum equation. The study revealed unique mechanisms for sustaining the meso-γ-scale convection under weak cold-pool conditions and for causing the tilt of the updraft. Results indicated that although the total buoyancy acceleration (ACCB) was essential in the updraft core generating the heaviest hourly precipitation, the total dynamic acceleration (ACCD) was crucial for the intensification of convection below 4 km and maintaining the quasi-stationary state. Positive ACCB was attributed to the thermal buoyancy after offsetting its induced negative vertical perturbation pressure gradient force (BVPPGF). The ACCD was produced mostly by the low-level vertical wind shear (VWS) and a meso-γ-scale vortex at 4 km; the averaged contribution of the former being about twice that of the latter. It is found that the deep upward acceleration on the forward (inflow) side and downward acceleration on the rear side of the updrafts caused by the reversed VWS above and below the low-level jet were conducive to maintaining the quasi-stationary convection, especially when the cold pool was relatively weak (different from the Rotunno-Klemp-Weisman theory). Furthermore, the vertical structure of the positive vorticity pair produced by the interaction between the VWS and updraft led to the upward flow tilting southward with increasing height. These results promote our understanding of the evolution mechanisms of strong convection from the perspective of storm-scale processes.