Enhancement Mechanism of a Quasi-Stationary MCS Triggered by a Sea Breeze Front Leading to Extreme Heavy Precipitation

This study investigates the maintenance and intensification mechanisms of a destructive, quasi-stationary mesoscale convective system (MCS) that caused extreme heavy rainfall (EHR) in North China on 17 May 2019. Occurring under weak synoptic forcing, the event was initiated by a sea breeze front (SBF) and exhibited distinct back-building propagation characteristics. The storm evolution presented two heavy rainfall phases. The first phase (rainfall rate > 78 mm h⁻¹) was driven by the merger of convective cells. Rather than immediate intensification, the merger deepened the low-level cold pool and induced structural reorganization. The second phase, which produced 100 mm of precipitation in 1.5 hours accompanied by hail, marked a transition to a supercell structure containing a low-level mesovortex. Vorticity budget analysis reveals that strong vertical wind shear facilitated the tilting of horizontal vorticity into the vertical, which was subsequently amplified by stretching, establishing a stable rotating updraft. The quasi-stationarity was maintained by the interaction between the SBF and the cold pool outflow, which continuously triggered new convective cells on the storm's rear flank, effectively counteracting the mean steering flow. These findings highlight the synergistic role of boundary interactions and shear-induced vorticity dynamics in driving severe convection in environments with weak large-scale forcing.