Two Modes of Self‐Organizing Development of Extreme Rainfall‐Producing Storms in South China

Abstract The convective‐scale dynamics of extreme rainstorms on monsoon coasts (South China) remain poorly understood. This study uses the advanced coupled model WRF‐Chem‐MOSAIC for a 90‐hr, 3‐km simulation, reasonably capturing the thermodynamic conditions, inhomogeneous near‐surface PM2.5, and the dynamical and microphysical features of two storms with contrasted convective intensities during the Guangzhou “May 7” record‐breaking rainfall event. Two modes of the storms' rapid self‐organizing development are unraveled via the vertical momentum equation. Storm1 (strong convective intensity) develops in an “effective buoyancy dominantly driving mode,” where a deep inflow layer with high convective available potential energy (CAPE) from the South China Sea is lifted by a convectively‐generated shallow cold pool. Strong effective buoyancy accelerates ascent, generating intense updrafts that advect upward above the melting level, with minor aid from dynamic vertical pressure perturbation forcing below 3 km. Under reduced boundary‐layer CAPE due to nocturnal cooling, Storm2 forms when a convective region merges with a northward‐moving convective cell and follows a “dynamic‐buoyancy jointly driving mode.” Strengthened southerly inflows below 3 km interact with the overturning outflow from lower‐topped Storm2, lifting inflow air via vertical twisting acceleration. Rear low‐level downdrafts enhance the cold pool and near‐surface northerlies, which converge with strengthened southerlies to form a strong horizontal convergence zone centered at 300 m. The sharp vertical decrease in convergence above this level forces the near‐surface moist air upward via vertical extension acceleration, intensifying sub‐4 km updrafts, boosting southerly moisture transport and in‐cloud latent heating, and thus elevating effective buoyancy up to ∼7 km.