Impact of Mesoscale Convective Systems on Moisture Budget During a Heavy Rainfall Event Over South China

ABSTRACT South China experienced substantial rainfall from April 19 to April 22, 2024, with accumulated precipitation ranging from 200 mm to 350 mm. This significant precipitation was primarily driven by the combined synoptic influences of the southern branch trough, low‐level jet stream, and the Jiang–Huai cyclone, affecting northern and central‐eastern Guangxi as well as most parts of Guangdong. These synoptic conditions facilitated the initiation, development, and propagation of mesoscale convective systems (MCSs). Specifically, an MCS initiated in the evening over the eastern edge of the Yunnan–Guizhou Plateau (YGP), leading to the formation of a mesoscale vortex with closed cyclonic circulation at 850 hPa, which further promoted the merging of convection cells into an organized MCS. The intensified southerly winds appeared over the eastern parts of the mesoscale vortex, preceding the peak meridional component of moisture convergence by approximately 4–5 h. Moisture budget analysis revealed that the meridional component of horizontal moisture convergence was the primary contributor to the moisture increase over Guangxi and Guangdong provinces. Consequently, the enhanced mesoscale systems (vortices and MCSs) significantly increased horizontal convergence at lower levels, contributing substantially to the moisture accumulation over South China. The intensified low‐level southerly wind associated with mesoscale vortices can be considered a potential forewarning parameter for short‐range precipitation forecasting in such heavy rainfall events.