Contrasts in the Evolution and Microphysical Features of Two Convective Systems during a Heavy Rainfall Event along the Coast of South China

On 1 June 2021, a heavy rainstorm hit the coast of South China (148.6 mm in 1 h, 361 mm over 12 h). The storm process was successively affected by two convective systems (CSs). The initial convection of the two CSs occurred at a similar location; however, they subsequently showed different evolution characteristics. Based on multi-source data, including dual-polarimetric radars, wind profiling radars, sounding, and automatic weather stations, we explored the differences in the key characteristics of these two CSs. It was found that the convection was initially triggered at a similar location for both CSs, closely related to the mesoscale boundary and the hilly terrain. After formation, CS1 moved eastward to the regions with lower surface temperature and weaker lower-level convergence but similar humidity, which means the environmental conditions for sustaining the CS became less favorable. As a result, CS1 dissipated rapidly and only lasted for about 90 min, resulting in 5% of the total precipitation of the overall storm. In contrast, during the lifespan of CS2, the southerly wind over the South China Sea became stronger. This caused an intense lower-level convergence zone along the coastal region of Guangdong Province, which provided favorable dynamic conditions for maintaining CS2. Favored by the strong coastal convergence and abundant moisture, new convective cells (CCs) were generated continuously and merged with CS2, acting as another favorable condition for its sustainment. Overall, CS2 lasted for 8 h, and its precipitation accounted for 95% of the total rainfall. In CS1, CCs showed a notable evaporation process below 4 km, manifested by the large raindrops. However, in CS2, the CCs had a higher concentration of small raindrops and higher ice and liquid water content. Since CS2 was close to the coastal region, the warm local environment promoted convection, leading to intense precipitation. In addition, the riming and melting processes were active, leading to a high precipitation efficiency and strong local precipitation during a short period of time.