The Influence of South China Sea Summer Monsoon Onset on Mesoscale Convective Systems in Southern China

Mesoscale convective systems (MCSs) frequently occur over southern China during early summer, often leading to significant precipitation and associated socioeconomic impacts. This study investigates the differences in MCS frequency and related precipitation in southern China before and after the onset of the South China Sea summer monsoon (SCSSM) during 2001－2020, using high-resolution satellite data from the Global Precipitation Measurement mission and iterative rain cell tracking (IRT) method that combines cloud and precipitation criteria.

Our analysis indicates that during two pentads after the SCSSM breaks out, the frequency of MCSs significantly decreases in southern China, especially over the middle and lower reaches of Yangtze River basin (MLYRB). Accordingly, the heavy rainfall amounts decrease sharply. For instance, the number of grids with hourly precipitation between 10 and 30 mm drops by over 40% over MLYRB after the monsoon onset. It is found that the remarkable weakening of lower-level vertical wind shear and abnormal descending motion over southern China are unfavorable for the formation of MCSs. Corresponding to the SCSSM onset, on the one hand, atmosphere warms much less over the tropical oceans, including the SCS, than over the extratropical lands, resulting in smaller magnitude of meridional air temperature gradient and a subsequent decrease in vertical wind shear. On the other hand, the lower-level northerly winds induced by the SCSSM onset result in the suppressed ascent flows over southern China.