LAUSR

The diurnal variations of coastal boundary layer jets (CBLJs) over the northern South China Sea (SCS) and their effects on the diurnal cycle of rainfall over southern China during the early-summer rainy season of 2010–2016 are examined based on the merged hourly gauge and satellite rainfall dataset, ERA5 reanalysis data and numerical simulations. CBLJs over the northern SCS exhibit two high-frequency regions with different diurnal cycles: one shows a nocturnal peak, located at the Beibu Gulf (WCBLJ); the other shows a double peak in the early morning and late afternoon, located over the coastal area of Guangdong (ECBLJ). Both of CBLJs are occurred more frequently near Hainan Island. The inertial oscillations triggered by the land–sea thermal contrast, dynamic effects of Hainan Island and effects of mountains over Vietnam are mainly attributed to the nocturnal WCBLJs, whereas the inertial oscillations induced by the large-scale sea breezes are responsible for nocturnal ECBLJs. The formation of afternoon ECBLJs mainly result from the thermal effects of Hainan Island. Nocturnal and morning rainfall over southern China are pronounced near the coast and the windward slope, while the afternoon rainfall peaks over the inland region in Guangdong Province. Due to dynamically forced lifting and moisture transport, WCBLJs and ECBLJs are essential for the formation of nocturnal and morning rainfall over Guangxi and Guangdong Province, respectively. In addition to the effects of solar heating, the afternoon rainfall over inland Guangdong is mainly affected by the ECBLJs and eastward-propagating rainfall belts from Guangxi Province, which are highly related to the occurrence of WCBLJs.