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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 2010–16 are examined based on the merged hourly gauge and satellite rainfall dataset, ERA5 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 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. [ABSTRACT FROM AUTHOR]

Based on the observational hourly precipitation data with high-resolution and ERA5 reanalysis, the thermodynamic characteristics of warm sector heavy rainfall (WSHR) events over coastal areas of Guangdong are studied by composite analysis. The results show that the WSHR water vapour is mainly concentrated below 800 hPa in the lower troposphere with the maximum center located at 900 hPa, which is favorable for the warm and moist energy accumulation. The boundary layer jet (BLJ) and synoptic-system low-level jet (SLLJ) prevail over northern part of the South China Sea before precipitation happens, and the exit of BLJ locates at the entrance of SLLJ. The suction effect, together with the coastline convergence, tends to increase local upward motion and trigger local convection over coastal areas of Guangdong. The warm and wet tongue indicated by θ se over northern part of South China Sea, which represents the warm and moist energy at low levels, has close relationship with WSHR events. According to analyzed thermodynamics of WSHR, the objective identifications of precursor signals for WSHR over coastal area of Guangdong are proposed from perspectives of low-level jet, warm and moist tongue and dynamic convergence in the lower troposphere, which are further demonstrated effective via hindcasts for 2008 and 2009 and can be pragmatically employed as a scientific reference for the local WSHR forecasting. • Objective identifications of WSHR precursor signals over Guangdong coast are proposed via the thermodynamic characteristics. • Identification criteria include aspects of low-level jet, warm-moist tongue and dynamic convergence in the lower troposphere. • The proposed identification procedures of WSHR precursor signals are demonstrated effective by hindcast experiments. [ABSTRACT FROM AUTHOR]