Organized Warm-Sector Rainfall in the Coastal Region of South China in an Anticyclone Synoptic Situation: Observational Analysis

Organized warm-sector rainfall (OWSR) near the coast of South China tends to occur in certain synoptic situations characterized with either a low-level jet or an anticyclone, with the latter being less investigated. This paper fills the gap by analyzing 15 OWSR events that occurred in an anticyclone synoptic situation during the pre-summer rainy season of 2011–2016, based on high-resolution observational and reanalysis data. The results show that the anticyclone synoptic situation produces marked northerly boundary-layer winds inland and obvious northeasterly, easterly/southwesterly, and southeasterly boundary-layer winds near the coasts of eastern Guangdong, western Guangdong, and Guangxi, respectively. The coastal boundary-layer winds promote favorable environmental conditions and strong convergence for convection initiation; consequently, OWSR is prone to occur near the coasts of western Guangdong and Guangxi, but exhibits different formation and propagation features in the following two sub-areas. (1) The southeasterly boundary-layer winds tend to converge near the border area between Guangxi and Guangdong (BGG), promoting the formation of a stable convective line along the mountains. The convective line persists with support of upper-level southwesterly winds that facilitate convective cells to propagate along the convective line, producing heavy OWSR along the mountains near BGG. (2) In contrast, a west-east convective line tends to form and maintain near the coast of Yangjiang (YJ) area, about 200 km east of BGG, owing to stable convergence between the easterly (or southwesterly) and the northerly boundary-layer winds reinforced by the mountains near YJ. Moreover, the coupling of upper-level westerly winds with the easterly (southwesterly) boundary-layer winds facilitates expansion (eastward propagation) of the convective line, causing west-east-oriented heavy OWSR near the coast of YJ. In a word, this study reveals refined properties of OWSR initiation and development in the anticyclone synoptic situation, which may help improve the forecast skill of OWSR during the pre-summer rainy season in South China.