Characteristics, Physical Mechanisms, and Prediction of Pre-summer Rainfall over South China: Research Progress during 2008-2019

The pre-summer rainy season (April to mid-June) over South China (SC) is characterized by a high intensity and frequent occurrence of heavy rainfall in the East Asian monsoon region. This review describes recent progress in the research related to this phenomenon. The mechanisms responsible for pre-summer rainfall consist of multiscale processes. Sea surface temperatures over the tropical Pacific and Indian Oceans arc shown to have a great influence on the interannual variations of pre-summer rainfall over SC. Synoptic disturbances associated with regional extreme rainfall over SC are mainly related to cyclone- and trough-type anomalies. Surface sensible heating and mechanical forcing from the Tibetan Plateau can contribute to the formation and intensification of such anomalies. On a sub-daily scale, double rain belts often co-exist over SC. The northern rain belt is closely linked to dynamic lifting by a subtropical low pressure and its associated front/shear line, whereas westward extension of the western North Pacific high and intensification of the southwesterly monsoonal flows play important roles in providing high-equivalent potential temperature air to the west- and east-inland regions, respectively. The southern rain belt, with a smaller horizontal span, exists in the warm sector over either inland or coastal SC. The warm-sector rainfall over inland SC results from surface heating, local topographic lifting, and urban heat island effects interacting with the sea breeze. The warm-sector rainfall over coastal SC is closely associated with double low-level jets, land-sea-breeze fronts, and coastal mountains. A close relationship is found between convectively-generated quasi-stationary mesoscale outflow boundaries and continuous convective initiation in extreme rainfall events. Active warm-rain microphysical processes can play an important role in some extreme rainfall events, although the relative contributions of warm-rain, riming, and ice-phase microphysical processes remain unclear. Moreover, to improve rainfall predictions, efforts have been made in convection-permitting modeling studies.