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

Abstract

Pre-summer rainy season (April to mid-June) over South China (SC) is characterized by high intensity and frequent occurrence of heavy rainfall in the East Asian monsoon region. This review describes recent progress in the research related to such a phenomenon. The mechanisms responsible for presummer rainfall consist of multi-scale processes. Sea surface temperatures over tropical Pacific and Indian Ocean are 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. Formation and intensification of such anomalies can be contributed by surface sensible heating and mechanical forcing from the Tibetan Plateau. 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, while the westward extension of the western North Pacific high and the intensification of the southwesterly monsoonal flows play important roles in providing highequivalent potential temperature air to the west-and east-inland regions, respectively. The southern rain belt with a smaller horizontal span is 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 effect 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 convectivelygenerated 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 by warm-rain, riming and ice-phase microphysical processes remain unclear. Moreover, to improve the rainfall prediction, efforts have been made in convection-permitting modeling studies.

The Synoptic Impacts on the Convection Initiation of a Warm‐Sector Heavy Rainfall Event Over Coastal South China Prior to the Monsoon Onset: A Numerical Modeling Study

“…Efforts have also been made to develop numerical weather prediction (NWP) technique including data assimilation (Bao et al., 2017; Zhang et al., 2016), model physic parameterization (Furtado et al., 2018, 2020; Qian et al., 2018), and perturbation‐generation methods for ensemble forecast (Zhang 2018, 2019). Despite all these research progresses (summarized by Luo et al., 2020), it remains challenging to make timely and accurate forecast for the pre‐summer warm‐sector heavy rainfall over coastal SC under relatively weaker large‐scale dynamic lifting. Improving the forecast skill requires better understanding the processes governing the convection initiation (CI) of such coastal heavy rainfall.…”

Convection Initiation at a Coastal Rainfall Hotspot in South China: Synoptic Patterns and Orographic Effects

“…The coastal regions of South China witness the most active convection in China, especially after the onset of the East Asian summer monsoon (Luo et al, 2017(Luo et al, , 2020. The orographic effects and the convergence between land breezes/downslope winds and low-level onshore flows (especially the onshore marine LLJ) play an important role in the regional hydrologic cycle (Bao et al, 2021;Luo et al, 2020;Wang et al, 2021). The CI occurrences in this region are pronounced from June to August, with a primary peak in the early afternoon and a secondary peak at midnight (Bai et al, 2020a).…”

Trends in Diurnal Cycle of Summertime Rainfall Over Coastal South China in the Past 135 Years: Characteristics and Possible Causes

“…The mean R in (Figure 2c) of pre‐summer rainfall increases from a minimum of 2.5 mm/hr after midnight to a maximum of 3.8–4.0 mm/hr in the morning (06–09 HKT), and then quickly declines until the night. This is associated with the enhanced transport of warm and moist air of tropical origin and the induced low‐level convergence during the onset of the East Asian summer monsoon (Luo et al., 2020). The mean R in of mid‐summer rainfall also increases after midnight to a peak of 3.8 mm/hr in early morning (09 HKT), however, nocturnal rainfall of mid‐summer is substantially stronger than that of pre‐summer, which explains the higher nocturnal R cu in mid‐summer (Figure 2a).…”

“…In this section, a further analysis of the possible contributors is presented to probe the possible mechanism behind this. Previous studies have demonstrated that the DCR in South China is due to multi‐scale conditions and in‐between interaction, the key factors include temperature, moisture availability, low‐level monsoonal flow, and land‐sea breeze (Chen et al., 2018; Luo et al., 2020). The variability of these factors and their correlations with the changes in rainfall over CSC during stage III* are examined based on the ERA5.…”

“…Pre‐summer rainfalls are associated with the activities of the East Asian summer monsoon during its establishment (R. Huang et al., 2012; B. Wang 2006), whereas mid‐summer rainfalls are more dominated by thermal convections, tropical depressions and cyclones (Lee et al., 2010). Most precipitation during summertime is of convective nature with mesoscale organizational characteristics, especially during heavy rainfall events (Luo et al., 2020; Xu et al., 2009). South China is also known to exhibit pronounced DCR with significant regional and seasonal variations.…”