Statistical Characteristics of Pre-summer Rainfall over South China and Associated Synoptic Conditions

Abstract

Climatological characteristics of pre-summer (April-to-June) rainfall over Southern China (SC) and associated synoptic conditions are examined using 1980-2017 hourly rainfall observations and reanalysis data. The rainfall amount, frequency, and intensity show pronounced regional variations and substantial changes between pre-and post-monsoon-onset periods. Owing to more favorable thermodynamic conditions after monsoon onset over South China Sea (SCS), rainfall intensifies generally over entire SC irrespective of the rainfall-event durations. Increased magnitudes of rainfall amount in longerduration (>6 h) events are found over a designated west-inland region (west of 111ºE), which are partially attributed to enhanced dynamic instability. In addition, rainfall events occur more frequently over the west-inland region, as well as coastal regions west side of 118ºE, but less over a designated east-inland region. The inland-region rainfall is closely linked to dynamic lifting driven by subtropical synoptic systems (low pressure and an associated front or shearline). The westward extension of the western North Pacific high and the eastward extension/movement of the front or shearline, interacting with the intra-period intensification of the southwesterly monsoonal flows, play important roles in providing high-θe (equivalent potential temperature) air to the west-and east-inland regions, respectively. The warm-sector, coastal rainfall is closely related to the deceleration of the southerly boundary layer (BL) jet (BLJ) over the northern SCS and associated convergence of BL high-θe air near the coast. Meanwhile the southwesterly synoptic-system-related low-level jet in the lowerto-middle troposphere to the south of the inland cold front can contribute to the coastal rainfall occurrence by providing divergence above the BL convergence near the coast. The BLJ often simultaneously strengthens with the lowertroposphere horizontal winds, suggesting a close association between the BLJ and the synoptic systems. The quantitative statistics provided in this study complement previous case studies or qualitative results and thus advance understanding about pre-summer rainfall over SC.

Roles of Double Low‐Level Jets in the Generation of Coexisting Inland and Coastal Heavy Rainfall Over South China During the Presummer Rainy Season

“…The importance of tropical‐originating airflows in the BL and lower troposphere, not necessarily reaching an LLJ's minimum intensity (peak wind speed >10 m s −1 ), in generating inland and coastal heavy rainfall over south China during presummer rainy season (April–June) has recently been documented in the literature (Chen et al, 2017; Huang & Luo, 2017; Li et al, 2020; Wang et al, 2014). A few case studies have shown the significant contributions of coupled SLLJ and BLJ to the development of dual coexisting rainbelts, that is, with extensive heavy rainfall over inland and concurrent extreme (yet localized) coastal rainfall (Du & Chen, 2018; Shen et al, 2020; Zhang & Meng, 2018).…”

“…Du and Chen (2018, 2019a) pointed out that, for coexisting inland synoptic and coastal warm‐sector heavy rainfall over south China, an SLLJ and a BLJ are more closely related to the former and latter, respectively. Climatological analyses (Li et al, 2020; Zhang & Meng, 2019) and another case study (Shen et al, 2020) confirm that the coupling of BLJs and SLLJs favors the initiation of coastal convection leading to warm‐sector heavy rainfall over south China through BL convergence and lower level to midlevel divergence. Differences between the BLJ and SLLJ influencing rainfall over south China are also noticed in terms of their spatial distributions and associated physical mechanisms (Du et al, 2014; Du & Chen, 2019b).…”

“…The warm‐sector rainfall over the south China coasts occurs either by its own or concurrently with synoptic rainfall over inland south China (Li et al, 2020; Wu et al, 2020). Its formation is closely associated with oceanic southerly airflows in the boundary layer (BL) (Du & Chen, 2018; Du & Rotunno, 2018; Luo et al, 2017), convectively generated cold pools (Liu et al, 2018; Wu & Luo, 2016), and surface forcing such as land‐sea contrast (Chen et al, 2016) and orographic lifting (Wang et al, 2014; Yu & Cheng, 2013).…”

Convection Initiation in Monsoon Coastal Areas (South China)

“…During the warm season, an abundance of organized storm types is observed, such as long‐lived mesoscale convective systems (Huang, Liu, et al, 2019), squall lines (Meng et al, 2013), supercells, and tornadoes (Bai, Meng, et al, 2020). As a result, monsoon rainfall exhibits a complex spatiotemporal nature along the coasts of South China (e.g., Chen et al, 2018; Li et al, 2020). In particular, the pronounced subseasonal/diurnal variations in convection and rainfall are demonstrated to be highly related to the summer monsoon, complex terrains (island, peninsula, and mountain), and land‐sea breeze circulations (e.g., Chen, Zhao, & Xue, 2014; Houze et al, 1981; Luo et al, 2017).…”

Observational Analysis of the Characteristics of the Synoptic Situation and Evolution of the Organized Warm-Sector Rainfall in the Coastal Region of South China in the Pre-Summer Rainy Season

“…The convergence can be enhanced when the BLJ collides with the coastal mountains and produces strong boundary-layer upward motion, providing favorable conditions for the initiation of the OWSRs on the windward side of the coastal mountains. This mechanism has been demonstrated for a heavy warm-sector rainfall at the coast of South China by Du and Chen [44] and a large ensemble of rainfall events over South China coasts during Apr-Jun 1980-2017 by Li et al [78]. Regarding the ACR situation, the boundary-layer convergence between northerly and southeasterly winds near the coastal mountains that form under this type likely promotes the initiation of the coastal OWSRs (Figure 8b).…”