Some Statistical Characteristics And Case Study Of Rainfall And Convection Over South China And Yangtze-huai River Basin During Summer Monsoon Period

Largescale circulation, weather systems, and horizontal distribution of rainfallassociated with the Asian-Pacific summer monsoon have been extensively studied, but therehave been few studies on characteristics of rainfall and convection of the Asian-Pacificsummer monsoon, especially fewer for the East Asian summer monsoon. The present studymakes compensation to previous studies on characteristics of precipitation and convectionover the East Asian summer monsoon region, and focus on two north and south adjacentsubregions of the East Asian summer monsoon region: South China (SC) and Yangtze-HuaiRiver Basin (YHRB) in the southeastern China. Comparisons of rainfall characteristics andconvective properties of precipitation systems over SC and YHRB regions during theirmonsoon active periods are made to reveal variations of monsoonal precipitation systemsalong with the northward march of the East Asian summer monsoon. In each region, thesubseasonal variations of rainfall characteristics and convective properties of precipitationsystems during from pre-monsoon, monsoon active and break, further to post-monsoonperiod are also compared to unveil subseasonal transitions of precipitation systems, based onmultiple years of the Tropical Rainfall Measuring Mission (TRMM) dataset and stationrainfall observation. Meanwhile, the characteristics of extreme convection over SC andYHRB, spatial distribution and diurnal variations of extreme convection in the southeasternChina are studied based on multiple years of TRMM dataset. Moreover, the synoptic patternand weather phenomena is associated with extreme wide convection (WC in short, defined asTSYN ZTZX TS\YN\40dBZ echo with near surface rainfall exceeding1000km2) overthe key region, Yangtze-Huai Plain, are further investigated. In the last, the process andmechanism of initiation and evolution and convective properties of a long-lived extremerainfall producing mesoscale convective system (MCS) developed over western coastalregion of Guangdong on10May2013are analyzed using multiple observations from SouthChina Monsoon Rainfall Experiment (SCMREX). The main conclusions are as follows.(1) By comparisons of rainfall characteristics and convective properties of precipitationsystems over SC and YHRB regions during their monsoon active periods of1998-2010, it is found that the differences in precipitation characteristics and convective properties betweenSC and YHRB are closely related to the topographic features, environmental atmosphericconditions and major weather systems in the two regions. The stronger convection in theYHRB during the monsoon active period is found to coincide with higher concurrentpresence and stronger intensity of the surface front and lower-level vortex in that region. Onthe other hand, the more frequent occurrence of precipitation systems and larger rainfallaccumulation in SC can be traced to the more mountainous land closer proximity to theocean.(2) Comparisons of subseasonal variations of rainfall characteristics and convectiveproperties of precipitation systems over SC and YHRB regions from pre-monsoon, monsoonactive and break, further to post-monsoon period suggest that there is a scale difference in thedriving mechanisms for rainfall production during the four periods over both SC and YHRB:the precipitation systems are more controlled by larger-scale weather systems (dynamicallydriven) during pre-monsoon and monsoon active periods, but more by local instability due tosolar heating (thermodynamically driven) during the post-monsoon and monsoon breakperiods. This scale difference can largely explain the smaller horizontal extents of theprecipitation systems in the post-monsoon and monsoon break periods. It can also contributeto the more pronounced afternoon-peaks in the diurnal cycles of the occurrence of thepost-monsoon convective storms.(3) By utilizing TRMM dataset of1998-2010, two definition methods of extremeconvection are applied according to proxies of convective intensity and three dimensionstructure of convection (classified into extreme deep convection, WC, and broad stratiform).The geographical distributions and diurnal variations of these kinds of extreme convectionsshow that the Yangtze-Huai Plain is the largest area with the maximal frequency ofoccurrences of all extreme convections (exclude broad stratiform) over the southeast Chinaduring the summer monsoon period. Diurnal variations of extreme deep convection andextreme convections defined according to proxies of convective intensity are all with asignificant peak in the afternoon, while extreme WC has two peaks in early morning andafternoon, suggesting the different formation mechanism between the two kinds of extremeconvections. Compared with the extreme deep convection, the extreme deep convection is with larger area and stronger rainfall intensity due to more favorable atmospheric circulationconditions for the formation of precipitation with larger area and stronger rainfall intensity,however, the extreme deep convection is with stronger convective instability condition.(4) Based on geopotential height on500hPa from ERA-interim dataset, an impersonalanalysis method is adopt to classified the environmental fields of extreme WCs inYangtze-Huai Plain during summer monsoon periods of1998-2010into three weatherpatterns: deep-trough-control (DTr), subtropical-high-maintenance (STH) and typhoon-effect(Typh), and the STH is the most popular. Among the three patterns, the extreme WCs underthe DTr pattern start to emerge the earliest (16-31May); the extreme WCs under the STHpattern have a significant peak in the first half of July; Those under the Typh pattern mainlyoccur in July and August. Nearly all of the extreme WCs in this region accompany withthunderstorms supported by large convective available potential energy and abundantmoisture. Among the three synoptic patterns, the DTr pattern features the driest and coldestair in the region, leading to the least occurrences of short-duration heavy rainfall. Gales occurthe most often under the DTr pattern, probably owing to the largest difference in air humiditybetween mid-and low-level layers. Hail at the surface is rare for all extreme WCs, which isrelated to the humid environmental air under all weather patterns and the high (>5km)freezing level under the STH pattern.(5) A long-lived mesoscale convective system (MCS) with extreme rainfall (451mm in19hours) over the western coastal region of Guangdong on10May2013during theSouthern China Monsoon Rainfall Experiment (SCMREX) is studied. The environmentalconditions are characterized by little convective inhibition, moderate convective availablepotential energy, moderate precipitable water, low lifting condensation level, and lack oflow-level jets from the tropical ocean. Repeated convective back-building and subsequentnortheastward "echo training" of convective cells are found during the MCS’s earlydevelopment and mature stages. However, the earlier and later stages possess distinctiveinitiation/maintenance factors and organization of convection. During the earlier stage frommidnight to early-morning, convection is continuously initiated when weak southeasterly andsoutherly flows near the surface impinge on the east sides of Mts. Longgao and Ehuangzhang(with their respective peaks of approximately800and1300m) and moves northeastward, leading to formation of two quasi-stationary rainbands. During the mature stage fromearly-morning to early-afternoon, new convection is repeatedly triggered along aprecipitation-induced cold outflow boundary, resulting in the formation of several rainbandsthat are quasi-stationary and move eastward in later times. Individual rainbands during thestages similarly consist of northeastward training of convective cells and a stratiform regionto the northeast. While the MCS dissipates, a stronger squall line moves into the region fromthe west and passes over within about3.5hours, contributing about10%-15%to the totalrainfall amount. It is concluded that terrain, near surface winds, and convectively generatedcold outflows play important roles in initiating and maintaining the extreme-rain-producingMCS.