Temporal and spatial characteristics of mesoscale convective systems associated with abrupt heavy rainfall events over Southwest China during May—August

Based on temperature of brightness blackbody (TBB) data from FY-2E (2010—2014) and FY-2G (2015—2018) and 468 rain gauges, temporal and spatial characteristics of the mesoscale convective systems associated with abrupt heavy rainfall events (AHR-MCS) over southwest China (SWC) in May—August of 2010—2018 are investigated according to whether the hourly cloud tops of mesoscale convective systems (MCS) cover the gauged-based abrupt heavy rainfall. AHR-MCSs appear most frequently in July with high frequencies over 4 discrete subregions located in Sichuan basin (SR-A), western Hunan (SR-B), northern Guangxi (SR-C) and southwestern Guangxi (SR-D). Abnormal meridional circulations, large divergence anomalies of vertically integrated water vapor flux, high moist static energy anomalies and strong wind shear (June—August) between surface and 700 hPa (especially in June—August) are favorable for AHR-MCSs. AHR-MCSs are the MCSs that have longer lifespan over SWC. They mainly move eastward, but those located in the western (eastern) part of SWC over SR-A and SR-D (SR-B and SR-C) more frequently move southeastward (northeastward). When they are mature, the MCSs in SR-A usually have smaller cloud areas and lower cloud tops, while those in SR-B and SR-C have larger areas and those in SR-D have higher cloud tops. AHR-MCSs appear most frequently during 20:00—23:00 BT with diurnal unimodal structure, and the peak occurs later as the lifespan is longer. There is only one peak over both SR-A and SR-D, but the peak occurrence time of the former is 6—8 h later than that of the latter. Besides, multi-peak structure of AHR-MCS frequency appears over SR-B and SR-C. The maximum hourly precipitation mostly appears during the development stage of the AHR-MCS, and often corresponds to the time when the minimum TBB shows up, earlier than the mature time. Gauges within maximum hourly precipitation are often located in the Ⅰ and Ⅲ quadrants of clouds when the minimum TBB occurs. The maximum hourly precipitation not only frequently appears in the cloud areas of lower TBB (＜−51℃), but also appears in the cloud areas of higher TBB gradient (＞ 0.4℃/km), and these two make similar contributions to heavy rainfall.