Characteristics of newly formed mesoscale convective systems during the abnormal precipitation over the Yangtze River basin from June to July, 2020

A 62-day “extremely violent Meiyu” occurred in the Yangtze River basin from June to July in 2020, and its abnormal precipitation caused serious losses to the social economy. In this paper, the characteristics of newly formed mesoscale convective systems (MCSs) and the environmental conditions affecting the MCS formation during this abnormal precipitation event are investigated based on hourly datasets, including the temperature of brightness blackbody (T_BB) data derived from the FY-2F geostationary meteorological satellite, and the three-source merged precipitation products from China Meteorological Administration. The results show that there were 31,778 times of short-duration heavy precipitation (hourly precipitation exceeding 20 mm) occurred in the Yangtze River basin from June to July in 2020. The 69.3% of short-duration heavy precipitation (hourly precipitation exceeding 20 mm) are closely related to the MCSs. The increase of newly formed MCSs leaded to more short-duration heavy precipitation. The abnormal precipitation in the Yangtze River basin from June to July is mainly caused by elongated MCSs with an average lifetime of 6 h. The number of the newly formed MCSs presents obvious diurnal variations, with the most frequent at 20:00 (Beijing Time, the same below). The newly formed MCSs are prone to appear near the Yangtze–Huaihe shear line, and the magnitude of southerly wind at 500 hPa determines the distance between the MCSs and the shear line. The enhancement of vorticity advection at 500 hPa and water vapor convergence are conducive to the MCS formation. The weakening of low-level jet corresponds to the increase of newborn MCS.