What caused the rainstorm in the Yangtze River basin in 2020?

In July, Guizhou, Chongqing, the middle and lower reaches of the Yangtze River and other places ushered in a new round of heavy rainfall, which seems to be leaking water and crazy in the Yangtze River basin and other places? Splash water? The precipitation in many places has broken records, the rivers have exceeded the police, and the water situation is grim. ? From June/KLOC-0 to July 7, there were six heavy rainfall processes in the middle and lower reaches of the Yangtze River. The average precipitation in the Yangtze River basin (346.9mm) is the second highest since June 196 1 (the first is 2016 376.3mm, and the third is1998 3366mm). Among them, from July 4 to 7, heavy rain continued to appear along the main stream of the middle and lower reaches of the Yangtze River, with a cumulative rainfall of 100 ~ 250 mm, 300 ~ 500 mm in parts of northwestern Hunan, southeastern Hubei, southern Anhui and northwestern Jiangxi, and more than 800 mm in Huanggang, Hubei. During the period, the daily maximum rainfall in Lianhuashan Township, Poyang County, Jiangxi Province reached 538 mm (08: 00 on the 7th to 08: 00 on the 8th). The daily rainfall of four national weather stations in Xishui, Hubei (19 1mm), Huangmei (332mm), Jiujiang County, Jiangxi (149mm) and Xinjian (215 mm) exceeded historical extremes, and the national meteorological observation1/. The recent rain belt stranded in the Yangtze River basin is mainly due to the relatively stable weather system related to Meiyu. The western Pacific subtropical high, which is closely related to the distribution of rain belts in China, has remained stable for some time. The warm and humid air flow at the edge of subtropical high is strong, and the low vortex shear system in the middle and lower reaches of the Yangtze River is active, which leads to long-term rainy weather in the middle and lower reaches of the Yangtze River. With the maintenance of such a long rainfall process in the Yangtze River basin, the extreme situation that the local rainfall exceeds the extreme value is also closely related to the local topographic characteristics. In fact, the plain area in the middle and lower reaches of the Yangtze River has always been one of the areas with the highest rainstorm intensity in China, and it is also the area with frequent rainstorm and flood disasters. Especially in low-lying areas, such as the Yangtze River's main stream from Jingzhou, Hubei to Tongling, Anhui, and the central plain of Anhui, because of its large confluence area, many tributaries, flat terrain, curved rivers and poor drainage, it is most prone to rainstorm and flood. Historically, serious floods have occurred many times in the Yangtze River basin, causing dike burst and flooding disasters, most of which occurred in this area.