What is the phenomenon called when there is often a period of heavy rainfall in early summer in the Jianghuai River Basin?

In the Jianghuai River Basin, there is often a period of heavy rainfall in early summer every year, which is called Meiyu. It is the ripening season of plums in the south of the Yangtze River, so it is called plum rain.

In early summer, a long period of gloomy and rainy weather often occurs in the Jianghuai River Basin. At this time, utensils are prone to mold, so it is also called "mold rain", or "mold" for short. It is also the time when plums in the south of the Yangtze River are yellow and ripe, so it is also called "plum rain" or "yellow plum rain". After mid-June, the rain belt remains in the Yangtze-Huaihe River Basin, which is the Meiyu period.

Conditions for the formation of Meiyu:

1. In the high latitudes of Asia, there is blocking high pressure or a stable high pressure ridge in the middle of the troposphere, and the atmospheric circulation is relatively stable and less changeable.

2. The westerly circulation in mid-latitudes is flat, and frequent shortwave activities provide cold air conditions for the Jianghuai region.

3. The western Pacific subtropical high has an obvious westward and northward jump process. The 500hPa subtropical high ridge is stable between 20 degrees and 25 degrees north latitude, and warm and moist air flows from the edge of the subtropical high to the Yangtze-Huaihe River Basin.

Under such circulation conditions, the Meiyu front lingers in the Yangtze-Huaihe River Basin, often accompanied by southwest vortices and shear lines, and mesoscale systems are active above the Meiyu front. It not only maintains continuous rainfall during the Meiyu period, but also provides abundant water vapor for heavy rains.