How is the plum rain formed? This kind of weather is easy to form in the rainy Jianghuai area. Is it controlled by cold and low pressure?

Meiyu (Huangmeitian) refers to a cloudy and rainy climate phenomenon in the middle and lower reaches of the Yangtze River, Taiwan Province Province, south-central Japan and south Korea every year from the middle and late June to the first half of July. This period is the maturity of plums in Jiangnan, so it is called Meiyu.

Meiyu weather process in Jianghuai Valley of China —— The middle and lower reaches of the Yangtze River are located in the middle latitude of the eastern part of Eurasia, which is affected by the cold air from the cold zone to the south and the warm and humid air from the tropical ocean to the north. Since the beginning of spring every year, the influence of warm and humid air flow has gradually strengthened, entering the mainland from the sea, reaching South China first, and then further northward. Early summer often extends to the middle and lower reaches of the Yangtze River, and sometimes reaches the Huaihe River and its north. Especially at the low altitude of two or three kilometers, there is often a very humid strong southerly airflow blowing from the ocean, and the wind speed reaches about ten to twenty meters per second. When it entered Chinese mainland, it encountered cold air from north to south. Cold and warm air meet, forming a front at the junction, and precipitation appears near the front. Meiyu belongs to frontal precipitation.

If the cold air is strong, the cloud and rain area will move southward with the cold air; If the warm air is strong, the cloud and rain area will move northward with the warm air. Obviously, in both cases, they will not stay in one area. However, in early summer, in the middle and lower reaches of the Yangtze River, on the one hand, the warm and humid airflow is quite active, on the other hand, the cold air from north to south still has a certain intensity, especially in the air layer near the ground, and there is often a small stream of cold air going south. In this way, cold and warm air will confront and compete with each other in this area, forming a stable rainfall belt. This rain belt is only two or three hundred kilometers from north to south, but it can reach about two thousand kilometers from east to west. It spans the middle and lower reaches of the Yangtze River and extends eastward to Japan. It is the influence of this rain belt that makes the plum rain in Japan obvious.

This rain belt tends to swing slightly from north to south in a short time. When the cold air is strengthened, it moves slightly south; When the warm air strengthens, it rises northward again. When this narrow rain belt swings slightly in the north-south direction, there will be sunny and rainy weather in the area near the rain belt. On this rain belt, there are also centers with relatively large rainfall intensity from time to time. There is often heavy rain or heavy rain in the area where the rainfall center passes.

In fact, this rain belt is also the product of the interface between cold and warm air fronts-the "front" commonly used in meteorological broadcasting. However, there are many differences between this front and the general front. First, this front is particularly stable. It not only has no obvious movement like "cold front" and "warm front", but also is different from the general "static front". Usually, the "static front" can only stay in one area for one or two days, up to three or four days. The active period of Meiyu front in the Yangtze River basin is precisely the period between two jumps of atmospheric movement in the vast area of East Asia. During this period, cold and warm air met for a long time in the middle and lower reaches of the Yangtze River, and the two sides were evenly matched and were in a state of seesaw. Therefore, this front and its rainfall belt have been particularly stable for a long time, bringing continuous rainy weather to the middle and lower reaches of the Yangtze River. Secondly, the difference between the cold and warm air on the north and south sides of Meiyu front is mainly manifested in the humidity of the air, that is, the humidity of the air from the ocean in the south is higher, which is quite different from the dry cold air in the north. However, the temperature difference between the two sides of the front is much smaller than that of the front in other seasons, and the cold air has not cooled significantly after crossing the border; Third, its rain area is very narrow in the north-south direction, unlike the front in winter and spring, and the rain area is very wide. But its precipitation intensity is much stronger than other seasons. Because these characteristics are unique in the Meiyu period, this front is called "Meiyu front" by the meteorological community and "quasi-stationary peak" in technical terms.