What does the rainy season mean?

The formation of Meiyu is the result of long-term stagnation of cyclones or fronts in the local area. During May and June each year, the subtropical high in the western Pacific is rapidly strengthened, and the humid tropical marine air mass and equatorial air mass expand to Nanling area of China. The following is what I call rainy season, for reference only. I like to share with you!

What does the rainy season mean?

Meiyu season is a warm and humid air flow in the Pacific Ocean brought by the southeast monsoon in June and July every year, which passes through the middle and lower reaches of the Yangtze River. Because it is the mature period of plums in Jiangnan, it is called "Meiyu", and this period is called Meiyu season. Note: At present, the academic circles believe that the East Asian summer monsoon started from the South China Sea summer monsoon (in late May on average) and belongs to the category of tropical southwest monsoon. The South China Sea summer monsoon gradually strengthens and moves northward, and moves northward to the middle and lower reaches of the Yangtze River during the meiyu period, and merges with the subtropical southwest monsoon on the northwest side of the subtropical high to form the meiyu belt.

In the rainy season, the air humidity is high and the temperature is high, so clothes are easy to get moldy, so some people call the rainy season "moldy rain". After the rainy season, the weather began to be dominated by the Pacific subtropical high and officially entered the hot summer.

There is no plum rain in other areas at the same latitude. There is no obvious plum rain in South China.

Every year from late April to early May, cold air from the north and warm air from the south meet in South China, forming a quasi-static front in South China. By the end of May, the influence of warm air was strengthened, and the quasi-static front moved northward to Jianghuai area, becoming the quasi-static front of Jianghuai (also known as Meiyu front). Because the warm air from the south carries a lot of water vapor, when it meets the cold air mass, it will produce a lot of convection activities. Because of the equal influence of cold and warm air during this period, the front stays in Jianghuai area.

Typical plum rains generally "enter plum blossoms" from mid-June to late June, and bloom in early to mid-July. Meiyu refers to the rainy days before plum blossom, which generally starts in mid-April and lasts for about half a month.

The cause of rainy season

The name of Meiyu has a long history, which was recorded as early as the Han Dynasty. It was called "Huang Meiyu" in ancient times. Among them, there are many accounts related to the occurrence time of plum rain: for example, it was called "summer rain" in the Jin Dynasty, "welcoming rain" in the Tang Dynasty, "plum yellow rain" in the Song Dynasty, and "plum ripens in May, and the rain lasts for ten days, so on. From these records, we can know that Meiyu got its name because it happened in the mature period of plums. In addition, due to the continuous rainy and humid air during the Meiyu period, things are prone to mildew, so Li Shizhen, a medical scientist in the Ming Dynasty, said that "Meiyu or mildew rain" caused black mold to appear on clothes and things. Therefore, Meiyu has another nickname: "light rain".

Meiyu is a process of continuous precipitation and rainy weather from June to July in China Jianghuai area and even in southeastern Japan. Meiyu period concentrates more than 40% of the annual precipitation in Jianghuai area, so it is also called Meiyu period in Jianghuai area. Meiyu in China mainly occurs in the Jianghuai Valley east of Yichang, Hubei, between 26 and 34 north latitude.

Meiyu weather is characterized by abundant rainfall, high relative humidity, short sunshine time, low ground wind and continuous precipitation. Because the rain belt swings back and forth, and many frontal cyclones move eastward, thunderstorms often occur during the meiyu period, and sometimes even regional heavy rains and heavy rains occur.

In normal years, in the middle of June every year, the circulation situation in East Asia obviously changes from the rainy season in Jiangnan in spring to the rainy season in Jianghuai area. Before the rainy season, the ridge of high pressure is located in West Asia or Eastern Europe, and the trough of low pressure is located on both sides of Eurasia. There are two branches of westerly jet in the north and south of East Asia. At the beginning of plum blossom, the ridge of westerly belt was in a state of disorder for a while, but soon a long-wave pattern of stable plum rain period appeared. The Okhotsk Sea High appears in the range of east longitude 130 ~ 150, and its two sides are two stable depressions in the range of east longitude 100 and east longitude 180 respectively. The subtropical ridge of westerly jet and Pacific moved northward, easterly winds appeared over South Asia, and southwest monsoon broke out. At the same time, from time to time, mid-high latitude cold air enters the Huanghuai area through Hexi or North China, and the cold air meets the southwest airflow that has gone north, forming a more lasting front area and rainy weather from Jianghuai area to Japan. When the Meiyu front was formed, small fluctuations from west to east made some smaller cyclones appear on the front belt, and the rainfall where the cyclones passed was relatively large. After mid-July, the westerly belt moved northward again, the Pacific subtropical ridge jumped 27 north latitude, the ground frontal rain belt moved northward to North China, the meiyu ended, and the Jianghuai area entered the summer season.

Jianghuai area is an important agricultural area in China. Whether the rainfall in rainy season is normal or not directly affects the agricultural production and national economic development in the Jianghuai basin. There is a saying among farmers in the south of the Yangtze River that there is no rain in Huangmei for half a year. The length, morning and evening, precipitation and the location of the rain belt are different every year, which depends on the intensity and location of the Asian atmospheric activity center and the strength of the monsoon, that is, on the interaction of the westerly belt, subtropical and tropical airflow. If the subtropical high is strong and moves northward quickly, there will be "less plum" or "empty plum", which is easy to cause drought; On the other hand, if the cold wave force in Meiyu period is so strong that the rain belt still stays in South China, it will cause the late Meiyu period or the interruption of Meiyu. Therefore, in order to develop production in Jianghuai area, ditches must be dredged according to the changing law of meiyu weather to facilitate drainage and flood control and drainage. Attention should also be paid to protecting irrigation water sources and preventing drought.

Weather characteristics of plum rainy season

In the rainy season, the air humidity is high and the temperature is high, so clothes are easy to get moldy, so some people call the rainy season "moldy rain". After the rainy season, the weather began to be dominated by the Pacific subtropical high and officially entered the hot summer.

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