Why is there a "Huangmeitian"?

Meiyu-A continuous rainy period from the Yangtze River valley in China to southern Japan in early summer. Because the plum is yellow and ripe, it is named.

Meiyu is produced in the front area of the western Pacific subtropical high edge (called Meiyu front), which is the product of the interaction between polar air mass and subtropical air mass. The location and stability of the Meiyu belt are closely related to the location and intensity of the subtropical high (the ridge line is generally stable between 20 and 25 north latitude), and also related to the circulation situation that the westerly belt is not conducive to the cold air moving south to the Yangtze River basin. At the beginning of June every year, when the atmospheric circulation situation is greatly adjusted, the ridge line of the western Pacific subtropical high jumps to the north of 20 north latitude, and people begin to blossom. When the ridge line of the western Pacific subtropical high jumps further northward and crosses 25 north latitude, the meiyu period ends and the Yangtze River basin enters the summer drought period.

Meiyu in Meiyu season

Meiyu refers to a period of continuous rainy weather from mid-June to mid-July and early summer. The middle and lower reaches of the Yangtze River in China refer to the range of 28-34 N east of Yichang, or the narrow area from Jianghuai Valley to southern Japan. During this period, due to the monsoon adjustment of atmospheric circulation, the warm and humid airflow from the ocean and the cold air from north to south continue to meet in the Jianghuai Basin, forming an east-west quasi-static front, commonly known as Meiyu Front, which makes the rain continuous and the rainstorm concentrated. It is called "Meiyu" or "Meiyu weather" because it is in the yellow ripening period of plums in the south of the Yangtze River. Because of the high temperature and humidity at this time, clothes are prone to mildew, also known as "mildew rain".

During the meiyu period, a stable and lasting rain belt was maintained from Jianghuai Valley to southern Japan. The distribution of rainstorm in rain belt is uneven, and there are often multiple rainstorm centers. The rainstorm intensity of Meiyu front is generally much smaller than that of typhoon, but the flood disaster area caused by Meiyu front is generally larger than that of typhoon because of its long duration and wide rainstorm range. For example, 199 1 and 1999, the rainy season in the Jianghuai basin lasted for two months, and heavy rains and serious floods occurred in the lower reaches of the Yangtze River, Taihu Lake and Huaihe River, which is rare in history. Taihu Lake, Lixia River, Chuhe River and Qinhuai River all have the highest water levels in history.

The atmospheric circulation conditions that form the Meiyu front rainstorm generally include:

(1) There are blocking high pressure or stable high pressure ridges in the middle troposphere at high latitudes in Asia, and the atmospheric circulation is relatively stable and rarely changes;

(2) The westerly circulation in the mid-latitude area is flat, and frequent short-wave activities provide cold air conditions for the Jianghuai area;

(3) The western Pacific subtropical high has an obvious process of extending westward and jumping northward. The ridge line of the 500 HPA subtropical high is stable between 20 and 25 degrees north latitude, and the warm and humid air flow is transported from the edge of the subtropical high to the Jianghuai basin.

Under this circulation condition, Meiyu front lingers in Jianghuai basin, often accompanied by southwest vortex and shear line. Mesoscale systems such as frontal cyclones are very active on the Meiyu front. This not only maintains the continuous precipitation during the Meiyu period, but also provides abundant water vapor for the rainstorm. Meiyu front rainstorm is a special weather in a specific area formed by the interaction of circulation systems of different scales.

Due to the variability of atmospheric circulation, the "in" and "out" dates of Meiyu are different every year, and the difference between morning and evening can reach more than 40 days. In some years, the Meiyu front is particularly active, and heavy rains are frequent, causing floods. Sometimes the plum rain front is not obvious, and there will be a phenomenon of "less plum" or "empty plum". For example, in 1954, 199 1 year, and 1999, the meiyu period was particularly long, and there was a lot of precipitation, which caused floods in the Jianghuai basin. However, there were few plums in 1994, which made the Jianghuai area extremely dry. In other years, the Meiyu belt will move northward and then return to the Jianghuai basin to maintain relative stability, which is customarily called "inverted Huangmei". For example, Huangmei in August of 1980 has a great influence on agricultural production in various parts of Zhejiang. This abnormal situation in meiyu period is often related to the abnormal change of global atmospheric circulation.

Under normal circumstances, after the first ten days of July, the meiyu in Jianghuai Basin ended from south to north, the frontal rain belt moved to North China, and the Jianghuai Basin entered the summer drought weather with high temperature and little rain.