Meiyu is a unique weather phenomenon in which part of China.

In the middle and lower reaches of the Yangtze River in China, Taiwan Province Province, south-central Japan, South Korea and other places, there will be continuous rainy weather in June and July every year. Because it is the mature period of plums in Jiangnan, it is called "Meiyu", and this period is called Meiyu 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.

In early summer, there are often long rainy days in the Jianghuai basin. At this time, the utensils are easy to be moldy, so it is also called "moldy rain" or "mildew" for short; It is also called "Plum Rain" or "Huang Meiyu" because plums in the south of the Yangtze River are yellow and ripe. It is recorded in China's historical records. For example, The Book of Beginners quoted the compilation of Emperor Liang Yuan in the Southern Dynasties, "Plum blossoms have plum rains when they are ripe". Liu Zongyuan's Plum Rain in the Tang Dynasty: "Shi Mei welcomes the rain, and it is worth the late spring." Wait a minute. It is recorded in China's almanac that there is mildew rain all day: it is called "mildew" at the beginning and "mildew" all day long. The first day after the awn is planted, it is moldy, but it is not moldy on the first day after the hot summer. Mold always enters from June 6th to 15, and mold always leaves from July 8th to 19. There is an obvious rainy season in the eastern part of China, with a long rainy period and relatively concentrated rainfall, which is caused by the north-south displacement of the main rain belt in the east-west direction and the seasonal transformation of the atmospheric circulation in East Asia at the turn of spring and summer.

Intermittent rain in Mao Mao (middle and lower reaches of the Yangtze River)

The unique phenomenon during the period. After mid-June, the rain belt remained in the Jianghuai basin, which is Meiyu. "(Because language is used to language, Meiyu is not confined to the Jianghuai area. The continuous precipitation in the rainy season in eastern China, such as Fujian and central and southern Liaoning, is also called Meiyu. )

Intermittent rain in Mao Mao (middle and lower reaches of the Yangtze River)

1. The middle and lower reaches of the Yangtze River in China are usually rainy from mid-June to early July every year. In the past few days, the sky has been gloomy and the precipitation has been constant, sometimes big and sometimes small. Therefore, there is a popular proverb in southern China: "Rain hits plum blossoms, and there is no sun for forty-five days". Continuous rain, high temperature and high humidity are the main characteristics of plum rain.

Very different from the climate in the same latitude area, Meiyu refers to the weather and climate phenomenon that occurs in a certain area and a certain season. It is found that Eurasia is between 20 N and 40 N, which is alternately controlled by subtropical high and westerly belt. The west coast of the mainland is controlled by the descending airflow on the east side of the subtropical high in summer, with sunny weather, partly cloudy and dry climate; In winter, under the influence of westerlies, warm and humid air is brought from the Atlantic Ocean, resulting in more precipitation and a mild and rainy climate. That is, the subtropical Mediterranean climate is dry in summer and humid in winter.

The east coast of the mainland is controlled by the west side of subtropical high in summer, and the downward flow is dry, but it absorbs a lot of water vapor from the warm and humid sea surface, bringing abundant precipitation and producing a subtropical humid climate. Due to the strong contrast between land and sea, a unique monsoon climate has formed here, which is characterized by rainy summer and dry winter, with rainfall concentrated in summer, which is just the opposite of the Mediterranean climate.

If compared with the east coast of the United States at the same latitude, it is also a world of difference. There will be no rainy weather for a long time before and after the summer monsoon in the mid-latitude zone of the east coast of the United States, and people do not feel the feeling of sultry weather for a long time, so it is difficult to appear moldy. It can be seen that the precipitation season at the same latitude varies greatly. Therefore, only Huangmei appears in the middle and lower reaches of the Yangtze River in China, starting from the east of Yichang, at 29-33 degrees north latitude, in the southeast of Japan and at the southernmost tip of the Korean Peninsula. In other words, Meiyu is a unique weather and climate phenomenon in East Asia and the middle and lower reaches of the Yangtze River in China.

People living in the middle and lower reaches of the Yangtze River often have the experience that early summer will come as soon as the sunny and rainy spring is over, but soon, the sky will be overcast, rainy and sometimes it will rain heavily. This is what people often say, "Meiyu" is coming.

Meiyu refers to the period from mid-June to mid-July and early summer every year. The middle and lower reaches of the Yangtze River in China refer to the range of 28-34 N east of Yichang, or the Jianghuai Valley. The persistent rainy weather in this narrow area of southern Japan.

How did the name "Meiyu" come from? Initially, it originated from a meteorological term in China. In ancient times, Meiyu was usually called Huang Meiyu. As early as the Han Dynasty, there were many proverbs about Huang Meiyu. In Jin Dynasty, there was a record of "summer rain to the sun, named Huang Meiyu"; Since the Tang and Song Dynasties, there have been many interesting descriptions about Meiyu. Liu Zongyuan, a writer in the Tang Dynasty, once wrote a poem about plum rain: "Plum blossoms are ripe to welcome the rain, and they arrive in Koharu with great splendor. I am deeply worried about the night, and my dreams are broken. " The sea fog connects the South Pole, the river clouds darken the north of Tianjin, and plain clothes is gone now, not because of the dust in the Imperial Capital. Among them, "Shimei welcomes the rain" means that when plums are ripe, they are greeted by the rain after the summer solstice and at three o'clock after the solar term. Meteorological Meiyu refers to a rainy weather that transits from early summer to midsummer.

Zhu He was once called "He Meizi" in the Song Dynasty, which is said to be because he wrote such a famous sentence in the word "Jade Case": "A stream is full of smoke. It rains when the plums are yellow. " In the Song Dynasty, Chen also wrote in "Geng Shi Hua" that "when plum blossoms are ripe in the south of the Yangtze River in May, it rains for ten days, which is called it". In the Ming Dynasty, Xu wrote in Tang Yuzhi's Talk about Hui: "Plum blossoms stand after the awn, and plum blossoms are broken after the awn." The historical "Huang Meiyu" usually refers to the precipitation during the "Mei" Festival. People in the middle and lower reaches of the Yangtze River are used to taking the solar term of "Mangzhong" as the plum festival, which coincides with the mature season of plums, so it is also called "Huangmei".

In addition, due to the high humidity in the air during this period, everything is easy to be damp and moldy, so people gave Meiyu an individual name, called "Moldy Rain". Ming metabolic slaughter line "Five Miscellaneous Torches One Day Department" describes: "Every year in March and April in the south of the Yangtze River, there is bitter rain, and everything is moldy, commonly known as plum rain, and it is also covered when it is green and yellow." From Xu Huai to the north, it is always dry in spring and summer. At the turn of June and July, the rain is endless and things begin to rot. "Li Shizhen, an outstanding medical scientist in Ming Dynasty, pointed out more clearly in Compendium of Materia Medica:" Plum rain or mildew rain, when it comes to clothes and things, all produce black mold. "

It can be seen that the appellation of "plum rain" or "mildew rain" has a long history, and it began to spread in China, at least dating back to 1000 years ago.

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 mid-latitude area is flat, and frequent short-wave activities provide cold air conditions for 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 the Jianghuai basin, often accompanied by southwest vortex and shear line, and the mesoscale system on Meiyu front is active. It not only maintains the continuous precipitation during the Meiyu period, but also provides abundant water for the rainstorm.

Meiyu front rainstorm is a special weather in a specific area formed by the interaction of circulation systems of different scales. The variability of atmospheric circulation leads to the late start and short duration of meiyu period every year. In some years, the Meiyu front is particularly active, with frequent rainstorms, leading to flooding. In some years, the meiyu front is not obvious, and there are "empty plums", which leads to dry weather. In some years, the Meiyu belt will move northward and then return to the Jianghuai basin to maintain relative stability, which is customarily called "inverted Huangmei". After the meiyu in Jianghuai basin ended, the rain belt moved to North China, and the Jianghuai basin entered high temperature and little rain.

rainband

Although Meiyu is a unique weather and climate in the middle and lower reaches of the Yangtze River, its appearance is not isolated, but closely related to the north-south displacement of a large-scale rain belt.

In the eastern part of China, east of 1 10 E, the main rain belt swings in Nanling Mountains and its south area during the flood season from mid-May to early June. In individual years, although it moved to the north of Nanling in a certain period, it often stayed in the south of 28°-29° N according to a waiting period (every 5 days) or the multi-year average of 10 days. This period is called "rainy season in the south of the Yangtze River" or "pre-flood season in South China".

In the middle and late June, the main rain belt moved northward within the range of 29-33 N (that is, from Yichang, China in the west, to the Yangtze River estuary in the east, and then across the sea to Japan; From China Lake Basin in the south to the south bank of Huaihe River in the north), it is stable and less active. At this time, the area south of Nanling is outside the rain belt, and the rainy weather is over; The middle and lower reaches of the Yangtze River bid farewell to the sunny early summer and ushered in the rainy season. Heavy rain and rainstorm appeared from time to time and lasted until the beginning of July. This is a famous rainy season in the middle and lower reaches of the Yangtze River.

Beginning in mid-July, the rain belt moved northward again, reaching the area north of 33 N, and stopped and wandered in the Yellow River, Huaihe River Basin, North China and Northeast China, causing heavy rainfall again and again, which were called "Huanghuai rainy season" and "North China rainy season" respectively. At this time, the plum rains in the middle and lower reaches of the Yangtze River ended, the sun hung high and entered the hot summer season. This kind of weather will last until the end of August, and then the rain belt will quickly withdraw southward with the gradual activity of cold air. In less than a month, the rain belt will always retreat to the coastal areas of South China. The regular change of this rain belt shows that the plum rain in the middle and lower reaches of the Yangtze River is not an isolated and local weather and climate phenomenon, but an integral part of the main rainy season activities in eastern China, which reflects the stagnation of the main rain belt in the middle and lower reaches of the Yangtze River during its northward movement.

Main circulation situation

The subtropical high extends westward and jumps northward, controlling South China, and the whole East Asian circulation is adjusted from spring to summer. The rain belt jumped northward at the same time, the flood season in South China ended, the plum rain in Jianghuai began, the Indian monsoon broke out, the subtropical westerly jet jumped from northern India to the northern plateau, and the anticyclone axis of 100hPa jumped to 34 N. The plum rain began to coincide with the establishment of a stable and sustained southwest monsoon in this area.

1. Upper level: A strong warm and high pressure (South Asia High) is maintained in the Jianghuai area.

2. Intermediate layer (500 hectopascals):

① Subtropical zone: the subtropical high in the western Pacific is distributed in a belt shape, and its ridge line extends from southern Japan to southern China, slightly trending northeast-southwest, and the position of the ridge line of120 E is stable at about 22 N.

② Mid-latitude area: Two stable shallow troughs were established in Balkhash Lake and the east coast of East Asia (between Hetao and North Korea).

③ High latitude area: There are three types of high resistance in active area: (50-70 N) single resistance type, double resistance type and triple resistance type.

(3) Low level: The Jianghuai shear line is located at 850 hectopascals or 700 hectopascals, and there is a low-level southwest wind jet almost parallel to it in the south, and sometimes the southwest vortex moves eastward on the shear line.

Grounding:

① On the ground map, there is a stationary front stagnation in Jianghuai Basin. If the westerly wind at 500 hPa has a weak trough moving eastward, there will often be southwest vortex moving eastward along the shear line at low altitude, which will make the stationary front fluctuate on the ground and produce Jianghuai cyclone.

② When the mid-latitude westerly wind has a strong trough moving eastward, the stationary frontal wave can develop into a perfect frontal cyclone.

Meiyu front: The stationary front in Meiyu period is called Meiyu front. It is the convergence line of summer monsoon airflow and polar air mass or metamorphic polar continental air mass, and it has intertropical convergence zone property. Features: the horizontal temperature gradient on both sides of the front is small and the humidity gradient is large.