What kind of weather does "Shi Mei welcomes the real rain, and it's late spring" mean?
-Talking about Meiyu
"It rains at home in Huangmei season, and frogs are everywhere in the grass pond; If you don't come at midnight, knock off the chess pieces and die. " Zhao Shixiu's poem Inviting Guests in Song Dynasty brought us into a strange rainy night in the south of the Yangtze River in early summer.
Plums in the south of China are ripe, and it often rains continuously, sometimes the sun is not seen in most months. People are used to calling it rainy weather. Yao Bian in Nanliang, China, called it "plum ripe, rainy", and because it was wet and rainy during this period, clothes and utensils were prone to mildew, so it was called "mildew rain". Meiyu is a unique climate phenomenon in East Asia, and it is a long continuous rainy period from the middle and lower reaches of the Yangtze River and the Yellow Sea to southern Japan in early summer.
Liu Zongyuan, an astronomer in the Tang Dynasty, said in Meiyu that "it rains in Shi Mei, and it is late spring", which tells us roughly when Meiyu began. According to modern climate statistics, Meiyu began around mid-June. "Huang Meiyu is broken at three o'clock" refers to the time when Meiyu ends, usually in early July. Every year, the start date of plum rain is called entering plum or main plum, and the end date is called plum or broken plum. The time of entering the sea and plum blossoms varies with the climate and from year to year. In the middle and lower reaches of the Yangtze River in China, the average plum blossom date in the 1950s was June 10, July 12, and the plum rain period was 33 days. In 1960s, the average plum blossom date was June 17, the average plum blossom date was July 6, and the plum rain period was 20 days. Compared with the 1950s, plum blossoms bloom late and early.
The average total precipitation during the annual Meiyu period is about 300 mm. Climatologists in China divide the intensity of Meiyu into three grades according to the precipitation and the number of precipitation days during Meiyu every year. A year with rainy days and heavy precipitation is called Mei Feng, and the precipitation during the rainy season in Mei Feng can sometimes reach more than 400 mm, accounting for 1/3 to 1/2 of the total annual precipitation. 1954 The meiyu period in the middle and lower reaches of the Yangtze River lasted for two months from the beginning of June to the beginning of August, and the precipitation reached about 1000 mm, causing a once-in-a-century flood in the Jianghuai basin. The date of precipitation and the year with little precipitation are called withered plums. In a few years, the rain belt quickly jumped over the Yangtze River and Huaihe River from South China and entered the Yellow River Basin. There is no plum rain in the middle and lower reaches of the Yangtze River, or there is only a few days of precipitation, with little precipitation. It's called Kongmei this year.
Meiyu is a unique climatic phenomenon in the seasonal variation of East Asian atmospheric circulation at the turn of spring and summer. Meiyu on the edge of subtropical high in the northwest Pacific. In late spring and early summer, the cold air in the north, which affects East Asia, is like a spent force, and it can no longer sweep the south of the Yangtze River like winter. The subtropical high in the northwest Pacific began to extend westward and rise northward. Warm and humid air transported by southeast wind or east wind gathers in the middle and lower reaches of the Yangtze River along the subtropical high south or west, confronting the cold air in the north. Cold and warm air meet to form a Meiyu front, which extends from the middle and lower reaches of the Yangtze River to southern Japan, forming cloudy, rainy, foggy and thunderstorm weather. Light rain, moderate rain, heavy rain, heavy rain and heavy rain appear alternately. In the Meiyu front belt from the East China Sea and the Yellow Sea to the South China Sea of Japan, the visibility of the sea surface is extremely low, and frontal cyclones often appear, which increases the wind power on the sea surface.
As long as the Meiyu front can remain stable, the Meiyu period will last. When the subtropical high rises northward again and the ridge line moves to 25 degrees north latitude, warm and humid air flows through the Jianghuai area, and the cold air in the north also begins to retreat northward, ending the Meiyu period. When cold and warm air is mixed again in the Yellow River basin, the rainy season in the north begins.
Normal Meiyu precipitation is beneficial to the growth of crops and the normal life of people and animals; But if the precipitation is too long or there is heavy rain, it is easy to cause floods. The high temperature and humidity in rainy season are easy to cause a large number of molds, which is harmful to human and animal health. The sea fog and strong wind near the Meiyu front on the sea surface are also extremely unfavorable to maritime shipping and fishery production.