Causes and manifestations of rainy season in the middle and lower reaches of the Yangtze River

Meiyu, also known as Huangmeitian, refers to a cloudy and rainy natural 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. Because plum rains occurred in the mature period of plums in the south of the Yangtze River, people in China call this climate phenomenon "plum rains", and this period is also called "plum rains 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 in Central China, South China, Taiwan Province Province and other places began to be dominated by the Pacific subtropical high, and officially entered the summer heat.

Reason:

Every year from late May to early June, 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 June, 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.

Performance:

Meiyu is a unique weather and climate phenomenon in the middle and lower reaches of the Yangtze River in early summer. It is the result of the stagnation of the Yangtze River basin during the northward movement of the main rain belt in eastern China. Meiyu is over and midsummer is coming. The change of this season every year is roughly the same as the movement of the rain belt with the season, forming a certain climate regularity. However, the annual plum rains are not completely consistent, and there are great interannual changes. Meteorologically, the beginning and ending time of plum rain are called "entering plum" (or "standing plum") and "leaving plum" (or "breaking plum") respectively. In the middle and lower reaches of the Yangtze River in China, plum blossoms in mid-June and in early July every year, lasting for more than 20 days. But for each specific year, the beginning and end of meiyu and the intensity of meiyu are very different. As a result, plum rains are obvious in some years, not obvious in some years, and even empty plums appear. For example, the rainy season in 1954 was particularly long and lasted for more than two months, which led to a rare waterlogging year in the middle and lower reaches of the Yangtze River. However, the rainy season of 1958 only lasted for two or three days, and there was a rare dry year in history.

Normal plum rain

The normal Meiyu in the middle and lower reaches of the Yangtze River began in mid-June and ended in mid-July, which coincided with the two solar terms of "ear seed" and "summer solstice". The meiyu period is about 20-30 days, and the rainfall is between 200-400 mm. Before and after "Little Summer", the main rainfall belt moves northward to the Yellow River and Huaihe River basins, and then to Shandong and North China. The Yangtze River basin began to change from rainy and humid weather to sunny and hot summer. According to statistics, this normal plum rain accounts for about half of the total.

Early plum rain

In some years, the rainy season starts very early and suddenly comes at the end of May and the beginning of June. Meteorologically, the plum rain that began before "Mangzi" is generally called "early plum rain". The early plum rains will bring some abnormal phenomena. For example, since the cold air from north to south is still very frequent in the atmosphere near the ground at the beginning of the plum rain, the temperature is still relatively low and even a little chilly after the rainy day, which is the agricultural proverb: "You should freeze for three times after eating the Dragon Boat Festival brown". At the same time, there is no obvious damp phenomenon. Farmers in some areas in the middle and lower reaches of the Yangtze River call Huang Meiyu, where the temperature is relatively low, "cold water Huangmei". In the future, with the extension of rainy time and the strengthening of warm and humid air, the temperature will gradually increase, the humidity will continue to increase, and the inherent characteristics of Meiyu will become more and more obvious. The chance of early plum rain is roughly once in ten years. This kind of early plum rain often presents two situations. One is to start early and end late, even in late July. The rainy season lasts for forty or fifty days, and some years last for two months. The other is to start early and finish early. By the end of June, the middle and lower reaches of the Yangtze River entered midsummer. Due to the early arrival of midsummer, droughts of different degrees often occur in the middle and lower reaches of the Yangtze River.

Chi Meiyu

Contrary to early plum rain, late plum rain is usually called late plum rain in meteorology. There are more opportunities for late meiyu than early meiyu. Because the solar term is already late, at the beginning of the late Meiyu, once the warm and humid air flows northward, its power is very strong, and the solar radiation is also relatively strong. When the air is heated, it is prone to strong convection, so there are often thunderstorms and showers in the later period of Meiyu. People also call this kind of Huang Meiyu "Huangmei". The duration of the late Meiyu is generally not long, with an average of only about half a month. However, the rainfall of this plum rain is sometimes quite concentrated.

Ultra-long plum rain

During the period of 1954, a once-in-a-century flood occurred in the Jianghuai Valley of China, which was caused by the unusually long meiyu. This year, there were many spring rains in the middle and lower reaches of the Yangtze River in the second half of May before the beginning of Meiyu. Meiyu came very early and began in early June. The weather has been rainy and rainy, with heavy rain and heavy rain from time to time, which lasted for a long time until the beginning of August. When the rainy weather ended and turned to midsummer weather, it was near "beginning of autumn". This year, the whole meiyu period lasted for two months, and with the spring rain in May, it reached more than two and a half months. After entering the "light summer heat" and "great summer heat", the middle and lower reaches of the Yangtze River should have been sunny and hot, but it has been gloomy and difficult to see the sun. From time to time, pouring rain poured down to the ground, floods rolled in, and "cold" hit people in many areas. In this year, the rainfall in the middle and lower reaches of the Yangtze River from May to July is generally 800- 1000 mm, which is close to the annual rainfall in normal years in this area. The rainfall in some areas is as high as 1500-2000mm, which is equivalent to the rainfall in the same area for one and a half years, leading to flooding. China has a vast territory, and local floods often occur. Some may be caused by typhoon rainstorm, and some may be caused by several consecutive rainstorms brought by other weather systems, but their duration is not long, the flood recedes quickly, and the influence range is relatively small. It is extremely rare that the rainfall time like 1954 reaches more than two months, resulting in a basin-wide flood in the Yangtze River basin. This rare flood is usually associated with abnormal plum rains. Like the flood in 1998, it was also caused by the extremely long plum rain.

"Short plum" and "empty plum"

Contrary to the exceptionally long plum rains, in some years, plum rains are very inconspicuous. Like a passer-by in a hurry. After ten days in the middle and lower reaches of the Yangtze River, I hurried north. Moreover, there is not much rain during this period, and it is rare to have one or two heavy rains. This situation is called "short plum". What's more, there has been no continuous rainy weather in the Yangtze River basin since the early summer of some years. Most days are sunny, warm as spring, and cool in the morning and evening, so the weather of "Huangmei season is dry and loose" appears. Moldy clothes, which often happens in rainy season, almost never happen. As soon as the cool weather is over. Then it became midsummer. This year is called "empty plum". The probability of "short regret" and "empty plum" appears, with an average of l-2 times in ten years. In the years of "short plum" and "empty plum", summer drought often occurs, and some years can also cause serious drought.

Rice yellow plum

In some years, Huangmei days in the middle and lower reaches of the Yangtze River seem to have passed, the weather turned fine and the temperature rose, showing the characteristics of midsummer. However, a few days later, hot and humid thunderstorms and showers reappeared and remained for quite some time. This situation is like Huangmeitian retrogression, returning to the middle and lower reaches of the Yangtze River, so it is called "Huangmeitian". There is thunder in summer, and Huangmei is upside down. This is a weather proverb widely circulated in the middle and lower reaches of the Yangtze River. It means that after the meiyu has passed, if there is thunder in the "light summer heat", the meiyu will be reversed. There is some truth in this statement. Because after the meiyu, the weather in the middle and lower reaches of the Yangtze River is usually more and more stable, and thunderstorm is a symbol of weather instability. Besides, in the "little summer", cold air usually no longer affects the Yangtze River basin, and the occurrence of thunderstorms is often related to the southward movement of a small cold air in the north, which is conducive to the re-establishment of rain belts in the middle and lower reaches of the Yangtze River. Of course, "pour Huangmei" does not necessarily appear after the thunder in summer. Generally speaking, the duration of "pouring Huangmei" is very short, ranging from about one week to ten days and a half months. However, during the "Huangmei" period, due to many thunderstorms and showers, the rainfall is often quite concentrated, which needs attention. Because "inverted yellow plum" is a kind of plum rain, it usually turns into sunny and hot weather after it ends. From the various plum rains described above, we can see that Huang Meiyu, which is usually considered to be similar, is actually diverse, and the differences between them are sometimes quite large. In terms of "entering Mei", the earliest is May 26th, and the latest is July 9th. The earliest date of "plum blossom" is June 16, and the latest date is August 2, with a difference of one and a half months. The longest year of plum rains lasts for more than two months, which can cause rare floods, while the short year has only a few days, and some even have "empty plums", which brings serious drought. It can be seen that plum rain is a complex weather and climate phenomenon, which is far from being as simple as "entering the plum blossom" and "leaving the plum blossom" as stipulated in the lunar calendar. Compared with normal plum rain, early plum, late plum, extra-long plum rain, empty plum and severe yellow plum all belong to abnormal plum rain.

Edit the Meiyu weather process in Jianghuai Basin.

How is the plum rain formed? To answer this question, it is necessary to find out how the rain belt stagnation in the middle and lower reaches of the Yangtze River is caused. Therefore, it is necessary to understand the weather process in Meiyu period from the atmospheric circulation situation in the upper and lower layers. The middle and lower reaches of the Yangtze River are located in the middle latitude of the eastern part of Eurasia. On the one hand, it is affected by the cold air from the cold zone to the south, and on the other hand, it is affected by 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, the meteorological community calls this front "Meiyu Front".