Heavy rain is rain with strong precipitation intensity. It is raining cats and dogs. According to China's meteorological regulations, rain with a precipitation of more than 50 mm in 24 hours is called "rainstorm". According to its precipitation intensity, it is divided into three grades, that is, the 24-hour precipitation of 50 ~ 99.9 mm is called "rainstorm", and the rainfall below 100 ~ 200 mm is called "heavy rainstorm"; More than 200 mm is called "rainstorm". Due to the different characteristics of precipitation and topography, the standards of rainstorm and flood are different in different places. Rainstorm is a kind of disastrous weather, which often causes floods and serious soil erosion, leading to major economic losses, such as engineering accidents, dike breaches and crop flooding. Especially for some low-lying and closed terrain areas, rainwater can not be discharged quickly, resulting in farmland water accumulation and soil water supersaturation, which will cause more disasters.
The world's largest rainstorm occurred in Reunion Island in the South Indian Ocean, with a 24-hour precipitation of 1.870 mm, and the largest rainstorm in China occurred in Xinliao, Taiwan Province Province, with a 24-hour precipitation of 1.672 mm, all caused by tropical cyclone activities. China is one of the countries with the most rainstorms in the world, and almost all provinces (cities, districts) have rainstorms. Mainly concentrated in the second half of the year. The regional distribution of rainstorm days is obviously more in the south and less in the north; There are many coastal areas and few inland areas; There are many windward slopes and few leeward slopes. The annual rainstorm days in mountainous areas of Taiwan Province Province are more than 16 days, Dongxing, Yangjiang, Shanwei and some coastal areas of South China are more than 10 days, and the average in northwest China is less than 1 day.
Formation of rainstorm
The formation process of rainstorm is quite complicated. Generally speaking, sufficient and continuous water vapor, strong and lasting upward movement of airflow and instability of atmospheric structure are the main physical conditions for heavy rain. The favorable combination of various scales of weather system and underlying surface, especially topography, can produce heavy rain. The main weather systems that cause large-scale rainstorm in China are fronts, cyclones, shear lines, eddies, troughs, typhoons, easterly waves and intertropical convergence zone. In addition, thermal thunderstorms will also cause short-term and small-scale heavy rains in arid and semi-arid areas.
Rainstorms often fall from cumulonimbus clouds. The condition for the formation of cumulonimbus clouds is that the atmosphere contains enough water vapor, and there is a strong upward movement to transport the water vapor rapidly upward. The water droplets in the cloud are constantly affected by the upward movement until the upward airflow can't support them, and they drop sharply to the ground. Cumulonimbus clouds are usually very large. Cumulonimbus clouds are precipitation units in rainstorm areas. Although the horizontal range of each unit is only 1 ~ 20km, they are arranged in a rain belt with a width of 100 ~ 200km. Cumulonimbus clouds are like mountains, like mountains When they develop strongly, they extend from the height of 0.4 ~ 1 km above the ground to the height of 10 km. The higher the altitude, the lower the temperature, often reaching MINUS 10 degrees Celsius or even lower, and the water droplets at the top of the clouds will freeze. On the ground, people can see wisps of white bands at the top of the cloud with naked eyes, which is the result of ice crystals and snowflakes dancing in the sky. It is rainy summer on the ground, but snowy winter in the sky.
In China, the water vapor of rainstorm comes from the South China Sea or the Bay of Bengal. Second, from the East China Sea or the Yellow Sea. Sometimes, in a rainstorm, water vapor comes from the east and south, or mainly from the south in the early stage and from the east in the later stage. The spread of "southeast wind, rain ancestor" in the Central Plains is an objective reflection of the precipitation law.
Like running water, the motion of the atmosphere often produces fluctuations or eddies. When two air streams from different directions or different temperatures and humidity meet, fluctuations or vortices will occur. The largest is several meters in Qian Qian, and the smallest is only a few kilometers. In these fluctuating areas, with the operation of air flow, upward movement often occurs, and horizontal water vapor quickly concentrates in the same area, forming a rainstorm center.
In addition, topography also has an influence on the formation of heavy rain and rainfall. For example, due to the existence of mountains, the airflow is forced to rise on the windward slope, which increases the vertical movement and the rainstorm; On the leeward slope of the mountain range, the airflow sinks and the rainfall is greatly reduced. On some leeward slopes, the rainfall is only110 of windward slopes. In early August, at 1963, there was a wet air flow from the South China Sea to North China, which happened to intersect with Taihang Mountain. Due to the uplift of the mountain range, there is a rare rainstorm in the east of Taihang Mountain. The narrow tube action of the valley can also strengthen the rainstorm. 1In August, 975, a torrential rain occurred in Henan Province. Its center, Zhuang Lin, is surrounded by mountains in the south, north and west, and gradually forms a trumpet-shaped terrain in the east. Due to this terrain, the airflow rises faster and the rainfall suddenly increases. From August 5 to 7, the precipitation reached 1600 mm or more, and Zhumadian, which is located in the plain area, is less than 40 kilometers southeast of Zhuanglin.
In addition, when the rainstorm occurs, the lower air is generally warm and wet, while the upper air is dry and cold, which makes the atmosphere in an extremely unstable state, which is conducive to the release of energy in the atmosphere and the full development of cumulonimbus clouds.
Seasonal and geographical distribution
China is a rainy country. Except for some provinces and regions in the northwest, it is almost always heavy rain. Winter rainstorm is limited to the coastal areas of South China, with frequent rainstorm in South China from April to June. From June to July, persistent rainstorms often occur in the middle and lower reaches of the Yangtze River, with long duration, wide range and large amount of rainstorms. July-August is the main rainstorm season in northern provinces, and the rainstorm intensity is very strong. From August to 65438+ 10, the rain belt gradually withdrew southward. After summer and autumn, the typhoon rainstorm in the East China Sea and the South China Sea is very active, and the point rainfall of typhoon rainstorm is often very large.
China has a monsoon climate. From late spring to midsummer, the cold air in the north fought and retreated. Cold and warm air frequently meet, forming a series of heavy rains. The location of main rain belts in Chinese mainland also moves from south to north with the seasons. South China (Guangdong, Guangxi, Fujian and Taiwan Province) is the region with the most rainstorm in China. April to September is the rainy season. The second half of June to the first half of July is usually the rainy season in the Yangtze River basin. In late July, the rain belt moved to the north of the Yellow River. After September, the winter wind was established and the rain belt retreated south. Under the influence of summer monsoon, the distribution of rainstorm days and rainfall in China decreases from southeast to northwest inland, with mountains more than plains. Moreover, the southeast coastal islands and coastal areas have the largest number of rainstorm days, and the farther northwest, the less. In the northwest plateau, there is less than one day of heavy rain on average every year. Mountain slopes in the southeast or east, such as Taihang Mountain, Dabie Mountain, Nanling Mountain and Wuyishan Mountain, are the centers of rainstorm days in these areas.
Of course, there will be anomalies in some years. 198 1 year, rare rainstorm occurred in parts of northwest China. Rainy areas sometimes get dry.