What is the main cause of heavy rains in China

On May 8, 2020, the National Climate Center monitored that a weak El Niño event had formed, and that floods were expected to be worse than drought this summer.

On May 8, 2020, the National Climate Center monitored that since November 2019, the equatorial eastern Pacific Ocean has entered and continued to maintain an El Niño state, and the moving average of the NIO index for five consecutive months is .5 and. According to the national criteria for determining El Niño La Niña events, an El Niño event (with weak intensity) has been formalized.

El Niño is a cyclical natural phenomenon that occurs approximately every 7 years. Through the study of global climate, scientists believe that El Niño is not an isolated natural phenomenon, but an aspect of global climate anomalies. In a normal year, the Pacific coast of the west coast of Peru is controlled by a cold ocean current and has a large natural fishing ground.

In the event of a climate anomaly, the cold ocean currents in the eastern Pacific Ocean are replaced by a warm ocean current.

This book is a summary of the comprehensive research on the analysis and design of rainstorm characteristics and the calculation of rainstorm in China by the Chinese water conservancy and hydropower departments since the 50s of the 20th century. The characteristics of the distribution of rainfall on the ground in China are mainly discussed. Briefly describe the principles and calculation precautions of design rainstorm analysis. In this paper, the data basis, analysis methods and main research results of the research on the main factors of rainstorm related to water conservancy and hydropower projects (extreme value of rainstorm amount, statistical characteristics, time-surface depth relationship, time-surface rain pattern, number of rainstorm days and rainstorm water resources, seasonal variation, etc.) are expounded. It brings together a comprehensive chart of national rainstorm research (including more than 100 maps reflecting the distribution or contour of various rainstorm characteristics across the country), the analysis of rainstorm characteristics of representative locations and the characteristics analysis results of representative heavy rainstorms (including more than 80 heavy rainstorms and typical rainstorm isobath maps, etc.); The single research results on heavy rain were introduced, and more than 300 references were listed. The rainstorm division is given, and the regional rainstorm is briefly described.

In order to facilitate retrieval, a brief description of more than 300 heavy rains and floods is also listed in the appendix at the back of the book, which provides basic information and analytical clues for further research in the future.

This book can be used by hydrological analysis and calculation personnel engaged in planning, design and management, scientific research and teaching of water conservancy and hydropower systems, and can also be used as a reference for engineering and technical personnel and scientific research and teaching personnel of civil engineering, architecture, meteorology, geography departments and disciplines related to heavy rain and floods. The formation process of heavy rain is very complex, and from the macroscopic physical conditions, the generation of heavy rain requires a sufficient flow of water vapor, strong and persistent upward movement, and the instability of the atmospheric structure. Large, medium and small weather systems at various scales and a favorable combination of underlying surfaces, especially topography, can produce large rainstorms.

The weather systems that cause widespread heavy rainfall in China mainly include fronts, cyclones, shear lines, low vortices, troughs, typhoons, easterly waves, and intertropical convergence zones. In addition, in some arid and semi-arid regions, thermal thundershowers can also cause short, small areas of extremely heavy rainfall.

Normally, heavy rains fall from cumulonimbus clouds, which are formed by the presence of sufficient water vapour in the atmosphere and strong upward movements. Only in this way can the water vapor be transported upwards quickly, and the water droplets in the cloud will continue to increase due to the influence of the upward motion, until the updraft can no longer support it, and then it will fall sharply to the ground. Cumulonimbus clouds are usually very large, and the clusters of cumulonimbus clouds resemble high mountains, and when they develop strongly, they extend from a height of 1,000 kilometers above the ground to an altitude of more than 10 kilometers.

The higher you go, the cooler it gets, usually minus a dozen degrees Celsius or even lower, and the water droplets above the clouds freeze. The wisps of white ribbon we can see in the sky are caused by the ice crystals and snowflakes flying in the sky.

The process of rainstorm formation is quite complex, generally from the macroscopic physical conditions, the main physical conditions for the production of rainstorm are sufficient water vapor, strong and persistent air flow upward movement and the instability of the atmospheric structure.

The more abundant water vapour is in the atmosphere, the greater the chance of heavy rainfall.

Heavy rain also requires a strong sustained upward movement so that the water vapor in the atmosphere can continue to condense and form raindrops. The southwesterly or southerly airflow from the tropical ocean is the conveyor belt of water vapour, and the southwesterly airflow on the northwest side of the high pressure generally carries sufficient water vapour. When the low-level southwesterly airflow reaches a certain strength (wind speed exceeds 12 m seconds), it is called a low-level jet stream, and when other conditions are favorable, the low-level jet stream can generally form heavy rainfall.

1. Typhoon: Typhoon brings a large amount of water vapor from the ocean, adding to the atmosphere.

Strenuous exercise is prone to heavy rainfall.

2. Cold and warm: the cold air mass and the warm air mass meet to form a cold front.

Rainstorms form in a short period of time.

3. Temperature: In summer, the temperature is high, and the water vapor evaporates vigorously, forming convective movement and producing heavy rain.

Heavy rain: Heavy rain refers to rain with high precipitation intensity, often formed in cumulonimbus clouds. China meteorologically stipulates that 24-hour precipitation.

Heavy rainfall of more than 50 mm.

It's called "heavy rain". According to the intensity of precipitation, it is divided into three grades, that is, the 24-hour precipitation is called "heavy rain" when it is millimeters, and the heavy rain between millimeters is called "extremely heavy rain". However, due to the different precipitation and topographic characteristics in different places, the standards of heavy rain and flooding are also different in different places.

In business practice, heavy rainfall can be divided into local rainstorm, regional rainstorm, large-scale rainstorm, and extra-large rainstorm according to the size of the occurrence and impact range.

The rainfall of 50 mm means that in an hour, the precipitation on a square meter of unit area is 50 mm. Rainfall of more than 50 mm is considered heavy rain.

Light rain, moderate rain, heavy rain, heavy rain, etc., are generally measured in daily rainfall. Among them, light rain refers to daily rainfall of less than 10 mm; Moderate rainfall of 10 mm on a day; Heavy rainfall of 25 millimeters; Heavy rainfall of 50 mm; Heavy rainfall of 100 250 mm; Heavy rainfall is known to have rainfall of more than 250 mm.

Factors influencing precipitation.

1. Topography: more precipitation on the windward slope and less precipitation on the leeward slope; Tall terrain can also prevent water vapor from entering, for example, the reason for Xinjiang's dry climate is not only deep inland, but also because of the high mountains around it.

2. Ocean currents: warm currents have the effect of warming and humidifying the coastal climate; The cold current has a cooling and dehumidifying effect on the coastal climate. For example, Australia's deserts extend over a wide area on the west coast of the continent and are affected by trade winds and the Western Australian cold snap in addition to the subtropical high control.

3. Vegetation and hydrological conditions: In areas with high vegetation coverage and around lakes and marshes and reservoirs, the humidity of the air is relatively high, and the relative precipitation is more.