In recent years, the key word rainstorm appears more and more frequently in hot search, and also in our lives.

From 1961 to 2023, China will have an average of 38 rainstorm processes every year. From the data, the frequency and intensity of extreme precipitation increased significantly, and the short duration precipitation record breaking events tended to be frequent.

The heavier the rainstorm, the more obvious the impact and secondary disasters such as waterlogging, mountain torrents, mud rock flows caused by it, and constantly put forward new countermeasures to the government and the public. What is the trend of rainstorm in the context of climate change? With the continuous progress of technology, why can't the rainstorm forecast achieve "100 hits"?

Extreme precipitation frequency trend

China is a country with heavy rain. The rain belt moves from south to north in a vast area, resulting in different types of rainstorm in different regions - the rainstorm in the pre flood season in South China, the rainstorm in the Meiyu front in the Yangtze Huaihe River basin, the rainstorm in the low trough and vortex in North China... The rainstorm is mainly concentrated in the flood season from May to August, with large intensity, high extreme value, long duration and wide range.

This year, heavy rains continued in the pre flood season in South China. As of the middle of May, the precipitation in South China (Fujian, Guangdong, Guangxi, Hainan) was the highest in the same period since 1961. Eleven of the first 12 regional rainstorm processes occurred this year fell in the south, significantly more than 7.4 in the same period of the year.

At the same time, rainstorm, the stereotyped "specialty" of the south, seems to be more and more common in the north.

Zheng Zhihai, the chief forecaster of the National Climate Center, said that in recent years, the summer precipitation in Northeast China, North China and other places is on the interdecadal background, especially in North China, where there has been more precipitation in seven years since 2016.

Some of these extreme events are more impressive. In July 2021, a catastrophic rainstorm disaster occurred in Henan Province, and the hourly rainfall of Zhengzhou Meteorological Observation Station exceeded the historical extreme value since the record in the mainland of China with 201.9 mm. Last summer, affected by the residual circulation of typhoon "Dusurai", heavy rain occurred in Beijing, Tianjin and Hebei, and many places were affected.

"The increase of atmospheric water content and urban heat island effect caused by global warming will increase the frequency and intensity of extreme rainstorm in cities." Wan Rong, chief engineer of Wuhan Rainstorm Research Institute of China Meteorological Administration, said that although warming will reduce the number of tropical cyclones, it will increase their intensity and subsequent rainfall.

It can be said that the increased risk of extreme heavy rainfall has moved from academic research to the reality we will face in the long run.

Difficult to simulate accurately

The unpredictable falling area and unexpected precipitation are often the important reasons for disasters caused by rainstorm. Today, with advanced technology, torrential rain can not be accurately predicted at what time and where, which makes many people feel difficult to understand.

The factors that affect the occurrence and development of rainstorm are numerous and complex, and it is difficult to accurately quantify the changeable process and impact. Zhou Zhimin, director of the Heavy Rainfall Numerical Prediction Research Office of Wuhan Heavy Rainfall Research Institute, who has been engaged in heavy rainfall research for many years, compared the process of heavy rainfall numerical prediction model to solving a huge equation set.

"After assimilating satellite, radar and other detection data, we enter the equation set to get an initial solution, and then iterate continuously." Zhou Zhimin said that because the current understanding of the physical process of rainstorm is not complete, the equation set cannot accurately describe these natural phenomena and their interactions.

On the one hand, the equation set can not completely and accurately reflect the actual process of rainstorm occurrence and development, and in this huge equation set, the number of unknowns is far greater than the number of equations, so the definite solution cannot be calculated. On the other hand, the unknowns brought into this equation group, namely the actual values of the factors affecting the rainstorm, are also difficult to be accurately observed.

"The raindrop shapes of the coastal and Meiyu front rainstorm are actually different. There are many small raindrops along the coast, and the raindrop diameter of Meiyu front is larger, but these differences cannot be seen in the model." Zhou Zhimin said that these details are also the key to affect the accuracy of sudden and local rainstorm prediction.

Zhong Min, the chief forecaster of Wuhan Central Meteorological Observatory, who has nearly 20 years of forecasting experience, believes that sudden, local and extreme rainstorm is still the bottleneck of forecasting. "There is still room for optimization of the numerical prediction model, and the lack of real-time observation data also limits the improvement of short-term and imminent prediction advance," Zhong Min said.

In fact, scientists have been answering the question of what, why and how to do in the research of rainstorm mechanism and prediction. "What is the real state of the rainstorm? What are the reasons and what factors play a leading role? How will it develop later? We have been conducting research around these three issues," said Wan Rong.

Go deeper into the microcosm

At the field scientific test base for rainstorm monitoring in the middle reaches of the Yangtze River of the China Meteorological Administration in Xianning, Hubei Province, wind profiler radar, laser raindrop spectrometer, cloud height meter and other equipment capture the physical parameters, raindrop shape, cloud base height, etc. of the wind at different altitudes in real time. The Dahongshan experimental base, about 300 kilometers away, is equipped with a variety of meteorological observation equipment at 211 meters, 515 meters, 985 meters, and 1050 meters above sea level, trying to draw a clear picture of the formation and evolution of the rainstorm.

At present, China has carried out rainstorm observation system construction from the ground, radar, satellite remote sensing, sounding and other dimensions. According to statistics, there are 76245 automatic ground stations, 9 meteorological satellites, 252 new generation weather radars, 294 X-band weather radars, 225 wind profile radars and 120 sounding stations in the national meteorological department.

The improvement of the accuracy of the content and scope of observation elements has uncovered some previous cognitive blind spots.

"After more detailed observation data, we found that there is a low-level jet below one kilometer, with a scale of tens to one or two hundred kilometers, and its outlet is usually the center of heavy rainfall," said Wang Xiaokang, a researcher at Wuhan Institute of Heavy Rainfall Research.

For the world-class problem of rainstorm prediction, more accurate observation and more in-depth mechanism research have always been the difficulties and efforts of the academic community and the industry.

"How to configure the environmental field of rainstorm, what is the dynamic field and thermal field, and what are the water vapor conditions and topographic characteristics? They affect each other to produce uncertainty, so we need to go deep into the microscopic study and analysis." Wan Rong said.

Experts said that in addition to improving the accuracy of forecasting and early warning, we need to improve the level of water conservancy and flood control facilities, promote the construction of resilient cities, and comprehensively improve the ability of disaster prevention, mitigation and relief in response to heavy rains.

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