Why can’t rainstorm forecasts be 100% accurate?

Why are increasingly severe rainstorms difficult to accurately predict?

In recent years, the keyword "heavy rain" has appeared more and more frequently on hot searches and has actually happened in our lives.

From 1961 to 2023, China will experience an average of 38 rainstorms each year. The data shows that the frequency and intensity of extreme precipitation have increased significantly, and short-duration precipitation record-breaking events have become more frequent.

As the torrential rains get heavier, the impacts they cause and the secondary disasters they cause, such as waterlogging, mountain torrents, and mudslides, become more and more obvious, constantly presenting new challenges for the government and the public. What is the trend of torrential rains under the background of climate change? With the continuous advancement of technology, why can't torrential rain forecasts be "100% accurate"?

Extreme precipitation trends

my country is a country with many rainstorms. The rain belt advances from south to north across a vast area, creating different types of rainstorms in different regions - rainstorms in the pre-flood season in South China, rainstorms in the plum rain front in the Yangtze and Huaihe River basins, and rainstorms in the low trough and low vortex in North China... The rainstorms are mainly concentrated in the flood season from May to August, with high intensity, high extreme values, long duration and wide range.

This year, heavy rains continued in South China during the pre-flood season. As of mid-May, precipitation in South China (Fujian, Guangdong, Guangxi, and Hainan) was the highest since 1961. Eleven of the first 12 regional rainstorms this year occurred in the south, significantly more than the 7.4 times in the same period of normal years.

On May 8, Nanning, Guangxi suffered heavy rains, and citizens rode on the streets. Photo by Xinhua News Agency reporter Zhou Hua

At the same time, heavy rain, a stereotyped "specialty" of the South, seems to be becoming more and more common in the North.

Zheng Zhihai, chief forecaster of the National Climate Center, said that in recent years, summer precipitation in Northeast China, North China and other places has been on the higher-than-normal interdecadal background, especially in North China, with seven years of higher precipitation since 2016.

Some of these extreme events are even more impressive. In July 2021, a heavy rainstorm disaster occurred in Henan Province. The hourly rainfall at the Zhengzhou Meteorological Observatory reached 201.9 mm, breaking the historical record of mainland my country. Last summer, due to the residual circulation of Typhoon Dusurui, heavy rains occurred in Beijing, Tianjin, Hebei and other places, and many places were affected.

"Global warming will lead to increased atmospheric moisture content and intensified urban heat island effects, which will increase the frequency and intensity of extreme rainstorms in cities," said Wan Rong, chief engineer of the Wuhan Heavy Rain Research Institute of the China Meteorological Administration. Although warming will reduce the number of tropical cyclones, it will increase their intensity and the rainfall that comes with it.

It can be said that the increased risk of extreme heavy rainfall has evolved from academic research to a reality that we will face for a long time.

Difficult to simulate accurately

Unpredictable rainfall areas and unexpected rainfall amounts are often the main causes of rainstorm disasters. Even with advanced technology today, it is still difficult to accurately predict when and where a rainstorm will fall, which many people find difficult to understand.

There are many and complex factors that affect the occurrence and development of rainstorms, and the ever-changing process and impact are difficult to quantify accurately. Zhou Zhimin, director of the Heavy Rain Numerical Forecasting Laboratory of the Wuhan Heavy Rain Research Institute, who has been engaged in heavy rain research for many years, compares the heavy rain numerical forecasting model process to solving a huge set of equations.

"After various detection data from satellites, radars, etc. are assimilated, they enter the equation group to obtain an initial solution, and then iterate continuously." Zhou Zhimin said that due to the incomplete understanding of the physical process of heavy rain, the equation group cannot accurately describe these natural phenomena and interactions.

On the one hand, the equations cannot fully and accurately reflect the actual process of the occurrence and development of rainstorms, and in this huge equations, the number of unknowns is far greater than the number of equations, and it is impossible to calculate a definite solution. On the other hand, the unknowns brought into this equations, that is, the actual values ​​of the factors affecting the rainstorm, are also difficult to be accurately observed.

"The shapes of raindrops for coastal and plum rain front rainstorms are actually different. There are more small raindrops along the coast, and the diameter of raindrops on the plum rain front is larger, but these differences cannot be seen in the model." Zhou Zhimin said that these details are also the key to affecting the accuracy of sudden and local rainstorm forecasts.

This is a radar deployed at the China Meteorological Administration's Yangtze River Middle Reaches Heavy Rain Monitoring Field Scientific Experimental Base in Xianning, Hubei. Photo by Xinhua News Agency reporter Huang Yao

Zhong Min, chief forecaster at Wuhan Central Meteorological Observatory, who has nearly 20 years of forecasting experience, believes that sudden, localized and extreme rainstorms are still a bottleneck in forecasting. "There is still room for optimization of numerical forecasting models, and insufficient real-time observation data also limits the improvement of short-term forecast lead time," said Zhong Min.

In fact, in the study of rainstorm mechanism and forecast, scientists have been answering the questions of what, why and how to deal with it. "What is the real situation when rainstorm occurs? What are the reasons and factors that play a leading role? How will it develop in the future? We have been conducting research around these three questions," said Wan Rong.

Going deeper into the micro

At the China Meteorological Administration's Yangtze River Middle Reaches Heavy Rain Monitoring Field Scientific Experimental Base in Xianning, Hubei, wind profile radar, laser raindrop spectrometer, ceilometer and other equipment capture the physical parameters of wind at different altitudes, raindrop shape, cloud base height, etc. at real time. The Dahongshan Experimental Base, about 300 kilometers away, has deployed a variety of meteorological observation equipment at altitudes of 211 meters, 515 meters, 985 meters, and 1,050 meters, respectively, in an attempt to draw a clear picture of the formation and evolution of heavy rain.

At present, my country is developing a rainstorm observation system from multiple dimensions, including ground, radar, satellite remote sensing and sounding. According to statistics, there are 76,245 automatic ground stations, 9 meteorological satellites, 252 new generation weather radars, 294 X-band weather radars, 225 wind profiler radars and 120 sounding stations in the national meteorological department.

This is a microwave radiometer deployed at the Dahongshan test base in Suizhou, Hubei. Photo by Xinhua News Agency reporter Huang Yao

The improvement in the accuracy of the content and scope of observation elements has revealed some previous blind spots in cognition.

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

For the world-class problem of heavy rain forecasting, more precise observations and more in-depth mechanism research have always been the difficulties and directions of efforts faced by academia and industry.

"How the environmental field of heavy rain is configured, what the dynamic field and thermal field are like, what the water vapor conditions and terrain characteristics are like, etc., they all influence each other and create uncertainty, so we need to go deep into the microscopic research and analysis." Wan Rong said.

Experts said that in addition to improving the accuracy of forecasts and warnings, coping with rainstorms also requires improving the level of water conservancy and flood control facilities, promoting the construction of resilient cities, and comprehensively enhancing disaster prevention, mitigation, and relief capabilities.

Planning and production

Source: Xinhuanet

Editor: Yang Yaping

Proofread by Xu Lailinlin