Meteorologists and hydrologists explained in detail how the flood with the highest water level in history was formed.

Rainfall intensity is beyond history?

There are five Dongting Lake waters in the province.

The rainfall in Changsha is the largest in the whole province.

In the view of meteorological and hydrological departments, this flood is a huge threat, and the "biggest accomplice" should be the rainstorm that "wandered" in Hunan in June.

According to the data of the provincial climate center, the cumulative average precipitation in Hunan in June was 407. 1mm, which was 9 1.6% higher than the average in the same period of previous years, 5.9% higher than the 384.4mm in June of 1954 and 20. 5% higher than the 337mm in June of 1998. "The rainfall of 407. 1 mm means that 86 billion cubic meters of water fell on the land of 265.438+0.10.8 million square kilometers in Hunan, which is equivalent to the water volume of five Dongting Lakes (when the water level of Chenglingji Station is 33.5 meters)." Ning Qianjin, deputy director of the Hunan Provincial Hydrology Bureau, figuratively said.

Changsha is one of the worst areas in this round of heavy rain. According to the monitoring data of Changsha Hydrological Bureau, the average rainfall in June was 558 mm, which was 65,438 0.38% more than the historical average (234.4 mm), equivalent to 56% of the rainfall in the whole flood season. "In previous years, the flood season in Changsha was generally from the end of April to the beginning of July, but this year's rainfall was concentrated in June." Li Jingwei, deputy director of Changsha Hydrology Bureau, said.

Especially since June 22, the province has suffered the strongest rainfall this year. The total amount, range, intensity and duration of heavy rainfall are rare in history. According to the provincial defense index, during the period from June 22nd to July 2nd 1 1 day, the province's average accumulated rainfall was 270 mm, including 452.0 mm in Changsha, and the maximum accumulated rainfall was 830 mm at Liuyang Yuquan Station. Among the two sets of data, Changsha ranks first, and the average rainfall is almost twice that of the whole province, setting a historical record of rainfall intensity in Hunan.

How to understand the average rainfall of 452 mm? Li Jingwei explained that this is equivalent to Changsha 1. 1.8 1.9 million square kilometers of land, with an average depth of 0.452 meters. Converted into total precipitation, it can reach 5.34 billion cubic meters. "5.34 billion cubic meters of water fell into Changsha, just like people drinking water too fast and choking easily, which is the direct cause of such a fierce flood."

Why is the precipitation so intense?

Interaction between Northern Cold Vortex and Subtropical High

Cold and warm air meet frequently.

According to the data of the Municipal Meteorological Observatory, from June 29th to July 1, the average precipitation in the whole city was 265.9mm, which exceeded the maximum continuous rainfall in June 1998 and reached the first place in history. 62% of the stations in the city have accumulated rainfall of more than 200mm in three days, and 10% of the stations have accumulated rainfall of more than 300mm. "The main rainfall period formed by this extraordinary flood is from 8: 00 on June 30 to 8: 00 on July 2, and the rainfall in two days is 230.5 mm, which is equivalent to the total rainfall in June." Chen Chunming, section chief of Water Science Department of Changsha Hydrological Bureau, added.

What caused this year's heavy rain with large rainfall, wide range and high intensity?

"The continuous heavy precipitation in June was the result of the strong convergence of the two stable air currents over Hunan. On the one hand, the intensity of the western Pacific subtropical high is relatively strong, and its location is southwest than normal, which makes the warm and humid airflow from the South China Sea and the western Pacific continuously transported over Hunan along the periphery of the subtropical high, providing a rich source of water vapor for precipitation in Hunan; On the other hand, the middle and high latitudes in Europe and Asia maintain a stable blocking high in Lake Baikal, guiding a small stream of cold air to the south, and finally being blocked by the northwest side of the subtropical high and meeting with the southwest warm and humid airflow. " Liao, director of the Hunan Climate Center, said.

Chen Yong, chief forecaster of Changsha Meteorological Observatory, told reporters that Changsha is just on the edge of the subtropical high in the western Pacific Ocean and on the strong water vapor conveyor belt. The interaction between the cold vortex in the north and the subtropical high caused the frequent intersection of cold and warm air over Hunan, which led to the occurrence of heavy rain.

Why is the flood so fierce?

"One River and Six Rivers" Superimposed Flood Peak

The natural "dam" delayed the extraction of water.

At present, the water levels of Dongting Lake and its tributaries, Xiangjiang River, Zishui and Shui Yuan, continue to decline. However, during the previous heavy rainfall, nine hydrological control stations in the trunk and tributary of Xiangjiang River Basin exceeded the historical water level.

Why is the flood so fierce this year? Ning Qianjin explained that compared with previous years, this year's floods can be described as "in one step": heavy rains and flash floods frequently occur in hilly areas, heavy rainfall raises the water level of rivers, lakes and reservoirs, and the main streams of the four major rivers in Hunan and Dongting Lake area are "in an emergency", further lengthening the flood control front and greatly increasing the difficulty of defense. To forecast flood, it is necessary to comprehensively consider precipitation and runoff after precipitation, and combine multiple factors such as weather forecast, hydrogeological conditions and river flood situation. This is also why the arrival time of the first flood peak at Changsha Station of Xiangjiang River was pushed back: at 6 o'clock on July 2, 12, 39.438+09 meters; It was 39.47 meters at 2 July 19; 0: 00 on July 3rd12,39.438+0m ...

"The water level of Xiangjiang Changsha Station has continuously set a record high. On the one hand, the rainfall in Changsha exceeds history, and the rain belt swings back and forth twice in Hunan, which leads to the high water level of "one river and six waters" in Xiangjiang River; Second, Shuangpai, Ouyang Hai, Shuifumiao and other reservoirs in the upper reaches of Xiangjiang River opened floodgates and brought a lot of water; Third, Guangxi in the upper reaches of Xiangjiang River also transported a large amount of traffic to Hunan, reaching 5300 cubic meters per second; Fourth, there were heavy rains in the whole province, and major floods occurred in Minjiang River and Zijiang River, which caused the water level in Dongting Lake to be too high. The' bottom-up pressure' delayed the retreat time of Xiangjiang River. " Li Jingwei said.

In addition, Liuyang River, Weishui River and Laodao River, the main tributaries of Changsha section of Xiangjiang River, all exceeded the highest water level in history. Once in 50 years, it is equivalent to building a "dam" in Xiangjiang River to input a large amount of traffic. "The superposition of flood peaks and the confluence of floods form mutual support with the main stream of Xiangjiang River, delaying the retreat." Chen Chunming analyzed, "After the flood peak of 1998, it took four days for the water level to return to the warning water level. It may take longer this year. "

What is the weather trend in July?

Sunny, hot and hot weather is coming.

The risk of geological disasters is high.

On July 3, Changsha was covered with blue sky and white clouds, and the fine weather made citizens go out of their homes. According to the forecast of Changsha meteorological department, there will be showers in the middle and upper reaches of Xiangjiang River today and tomorrow. Because of the small rainfall, it is beneficial to the water level decline in Xiangjiang River Basin. There was a period of sunny and hot weather in the middle of last year. It is estimated that the rainfall this month is about 150.0mm (average in July 14 1.4mm), and the total monthly rainfall days are about 12 days (average in July 10 days).

However, meteorologists have warned that some rivers are still running at high water level, the soil moisture is saturated, and many rivers still exceed the warning water level, so the risk of geological disasters is high. While doing a good job in flood control and disaster relief, the public should pay attention to heatstroke prevention and cooling in sunny and hot weather, especially to geological disasters such as dam break, landslide and collapse after heavy precipitation.

Since June 22nd, the provincial and municipal hydrological departments have issued flood warning 107 times, including 8 red warnings, 32 orange warnings, 53 yellow warnings and 4 blue warnings 14 times, which provides strong information support for relevant units and the public to do a good job in risk avoidance and disaster relief along high flood level rivers.

The focus of flood control in the province is currently concentrated in the middle and lower reaches of Xiangjiang River and Dongting Lake area. Changsha Hydrological Bureau predicts that the second flood peak of Xiangjiang River is expected to pass through Changsha today. When the flood peak crosses the border, the water level of Xiangjiang River will not rise at the original water level, but the water level of Changsha section of Xiangjiang River will recede slightly. The next step is to focus on strengthening flood control and dike inspection along the main stream and tributaries of Xiangjiang River to prevent geological disasters in time.

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