Expert interpretation: Why is heavy rainfall “stuck” in Beijing, Tianjin and Hebei?

Affected by the residual circulation of Typhoon Dusurui, precipitation has gradually spread in North China and the Huanghuai area since July 29. From the night of the 29th to the 30th, heavy rain occurred in Beijing, with an average rainfall of 89.6 mm in the city, and a total of 251 stations with rainfall exceeding 100 mm. The Central Meteorological Observatory continued to issue a red warning for heavy rain and a yellow warning for severe convective weather at 10:00 on the 30th. As of the afternoon of the 30th, the local rainfall in Hebei has reached the level of extremely heavy rain.

What are the characteristics of this round of precipitation? How long will the rainfall last? On these questions, Science and Technology Daily interviewed Fang Chong, chief forecaster of the Central Meteorological Observatory.

Fang Chong introduced that this round of precipitation is mainly stable precipitation, with long duration, large accumulated rainfall, and high disaster risk. The residual circulation of "Dusurui" entered Henan on the night of the 29th and continued to move north. On the evening of the 29th, the two high pressures in the east and west of Beijing-Tianjin-Hebei were connected, forming a "high pressure dam" in northern North China, blocking the northward movement of "Dusurui".

Therefore, the stay time of "Dusurui" in the area from North China to the Huanghuai region will increase, resulting in a long duration of rainfall and heavy accumulated rainfall. Taking Beijing as an example, intermittent heavy rainfall will continue until the morning of August 2, and the duration of rainfall may exceed 70 hours.

Fang Chong further explained that Dusurui itself carries a large amount of water vapor and has sufficient supply. The subtropical high pressure on its east side makes the pressure gradient between the two very strong. The easterly and southeasterly winds in the Beijing-Tianjin-Hebei region have significantly increased, and the water vapor is transported northward unimpeded. At the same time, Typhoon Kanu in the western Pacific has become a typhoon, and the strong southeasterly wind has continuously transported the water vapor near Kanu to the North China Plain over a long distance. The two water vapor channels bring unusual water vapor conditions, resulting in extremely high precipitation.

"In addition, the Taihang Mountains to the west of Beijing-Tianjin-Hebei are in direct conflict with the easterly and southeasterly winds carrying the water system, and the Yanshan Mountains to the north also have an angle of intersection with the water vapor channel. The water vapor is blocked and lifted by the terrain, and concentrated in front of the mountains to form extremely heavy precipitation. The obstructed process, abundant water vapor and the blessing of the terrain have caused extreme precipitation in the Beijing-Tianjin-Hebei region." Fang Chong said.

Meteorological monitoring shows that from 9:00 on the 29th to 11:00 on the 30th, the maximum precipitation in the Beijing-Tianjin-Hebei region occurred in Liangjiazhuang, Zhaozhuang, Lincheng, Xingtai, Hebei, with a total of 583.8 mm. At 17:00 on the 29th, the maximum hourly rainfall intensity occurred in Juancheng, Shandong, with a total of 86 mm. From 20:00 on the 29th to 17:00 on the 30th, the average rainfall in Beijing was 89.6 mm, and the average rainfall in the urban area was 102.4 mm. The maximum rainfall occurred in Sanliushui, Fangshan, with a cumulative rainfall of 296.8 mm. From 8:00 on the 28th to 7:00 on the 30th, the precipitation at 313 stations in Tianjin reached heavy rain, 115 stations had heavy rain, and 7 stations had extremely heavy rain.

Experts warn that although the precipitation this time does not reach the intensity of severe convective weather, it must not be taken lightly. The meteorological risks of urban waterlogging, farmland waterlogging, and floods in small and medium rivers are high, and it is recommended to take defensive measures in advance. The public needs to pay attention to the upcoming forecasts and warnings, and take relevant defensive measures in a timely manner according to the changes in rainfall and flood conditions, avoid going to dangerous areas such as mountainous areas, rivers, and geological disaster risk areas. Vehicles should avoid sections with high risks of traffic and meteorological disasters, drive carefully, and avoid wading. At the same time, this round of precipitation is large and strong, and oversaturated soil can easily lead to slope instability, and the risk of geological disasters is relatively high. It is recommended to do a good job in defense and rescue of disasters such as mountain torrents, landslides, and mud-rock flows.

Source: Science and Technology Daily