Extreme weather event review: how the rainfall in Henan compares

Aerial photo taken on July 26, 2021 shows the rescue scene outside the First Affiliated Hospital of Xinxiang Medical University in Weihui, Xinxiang City of central China's Henan Province. (Xinhua/Hao Yuan)

From July 17 to 22, torrential rainstorms hit central China’s Henan Province. In its capital city, Zhengzhou, the equivalent of half its annual rainfall poured down in 24 hours between July 20 and 21. Following a record-breaking maximum rainfall of 201.9 millimeters in an hour, 1/6 of weather stations in the province renewed their daily rainfall records.

By August 2, the devastating flooding it triggered had claimed 302 lives and affected more than 14.53 million people in 150 county-level regions. According to official data, over 1.09 million hectares of crops were damaged, and over 30,600 houses have collapsed across the province.

The rainfall is deemed a rare weather event in climate history in terms of both intensity and influence. A review of the precipitation process and the characteristics of the rainfall is made for a clear understanding of the situation.

High extremity

Figure 1 (Source: National Meteorological Center of CMA)

During the process of the heavy rainfall, 20 national weather stations renewed their daily rainfall records, which account for one sixth of the total number of national weather stations in Henan. Figure 1 shows those national weather stations and a comparison of the new record with the old record. In the meantime, 32 weather stations in the province recorded more than 500 mm of precipitation within a day, and 5 recorded more than 600 mm.

Notably, the rainfall recorded by the station of Zhengzhou totaled 624.1 mm within 24 hours from 8:00 am, July 20, to 8:00 am, July 21, an amount almost equal to its annual average total.

Figure 2 (Source: National Meteorological Center of CMA, World Meteorological Organization)

On July 14 and 15, extreme rainfall affected Germany and led to flooding that caused more than 50 deaths. The incident was deemed to go beyond the description of the German language by German Chancellor Angela Merkel. According to the German Meteorological Service (DWD), about 100 to 150 mm of precipitation occurred in 24 hours. Figure 2 shows that the DWD weather station of Wipperfuerth-Gardeweg recorded the highest precipitation of 162 mm in a single day, which was followed by Cologne-Stammheim with 160 mm, Kall-Sistig with 152 mm and Wuppertal-Buchenhofen with 151 mm.

Figure 2 also lists the top four new daily rainfall records registered by the 20 national weather stations mentioned above. To make a horizontal comparison, they are paralleled with the rain intensity classification of the China Meteorological Administration (CMA) and the highest daily rainfall records of the four DWD weather stations.

According to the rain intensity classification of CMA, rainfall recorded by the four DWD weather stations are within the range of very torrential rain (when the precipitation rate is between 100 mm and 249.9 mm per 24-hour). Yet the four renewed rainfall records in Henan all exceed the level of extreme torrential rain (when the precipitation rate is more than 250 mm per 24-hour), and are more than two times heavier than the rainfall recorded by the DWD weather stations.

High intensity in short terms

Figure 3 (Source: National Meteorological Center of CMA, World Meteorological Organization)

1 - hour precipitation is also universally used to measure rainfall intensity. According to Wikipedia, the following categories are applied: 

• Light rain - when the precipitation rate is < 2.5 mm per hour 

• Moderate rain - when the precipitation rate is between 2.5 mm - 7.6 mm per hour 

• Heavy rain - when the precipitation rate is > 7.6 mm per hour 

• Violent rain - when the precipitation rate is > 50 mm per hour

During the rain event in Henan, 10 weather stations in Zhengzhou City, 5 in Xinxiang City and 1 in Hebi City recorded a maximum one-hour rainfall of more than 100 mm. Among them, the weather station of Zhengzhou ranked first with a staggering 201.9 mm of rainfall from 16:00 to 17:00, July 20, which was about three times higher than the violent rain intensity. It also broke the Chinese mainland’s record one of hourly precipitation - hourly rainfall of 198.5 mm per hour in Linzhuang, Henan on August 5, 1975, as shown in Figure 3.

Large accumulation

Figure 4 (Source: National Meteorological Center of CMA)

Figure 4 shows that from July 17 to 22, rainfall in central and northern Henan totaled 200 ~ 400 mm. In the cities of Zhengzhou, Xuchang, Pingdingshan, Xinxiang, Jiaozuo, Hebi and Anyang, accumulated rainfall in some areas (30% ~ 60% of area) reached 500 ~ 700 mm. In cities of Xinxiang, Zhengzhou and Hebi, accumulated rainfall in parts of areas ( < 30% of area) reached 800 ~ 965.5 mm, 800 ~ 993.1mm and 800 ~ 1122.6mm respectively. The maximum accumulated rainfall, 1122.6mm, occurred in the Qibin District in Hebi City.

According to data from CMA, the averaged rainfall in Henan Province reached 222.9 mm, and the averaged rainfall in Zhengzhou City reached 532.3 mm. Meanwhile, the accumulated rainfall at 10 national weather stations exceeded the annual averaged rainfall in the area. As shown in Figure 4, rainfall recorded in the weather station of Zhengzhzou totaled 820.5 mm within 5 days, which is more than 127% of its annual average of 641 mm.

Long duration

The heavy rains lasted six consecutive days from July 17 to 22. The most intensified rainfall occurred between July 19 to 21. Central and northern Henan was hit by very torrential rains for 4 consecutive days, while the cities of Zhengzhou and Xinxiang saw extreme torrential rains two days in a row.

Vertical comparison

Figure 5 (Source: World Meteorological Organization, National Meteorological Center of CMA, Xinhua, Reuters, NASA Earth Observatory, TIME )

Since 1994, the World Meteorological Organization has been publishing the WMO Statement on the Status of the Global Climate on an annual basis, which records major weather and climate events around the world. A few extreme rainfall events occurred in this century are listed for reference and comparison with the rainfall in Henan.