21 July 2021
The extraordinary, almost unbelievable, rainfall in Henan Province yesterday
There are widespread reports today of the terrible floods that affected Henan Province in China yesterday, triggering floods and landslides. There are some dreadful videos and images of the result of this rainfall, some of which are deeply harrowing, focusing mainly on the city of Zhengzhou. Xinhua is reporting that 25 people have been killed, but rescue operations continue and more heavy rainfall is forecast.
What has not been widely reported is the extraordinary nature of this rainfall event. The long term average annual rainfall in Zhengzhou is 640.8 mm. These are the stats for the current rainfall as at the end of the day yesterday:
- Max. 24 hour rainfall: 552.5 mm (from 20:00 on 19 July to 20:00 20 July);
- Max. 72 hour rainfall: 617.1 mm (from 20:00 on 17 July to 20:00 20 July);
- Max one hour rainfall: 201.9 mm (from 16:00 to 17:00 on 20 July).
This is extraordinary rainfall. To put this in context, there is a yellow weather warning for the UK for this coming weekend because there are fears of localised convective storms that might lead to locations receiving 100 mm of rainfall over the course of a 24 hour period.
As such, statistically, this rainfall in Henan was one in one thousand year event.
The highest one hour rainfall ever recorded is (I believe) 214.8mm in Penghu, Taiwan, but this was associated with a typhoon. To have >200 mm without the local presence of a tropical cyclone is very surprising.
The all time record one hour rainfall in China before yesterday was 168.3 mm, recorded at Maoming in Guangdong on 2 July 2002. Clearly the Henan event has soundly exceeded that value. It is worth noting that this rainfall event extended over a wide area, whereas many large one and 24 hour totals are highly concentrated. This may well also be the highest rainfall intensity ever recorded within a major city.
The cause of the heavy rainfall in Henan was a typhoon located to the east of Taiwan, with a high pressure system located to the north of Taiwan over the Sea of Japan. This fed an atmospheric river of water vapour into central China. This was intensified by another airstream from the south, pushed by a smaller tropical cyclone located off the south coast of China. The warm, moist air encountered the uplands of Henan, generating orographic rainfall in vast quantities.