21 July 2021
The extraordinary, almost unbelievable, rainfall in Henan Province yesterday
Posted by Dave Petley
The floods in Zhengzhou in Henan, China on 21 July 2021. Image via the Hong Kong Standard.
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);
And unbelievably:
- 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.
This is of course only one of a series of extreme weather events in the last week, with severe flooding also being seen in western Europe, New Zealand, Oman and elsewhere.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Terrifying footage on TV news of subway trains flooded neck-deep, hapless pedestrians washed off street down subway maelstroms, rescued by ‘skin of teeth’…
A few places in UK have had current heat-wave punctuated by localised thunderstorms with ‘golf-ball hail’…
Tangential, a surprising number of ‘eco-fridges’ seem unable to heat-pump their larder zone beyond a dozen degrees below ambient. So, now, much spoilage ensues…
25 1/2 inches of continuous rain. That is a world record and a disaster.
The previous 24 h rainfall records sounds surprisingly low. Even much smaller countries in temperate Europe have higher records, and China has seen a lot tropical cyclones. Surely some of them must have had a higher 24 h rainfall.
[Sorry, my post was not clear on one vs 24 hour rainfall. I have put this right. D.]
Its phenominal rainfall but it seems some issues with records. This report from Global time reports “The maximum precipitation in Zhengzhou reached 201.9 millimeters per hour, breaking the previous record of 198.5 millimeters in 1975, according to the national meteorological observatory.”
https://www.globaltimes.cn/page/202107/1229173.shtml
There is ampe observational bias that is glossed over in the search for new extremes!
I watched the videos coming in last night and found this very distressing. It seems to me that these are likely to be some of the most disastrous non coastal floods ever?
In Oman, the catchment area of various wadi systems is huge, so you don’t need that much rain, with run-off over bare rock, to soon fill wadis. Cyclone Gonu (2007) did major damage, with significant loss of life, but I wouldn’t class recent rainfall as “an extreme weather event”.
As far as point values of rainfall depth, the recorded events at Henan fall short of some recorded events (for 1h 24h and 72h durations) around China and the world (Refer NOAA https://www.nws.noaa.gov/ohd/hdsc/record_precip/record_precip_world.html).
They are however, still incredible events. Your point about the large inundation area of the intense rainfall is spot on – the large extent of high rainfall means that the total volume of rainfall over the area is immense.
I’m reminded of the 1975 flood disaster that occurred just to the south of this region, which similarly included extreme rainfall and dams breaching through either overtopping or the military blowing them up. Fatalities during that event were in the tens of thousands.
Henan province has form when it comes to disastrous dam collapses. https://en.wikipedia.org/wiki/1975_Banqiao_Dam_failure
The Banqiao Dam and other 61 dams in Henan, China collapsed (or were blown-up by the military) due to rainfall associated with Typhoon Nina in August 1975. This created the third-deadliest flood in history which inundated around 12,000 square kilometres and 30 cities and counties. Estimates for the death-toll range from 26,000 to 240,000. (Records are poor because the events were covered-up at the time – due to their potential to discredit the Cultural Revolution which was still raging at the time.)
A breezy account can also be found here https://youtu.be/iwX30hXw3CQ
… once in a thousand years huh? so, they have records and statistics since 1021 AD? any half-decent or dignified science blog shouldn’t quote such rubbish from China.
https://en.wikipedia.org/wiki/Hyperbole