At least 33 people are dead and 8 missing in Zhengzhou, China, after a July 20 extreme rainfall event of nearly unimaginable intensity. Zhengzhou, a megacity of more than 10 million – and the world’s biggest manufacturing base for iPhones and a major hub for food production and heavy industry – recorded an astonishing 644.6 mm (25.38 inches) of rain in the 24 hours ending at 21Z July 20. This is literally more than a year’s worth of rain: Its average annual precipitation (1981-2010 climatology) is only 640.9 mm (25.24 inches).

The disaster follows on the heels of the extreme rainfall event that killed more than 200 people in Germany and Belgium last week. Both disasters have preliminary damage estimates in excess of $10 billion.

https://twitter.com/RitaBai/status/1417473365200179203

According to meteorologist Minghao Zhou, a Ph.D. student at SUNY Albany, the city’s maximum one-hour rainfall rate between 8 and 9Z July 20 (4-5 p.m. local time) was a staggering 201.9 mm/hr (7.95 inches/hr). This amount shattered China’s national record for highest one-hour rainfall of 168.3 mm/hr (6.63 inches/hr) for all 2,418 national weather stations in mainland China, previously set at Maoming, Guangdong.

Heavy rains continued into Wednesday in Zhengzhou, with the city recording a total of 787.9 mm (31.02 inches) in the 72 hours ending at 12Z July 21. Fortunately, the latest run of the GFS model calls for no additional heavy rains in the city in the coming week.

Meteorology of the event

According to Zhou’s analysis (see Tweet above), much of the moisture for this record extreme rainfall event was supplied by a large-scale monsoon circulation off the coast of China, which was also feeding moisture to two typhoons: Typhoon Cempaka, a category 1 storm that was making landfall southwest of Hong Kong, and Typhoon In-fa, a category 2 storm that was headed westwards toward the Chinese coast, north of Taiwan. The monsoon moisture, boosted by the circulations of the two typhoons, fed into an upper-level trough of low pressure passing over northern China’s Henan Province, generating intense rainfall over Zhengzhou. The rainfall was likely increased as a result of air approaching Zhengzhou that travelled uphill into the foothills of the Tai-hang Mountains, causing extra cooling and condensation as the moist air rose upwards (upslope enhancement). A region of nearly stationary thunderstorms set up, leading to the astounding rainfall totals.

Henan Province’s worst flood catastrophe: Typhoon Nina of 1975

Henan Province’s deadliest flood disaster occurred in August 1975, when the meandering remnant of Typhoon Nina dropped 1060 mm (41.73 inches) of rain at Linzhuang in 24 hours, and 1606 mm (63.23 inches) in 72 hours, leading to a domino chain effect of 62 dam failures. The resulting catastrophic floods killed at least 26,000 people, with some estimates as high as 240,000 killed. The magnitude of the disaster was revealed only in the mid-1990s. The maximum rain rate observed during Typhoon Nina was 198.5 mm/hr at Linzhou, Henan, which is less than the 201.9 mm/hr at Zhengzhou on July 20, 2021.

Extreme rains in China expected to increase in warming climate

As the planet’s oceans and atmosphere warm more as a result of increased amounts of heat-trapping gases like carbon dioxide, more water vapor is entering the lower atmosphere, increasing the odds of extreme rainfall events.

Studies using high-resolution global climate models have found that both average and extreme precipitation from China’s summer monsoon rains will increase during the 21st century (see this 2011 paper in Climate Dynamics, and the 2015 book, The Monsoons and Climate Change: Observations and Modeling). The authors of the book, Hirokazu Endo and Akio Kitoh, concluded: “State-of-the-art climate models project that both the amount and intensity of Asian summer monsoon rainfall are likely to increase under global warming, and that the rates of increase will be higher than those in other monsoon regions.”

Authors of an August 2020 study published in the Bulletin of the American Meteorological Society, “Each 0.5°C of Warming Increases Annual Flood Losses in China by More than US$60 Billion,” found that annual average flood losses in China between 1984 and 2018 were $19.2 billion (2015 U.S. dollars), which was 0.5% of China’s GDP. Annual flood losses increased to $25.3 billion annually during the period from 2006 to 2018. The study predicted that each additional 0.5 degree Celsius of global warming will increase China flood losses by $60 billion per year. Losses of this magnitude will pose a significant challenge to the Chinese economy.

In 2020, Earth’s most expensive weather disaster was the summer monsoon flooding in China, which caused $35 billion in damage. According to statistics from EM-DAT, the international disaster database, that total ranks as the third-most expensive non-U.S. weather disaster since accurate records began in 1990, behind 1998 flooding in China ($48 billion 2021 dollars) and 2011 flooding in Thailand ($47 billion 2021 dollars).

July 23 update: According to Steve Bowen of insurance broker Aon, this week’s flood in Zhengzhou will cost at least $10.1 billion; losses elsewhere in the surrounding province will push the damage tally higher. China experienced $1.35 billion in flood damage in June, and another billion-dollar Chinese flood disaster may well occur early next week, when Typhoon In-fa is predicted to move very slowly near the megacity of Ningbo (population 8 million); In-fa is likely to dump over 20 inches of rain along the coast near Ningbo. The nearby city of Shanghai (population 26 million) will also be at risk of damaging rainfall from In-fa; the 12Z Thursday run of the HWRF model predicted Shanghai would receive over 20 inches of rain from In-fa through Tuesday.

Thanks go to Minghao Zhou, @Minghao_Zhou,  for providing much of the material for this post. Bob Henson also contributed. All dollar figures in given are in U.S. dollars.

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