What caused record rainfall in Beijing and northern China?

Extreme rain battered Beijing, Tianjin and the province of Hebei in the wake of Typhoon Doksuri in late July, causing widespread flooding and damage in a region the size of Britain.

The storms, which have killed at least 20 people and led to the displacement of hundreds of thousands of residents, were the worst to hit China in over a decade, with Beijing experiencing its heaviest rainfall in 140 years.

How severe was the rainfall?

The amount of rainfall since Saturday has breached many local meteorological records.

A reservoir in Beijing's Changping district logged a precipitation reading of 744.8mm (29.3 inches) between Saturday and Wednesday, the most in the city in over 140 years and far exceeding the previous record of 609mm set in 1891.

Also read | Typhoon Doksuri: Thousands flee their homes as heavy rain lashes Beijing

The persistent downpour prompted Beijing to use a flood storage reservoir for the first time since its establishment 25 years ago to divert floodwater.

In Hebei, one local weather station recorded 1,003mm of rain for a three-day period from Saturday to Monday, an amount normally seen over half a year.

How did the extreme rain happen?

Besides the remnants of Doksuri, warm and humid air-flows and water vapour brought by Typhoon Khanun slowly moving in the Western Pacific created the conditions for the heavy rains, according to Chinese meteorologists.

As the residual circulation of Doksuri's rain clouds headed north, a subtropical and continental high pressure system in the atmosphere also blocked their north and eastward passage, leading to the continuing convergence of water vapour that acted like a dam storing the water, the meteorologists say.

Topographic features in the area also contributed to the weather. As large amounts of vapour gathered in northern China, it was then lifted up by a low-altitude wind, shifting precipitation east of the Taihang mountain range, where the worst-hit areas - including Beijing's Fangshan and Mentougou districts - are located.

Meanwhile, a series of convective clouds gathered over the area, resulting in heavy rainfall for a protracted period of time, further exacerbating the damage and complicating rescue operations.

How damaging was the rain?

In the urban parts of Beijing, hundreds of roads were flooded, forcing parks and tourism spots to shut. Hundreds of flights were either delayed or cancelled at the city's two major airports. Some subway lines and trains were also suspended.

The impact was more pronounced in the city's western suburbs. In Mentougou and Fangshan districts, raging water coursed down roads, sweeping away cars. Villages in mountainous areas were cut off, prompting authorities to deploy helicopters to drop off food, water and emergency supplies.

Hebei's Zhuozhou, a city with more than 600,000 people to the southwest of Beijing, was half-submerged, with about 134,000 residents affected and one-sixth of the city's population evacuated.

Have similar weather events happened in the past?

Rain with intensity seen in the latest event following weakened typhoons is unusual in Beijing and its surroundings. The Chinese capital has observed at least 12 incidences of significant rain brought by typhoons since authorities started keeping records, according to state media.

In 2017 and 2018, Typhoon Haitang and Ampil both dumped over 100mm of rain on Beijing. One of the most significant rain events was caused by Typhoon Wanda in 1956, which unleashed more than 400mm of precipitation in the densely populated city.