Britain braced for more Boscastle-style summer floods

Summer flash floods caused by sudden, heavy downpours will be several times more frequent in southern Britain by the end of the century, climate scientists have predicted.

A warmer climate is likely to result in drier summers overall, but it will also generate four or five times as many flash floods in the summer months because of a significant increase in the frequency of extreme rainfall over relatively short periods, a Met Office study has found.

Climate researchers have generated the first hour-by-hour computer model of the British summer weather down to a scale of 1.5km, said Lizzie Kendon, the lead author of the study. Previous models were too imprecise to detect the kind of localised summer downpours that caused the 2004 flood in Boscastle, Cornwall, and the “Toon flood” in Newcastle in 2012.

“Until now, climate models haven’t been able to simulate how extreme hourly rainfall might change in future. The very high resolution model used in this study allows us to examine these changes for the first time,” Dr Kendon said.

The study used the Met Office supercomputer at is headquarters in Exeter to run the climate model for southern Britain to simulate two 13-year periods. One represented the current climate as it was between 1996 and 2009 and another depicting it as it is likely to be by 2100 as a result of global warming.

“It shows heavier summer downpours in the future, with almost five times more events exceeding 28mm in one hour in the future than in the current climate – changes we might expect theoretically as the world warms,” Dr Kendon said. “However, we need to be careful as the result is only based on one model. So we need to wait for other centres to run similarly detailed simulations to see whether their results support these findings.”

For the current climate, the Met Office computer predicted about six summer downpours greater than 30mm per hour, while at the end of the century this rose to about 22, Dr Kendon said.

Winter rainfall is generally the result of large weather systems moving over the country, usually from the Atlantic. Summer rainfall, however, can be caused by rising convective air currents caused by the heat of the sun, which can produce intense, localised downpours that can be easily missed by existing weather models.

In the Boscastle flood, the rainfall was estimated to be 90mm an hour over an eight-hour period while the “Toon flood” in Newcastle resulted from 50 mm of rain falling over 90 minutes, Dr Kendon added.