Heavier summer downpours in U.K. with climate change: study

A state-of-the-art climate model used in recent research led by the Met Office in the United Kingdom in collaboration with Newcastle University provides the first evidence that hourly summer rainfall rates could increase and extreme summer rainfall may become more frequent in the U.K. as a result of climate change.

While summers are expected to become drier overall by 2100, intense rainfall indicative of serious flash flooding could become several times more frequent, notes a statement issued this week by the Met Office, the U.K.’s National Weather Service.

Published in Nature Climate Change, research findings represent the first step towards building a more complete picture of how U.K. rainfall may change as the climate warms, the statement adds.

Working at coarse resolutions, climate models have been able to accurately simulate winter rainfall, suggesting generally wetter winters with the potential for higher daily rainfall rates in the future.

While in winter, it is the daily or multi-day rainfall totals that are important – because there tends to be steady, long-lasting periods of rain from large-scale weather systems – it is not the same in summer. In summer, the Met Office explains that hourly rainfall rates are more important since rain tends to fall in short, but intense, bursts.

Climate models have so far lacked the resolution to accurately simulate the smaller-scale convective storms that cause this type of rain, the statement notes.

“Until now, climate models haven’t been able to simulate how extreme hourly rainfall might change in future,” says Lizzie Kendon, lead author of the research at the Met Office. “The very high resolution model used in this study allows us to examine these changes for the first time,” Kendon adds.

The research used a climate model with a higher resolution than ever used before to examine future rainfall change – 1.5 km grid boxes instead of the usual 12 km or larger.

Kendon reports that findings show “heavier summer downpours in the future, with almost five times more events exceeding 28 mm in one hour in the future than in the current climate – changes we might expect theoretically as the world warms.” That said, it is necessary 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,” she says.

“We need to understand about possible changes to summer and winter rainfall so we can make informed decisions about how to manage these very different flooding risks in the future,” Professor Hayley Fowler, a member of Newcastle University’s School of Civil Engineering and Geosciences, says in the statement.

“The changes we have found are consistent with increases we would expect in extreme rainfall with increasing temperatures and will mean more flash floods,” Fowler continues.

It took about nine months for the Met Office supercomputer to run simulations of the southern half of the U.K. The simulations looked at two 13-year periods, one based on current climate and one based on expected climate around 2100.

“This model gives a realistic representation of hourly rainfall, allowing us to make future projections with some confidence,” the statement notes. “The next steps are to see if these changes are consistent with observed trends in summer rainfall extremes and changes projected by climate models in other parts of the world,” says Fowler.

University research efforts will continue over the next five years, jointly with the Met Office and other leading international scientists in the European Research Council.