Extreme summer rainfall could become more frequent in the UK due to climate change, according to new research led by Newcastle University and the UK’s Met Office.
The new study used a state-of-the-art climate model providing the first evidence that hourly summer rainfall rates could increase. It was so computer intensive that only the southern half of the UK could be studied and even then it took the Met Office supercomputer – one of the most powerful in the world – about nine months to run the simulations.
While summers are expected to become drier overall by 2100, intense rainfall indicative of serious flash flooding could become several times more frequent. The results from the study, published in Nature Climate Change, are the first step towards building a more complete picture of how UK rainfall may change as our climate warms.
Prof Hayley Fowler, from Newcastle University’s School of Engineering and Geosciences, said that it is important to understand possible changes to summer and winter rainfall so informed decisions can be made about how to manage very different flooding risks in the future.
"The changes we have found are consistent with increases we would expect in extreme rainfall with increasing temperatures and will mean more flash floods,” he said.
Dr Lizzie Kendon, lead author of the research at the Met Office, said that 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 allowed scientists to examine these changes for the first time.
“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.”
As the atmosphere warms it can hold more moisture and this is expected to intensify rainfall. However, research is needed to understand what this might mean for extremes and how this might affect the UK.
In winter, the daily or multi-day rainfall totals are important, because large scale weather systems produce steady, long-lasting periods of rain– similar to those seen during the winter floods of 2013/14.
Climate models, which generally work at coarse resolutions, have been able to accurately simulate winter rainfall and have suggested generally wetter winters with the potential for higher daily rainfall rates in the future.
In summer, however, the hourly rates are more important as rain tends to fall in short but intense bursts – as seen during the Boscastle flooding of 2004 and the ‘Toon Flood’, otherwise known as Thunder Thursday, in Newcastle in 2012. Climate models have so far lacked the resolution to accurately simulate the smaller-scale convective storms which cause this type of rain.
To deal with this issue, the study used a climate model with a higher resolution than ever used before to examine future rainfall change – using 1.5km grid boxes instead of the usual 12km or larger – the same as the Met Office weather forecast model. This model gives a realistic representation of hourly rainfall, allowing future projections to be made with some confidence.
The simulations looked at two 13-year periods, one based on current climate and one based on expected climate around 2100.
The researchers’ next steps are to see if the changes are consistent with observed trends in summer rainfall extremes and changes projected by climate models in other parts of the world. Newcastle University’s research will continue over the next five years, jointly with the Met Office and other leading international scientists in the European Research Council funded INTENSE project.