Extreme summer rainfall may become more frequent in the UK as climate change progresses.

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New findings from the Met Office and Newcastle University, which came out of the government-funded CONVEX research project, and are based on state-of-the-art climate modelling, provide evidence that hourly summer rainfall rates are set to increase.

Met Office experts said that, while summers are expected to become drier overall by the end of the century, intense rainfall could become much more frequent, leading to flash flooding.

Last year, the Met Office released statistics showing that UK weather is already becoming wetter. Figures released in the wake of 2012’s wash-out harvest, which severely impacted on crop yield and quality, leading the UK to become a net importer of wheat for the first time in over a decade, revealed that rainfall is increasing, but arriving in more intense bursts.

Of the UK's five wettest years on record, four have occurred since 2000, and 30-year long-term averages show an increase in annual rainfall of about 5 percent from 1961-1990 to 1981-2010. However, as a result of anthropogenic climate change, rain is falling in more concentrated downpours, making it harder for ground and surface waters to absorb with and increasing the likelihood of flooding.

The latest findings from the Met Office and Newcastle University's research were published in Nature Climate Change. They represent the first step towards building a more complete picture of how UK rainfall may change as our climate warms.

Dr Lizzie Kendon, lead author of the research at the Met Office, commented, "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.

"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. 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.

New detailed predictions took supercomputer 9 months to make

In winter it is the daily or multi-day rainfall totals that are important, because the UK is then affected by steady, long-lasting periods of rain from large scale weather systems - similar to those that caused the recent winter floods. Climate models have long been able to predict this kind of rainfall and current suggestions are that the UK will experience generally wetter winters with the potential for higher daily rainfall rates in the future.

In summer, however, it is the hourly rates that are more important as rain tends to fall in short but intense bursts - as seen during the 'Toon Flood' in Newcastle in 2012. Until now though, climate models have lacked the precision to accurately simulate the smaller-scale convective storms which cause this type of rain.

The method is so new, and so intensive, that it took the Met Office supercomputer - one of the most powerful in the world - about nine months to run the simulations, and even then only the Southern part of the UK could be studied. The simulations looked at two 13-year periods, one based on current climate and one based on expected climate around 2100.

Newcastle University Professor Hayley Fowler added, "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. The changes we have found are consistent with increases we would expect in extreme rainfall with increasing temperatures and will mean more flash floods.

"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. We will be looking at this over the next five years, jointly with the Met Office and other leading international scientists in the European Research Council funded INTENSE project."