Met Office Research Shows Increasing Extreme Rainfall Frequency
EXTREME RAINFALL EVENTS could be four times as frequent by 2080 compared to 1980s.
For the first time, a high resolution version of the Met Office Unified Model can capture the detail of extreme rainfall events and the future risk of intense rainfall that can cause flash flooding.
Extreme Rainfall Increase
Running 100-year climate projections, the occurrence of local weather extremes year-by-year are captured. The model accurately assesses how extreme downpours that can lead to flash flooding will change, for example when rain intensity exceeds 20mm/hour and these can be used for planning, such as surface water drainage and flood risk.
The research, published in Nature Communications, found that extreme rainfall events in the UK exceeding 20mm/hr could be 4 times as frequent by 2080 compared to the 1980s. Previous models predicted an increase of around two and a half times.
The model assumes greenhouse gas emissions keep accelerating. This is not inevitable, but a plausible scenario if we do not curb our emissions.
An example of an intense rainfall event with 20mm/hr is London in July 2021, when 40mm of rain fell over three hours at Kew Gardens, flooding the underground and other infrastructure.
The modelled increases were found to differ across the UK. Future extreme rainfall events could be almost 10 times more frequent in Northwest Scotland in 2080 compared to the 1980s, whilst in the south of the UK the value is closer to 3 times more frequent.
Projected Future Climate
As the atmosphere warms it can hold more moisture, at a rate of 7% more moisture for every degree of warming. On a simple level, this explains why in many regions of the world projections show an increase in rain as a consequence of human induced climate change.
This new study has shown that during extreme rainfall events in the UK the intensity of downpours could increase by 5-15% per °C of regional warming. This change is uneven across the UK, as the map below shows. The greatest change is indicated by the blues in the Northwest of the UK, with the red and orange colours showing less change.
Met Office Climate Scientist, Lizzie Kendon, said: “Being able to look at our projected future climate in such detail has unlocked an incredible amount of information and has shown how expected increases in intense rainfall events will actually manifest at local scale and for the coming years. Having this level of detail is crucial to ensure that we’re prepared for the possible extremes of the future.”
Although there is an increase in the number of extreme rainfall events in the UK as we go through the century, as shown by the white line in the graph above, the number of events per year remains erratic.
Professor Kendon explains: “This could mean we see clusters of record breaking intense rainfall events, followed by a period when no records are broken. Despite the underlying trend, these pauses in the intensification of local rainfall extremes can last a surprisingly long time – even multiple decades.
“Another concern is apparent sudden transitions to a much higher frequency of extreme events illustrated by the model output. This would suggest a sudden increase in the numbers of extreme rainfall events, outside of the experience of recent decades. If this scenario did happen it could lead to impacts where infrastructure was unprepared for such a change in our weather.”
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