Using a convection permitting model ensemble for projecting future change in precipitation extremes

Abstract:

For the first time internationally a model at a resolution on par with operational weather forecast models has been used for national climate scenarios. As part of the UK Climate Projections (UKCP) project, an ensemble of 12 projections at 2.2km resolution have been carried out over the UK. These were launched in September 2019, with the aim of providing an improved simulation of extreme precipitation and also other high-impact events at local scales for the coming decades. At such high (2.2km) resolution, convection can be represented explicitly ('permitted') without the need for a parameterisation scheme, leading to a much more realistic representation of hourly precipitation characteristics, including extremes. Here we present results looking at future changes in precipitation extremes. This includes new understanding of changes in winter, where future increases in precipitation are substantially greater in the 2.2km projections compared to the coarser resolution driving models. The difference appears to be explained by the better representation of convection in the 2.2km model, and its ability to advect showers triggered over the sea inland. This work reveals an important limitation of national climate scenarios based on traditional coarser resolution climate models. Such scenarios may underestimate future increases in winter precipitation, especially where wintertime convective showers are a key contributor, since the processes important for the advection and further triggering of showers are only well captured in convection-permitting models. This goes beyond the known deficiencies of coarse resolution models in representing short-duration rainfall extremes and has implications for future flood risk and water resource management.