Do future projections in windstorm severity depend on climate model resolution?

Extratropical windstorms can lead to considerable impacts in the UK. Anthropogenic climate change is likely to influence the frequency of extreme windstorms, although there is considerable uncertainty in the simulated response by climate models due to internal variability and differences in climate model formulations. A further source of uncertainty arises from model inadequacies in their representation of key processes that drive extreme winds and their future changes due to their spatial resolution. To assess this uncertainty, we study the UKCP18 climate projections, generated by the UK Met Office, which comprises a 12-member perturbed parameter ensemble from a 60km global climate model (GCM), downscaled by a 12km regional climate model (RCM) over Europe and a 2.2km convection-permitting model (CPM) over the UK. We study a control (1981-2000) and future (2061-2080, RCP8.5) period over a domain covering the UK and Ireland. Storm intensity is quantified using the storm severity index (SSI) which incorporates surface wind gusts, while we also assess wind speeds at 850hPa. The CPM and RCM project a future increase in the frequency of extreme windstorms and both are very similar in their representation of the SSI as well as extreme winds at 850hPa. In contrast, the GCM underestimates the severity of windstorms due to poorer representation of cold sector winds driven by cold conveyor belts and sting jets. Furthermore, the GCM projects little change in extreme windstorms in the future period resulting in a large underestimation in the future change compared to the CPM and RCM. Overall, little difference is seen between the CPM and RCM, with CPM added value largely restricted to fine-scale detail in the spatial structure of storms. This may in part be due to the small domain size of the CPM, and the relatively high resolution of the RCM. Results for the GCM may bring into question the usefulness of coarser resolution models, such as those used in CMIP5 and CMIP6, for studying future changes in windstorm severity.