Projections of Increased Intensity of Summer Rainfall over the UK from Very High Resolution Regional Climate Model Simulations

Abstract:

We have performed the first climate change experiments with a very high resolution (1.5 km grid spacing) regional climate model over a region of the UK and compared these to results for a coarser resolution climate model (12 km). This model is typically used for weather forecasting in the UK. Although observations show increases in heavy rainfall at daily timescales in many regions, how changes will manifest themselves on sub-daily timescales remains highly uncertain. A reanalysis-driven simulation shows realistic hourly rainfall characteristics, including extremes, unlike results for coarser resolution climate models. This gives us confidence in the very high resolution model's ability to project future changes at sub-daily scales.

The 1.5 km model shows increases in hourly rainfall intensities in winter, consistent with projections from the coarser 12 km resolution model and from previous studies at the daily timescale. However, the 1.5 km model also shows future intensification of short-duration heavy rainfall in summer with significantly more events exceeding the high thresholds set by UK flood forecasters as indicative of serious flash flooding. We conclude that accurate representation of the local storm dynamics is an essential requirement for predicting changes to convective extremes; when included we find for the model here that summer downpours intensify with warming.

We further explore some of the mechanisms causing the changes, including the relationships to temperature and humidity through mechanisms such as the Clausius-cCapeyron relationship, and larger scale circulation changes.