The Urban Influence on Extreme Temperatures in the UK Climate Projections (UKCP18)

Increasing summer temperatures in a warming climate will increase the exposure of the UK population to heat-stress and associated heat-related mortality. Urban inhabitants are particularly at risk because urban areas modify the local climate by altering the surface energy balance, which often makes them warmer than surrounding rural areas. Understanding how urban temperature extremes will change is essential to inform policy makers and help health agencies and urban planners to mitigate future climate risk. However, it is also crucial to understand how the representation of urban and rural processes in climate models influences projected temperature extremes. The latest UK Climate Projections (UKCP18) include an ensemble of convection-permitting model (CPM) simulations which provide credible climate information at the city-scale, the first of their kind for national climate scenarios. Using a newly developed urban signal extraction technique, we quantify the urban influence on present-day (1981-2000) and future (2061-2080) temperature extremes compared to the coarser resolution regional climate model (RCM) simulations. We find that the urban influence in these models is markedly different, with significant (up to 4C) overestimation of the night-time urban heat island in the RCM, while the CPM agrees much better with observations. This improvement is driven by the improved land-surface representation and more sophisticated two-tile urban scheme (MORUSES) employed in the CPM, which distinguishes street canyons and roofs. In future, a strong intensification of the night-time and weakening of the daytime urban heat island effect in the RCM is not seen in the CPM. The results indicate that the CPM provides more reliable urban temperature projections, but that future changes in soil moisture play in important role in the magnitude of the daytime urban influence.