Projected Evolution of the Urban Climate and Heatwaves using an Ensemble of Convection-Permitting Regional Climate Models

With climate change, the challenges associated with extreme heat in cities are becoming ever more pressing. There is a need for accurate climate change projections for cities, and for robust quantification of changes in extreme temperatures under the combined effect of global warming and urban heat effect. The present study takes advantage of an ensemble of existing high-resolution climate simulations for a multi-model analysis of the urban climate and its evolution in the Paris region (France). Seven members, based on different Convection-Permitting Regional Climate Models (CP-RCM), provide 10-year simulations for a common historical period and two future periods with the RCP8.5 greenhouse gas emission scenario. The ensemble was analyzed to evaluate the CP-RCMs depending on the configuration, especially the surface database and land-surface modeling including or not a dedicated urban parameterization (based on a slab approach or an urban canopy model). All urbanized CP-RCMs capture the urban heat island (UHI) and its seasonal variations. A more precise assessment of CP-RCMs according to the type of parameterization remains tricky, and very sensitive to the type and resolution of land cover database. The analysis of projections indicates that all models agree with an increase in heatwave occurrences over Paris region, with greater signal on the natural areas surrounding the city. Most projections suggest a significant mitigation in the daytime UHI at seasonal scale in summer, and a decrease in nighttime UHI during heatwaves. The multi-model approach therefore makes it possible to confirm previous results already obtained by analyses based on a single model.