A Regional Climate Model Ensemble Analysis of Urban Heat Island Intensity During a Heat Wave in Paris

With the increasing resolution of regional climate models (RCM), the representation of urban areas becomes ever more critical. However, even in a kilometer-scale resolution, many RCMs still neglect urban-specific processes and represent cities only by their specific physical parameters (e.g., albedo, roughness). The URB-RCC flagship pilot study under the framework of the CORDEX initiative aims to promote and analyze RCMs with explicit urban treatment. In the first phase, the study assesses the performance of high-resolution RCMs that include urban schemes in large cities under various weather conditions. This analysis focuses on a selected heat wave episode in August 2020 and analyzes how several RCMs capture it in the city of Paris. The urban heat island (UHI) is represented by the temperature contrast (UHI index) between several locations within and outside the Paris city center. The UHI index is calculated from station measurements and compared to the index calculated from RCM simulations in 3km resolution. Preliminary results show that while RCMs generally capture day-to-day variability and general weather patterns, there is quite a large uncertainty in the daily cycle. Some models agree well with the observations for both daily minima and maxima. On the other hand, a large set of the model ensemble tends to overestimate the UHI index derived from daily maxima. At the same time, they underestimate the UHI index in daily temperature minima. The coordinated nature of the multi-model ensemble experiment allows to address the uncertainty coming from different model formulations.