Observational evidence of urban heat island intensification during heatwaves in European cities

Heatwaves (HWs) are extreme weather conditions characterized by persistent high temperatures with considerable impacts on society in terms of mortality, thermal stress and energy demand of the population. One of the most interesting aspects of HWs concerns the interaction with the phenomenon of urban heat island (UHI). The UHI is the tendency of urbanized areas to have warmer temperatures than the surrounding rural areas, mainly due to the thermal properties of materials forming urban environment and the heat produced by human activities. Some studies analyzed the behavior of UHI during periods of extreme heat, showing an amplification of the gradient of temperature between urban and rural areas in HW conditions, but the results are often limited to case studies with a single HW and/or a specific city. Other papers dealt with the same topic by examining events on various cities using outputs of global models, but with resolution insufficient to include in detail urban-scale processes and therefore to take into account specific properties of the cities investigated. The approach of this work consisted in providing observational evidence and extending the aforementioned results, studying the effect of HWs on UHI in about ten European cities with different characteristics (geography, topography, urban planning) through the analysis of daily maximum/minimum temperatures data measured by meteorological stations for the summers of period 2006-2019. In particular, the intensity of UHI was assessed through the computation of a Composite UHI Index (UHII), defined as the difference between averaged urban and non-urban values. The different behavior of UHII during HWs compared to "normal" summer days (NO) in selected European cities was investigated, detecting an intensification of index values regarding periods of extreme heat for the majority of examined locations. More specifically, the analysis of temporal evolution of UHII was conducted, revealing an average increase of this index during the occurrence of HW events. Moreover, a correlation between UHI index and maximum temperature anomalies was examined, and HW days appeared to exhibit a larger percentage of positive UHII with respect to NO days, showing also higher absolute values. This work provides an indication of how European urban areas respond to severe hot periods and could be useful to validate numerical model simulations for more detailed analysis, for example regarding mitigation strategies. Finally, the emergence of some outliers, namely cities whose UHI manifested a different reaction to HWs, may deserve dedicated studies in the future.