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

Abstract

<p>Heat waves (HWs) are extreme weather conditions characterized by persistent high temperatures with considerable impacts on society in terms of<br>mortality, thermal stress and energy demand of the population. One of the most interesting aspects of HWs concerns the interaction with the phenomenon<br>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<br>properties of materials forming urban environment and the heat produced by human activities. Some studies analyzed the behavior of UHI during periods of<br>extreme heat, showing an amplification of the gradient of temperature between urban and rural areas in HW conditions, but results are often limited to case<br>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,<br>but with resolution insufficient to include in detail urban-scale processes and therefore to take into account specific properties of the cities investigated. The<br>approach of this work consisted in providing observational evidence and extending the aforementioned results, studying the effect of HWs on UHI in 41 European cities<br>with different characteristics (geography, topography, urban planning) through the analysis of daily maximum / minimum temperatures data measured by<br>meteorological stations for the summers of period 2000-2019. In particular, the intensity of UHI was assessed through the computation of a Composite UHI Index<br>(UHII), defined as the difference between averaged urban and non-urban values. The different behavior of UHII during HWs compared to "normal" summer days<br>(NO) in selected European cities was investigated, detecting an intensification of index values regarding periods of extreme heat for the majority of examined<br>locations. More specifically, the analysis of temporal evolution of UHII was conducted, revealing an average increase of this index during the occurrence of<br>HW events due to higher urban than rural temperatures. This work provides an indication of how European urban areas respond to severe hot periods and could<br>be useful to validate numerical model simulations for more detailed analysis, for example regarding mitigation strategies. Finally, the emergence of some outliers,<br>namely cities whose UHI manifested a different reaction to HWs, may deserve dedicated studies in the future.</p><p> </p>