Evidence of intensified Urban Heat Island during Heatwaves in European cities through observational data

Heatwaves (HWs) are meteorological extreme events on the scale of a few thousand kilometres characterized by persistent extremely hot temperature conditions. Due to their duration and intensity, HWs have been proven to affect thermal risk and mortality in different kinds of environment, but they result particularly dangerous in urban areas with high population density. Urban areas can be also exposed to the local scale phenomenon of Urban Heat Island (UHI), which occurs when a city experiences much warmer temperatures than nearby rural areas essentially due to the different surfaces’ heat absorption and retention capacities. The dynamical interaction of HW and UHI is not entirely understood. Many studies report partially incoherent outcomes, assessing both the existence and the absence of a synergistic behaviour exacerbating the urban-rural temperature difference during HWs. In this study, in situ air temperature measurements are analyzed to estimate the different UHI metrics during HWs events. Data provided by the European Climate Assessment and Dataset and the World Meteorological Organization relative to 41 European cities are examined for the 2000-2019 summer periods. The intensity of the UHI effect is estimated through the Composite UHI Index (UHII), defined as the difference between averaged urban and non-urban air temperatures. Regarding the nighttime, 28 cities present positive UHII increase ranging from 0.2 to 2.0 ºC, which means an average UHI intensification of about 0.7ºC during the HW events. Also, an increment in the percentage of days with positive UHII is observed. The analysis of the temporal evolution of UHII during HW periods highlights that the main responsible for this exacerbation is the rural temperature which tends to amplify less than rural in HW conditions. Furthermore, a composite analysis using the UHII and the temperature anomaly during HW events shows UHI amplification directly dependent on the HW intensity. This study is placed in the wide context of how urban areas respond to severe hot periods, and it could be useful to improve adaptation strategies aimed at reducing HWs’ impacts.