Observational evidence of intensified nocturnal urban heat island during heatwaves in European cities

Abstract

A Heatwave (HW) is a large-scale meteorological event characterised by persistent and extremely high-temperature condition. At the local scale, the Urban Heat Island (UHI) is another thermal-related phenomenon defined as an urban area warmer than its surrounding regions due to different surfaces’ capabilities to absorb and store heat. However, the assessment about the effect produced on UHI by HW events is not homogeneous. Indeed, regarding the capability of HWs to influence the urban-rural temperature difference, several studies report different conclusions describing both an exacerbation and a reduction of UHI during HW events. In this context, the present study analyses in situ long records of temperature measurements (20 years) to provide observational shreds of evidence of UHI modification under HW conditions. We examine data from the European Climate Assessment & Dataset and World Meteorological Organization computing the UHI Index (UHII) to quantify the UHI effect intensity in 37 European cities during the last 20 summers. The results show a UHII intensification for 28 of the 32 cities affected by positive UHI during extremely high temperatures at night, while substantial variations are not observed during the daytime. The time evolution of UHI during a heatwave highlights that a more significant and persistent urban-rural temperature gradient explains the UHI intensification. Finally, the relationship between the large and local-scale temperature phenomena reveals that continental high-temperature periods are often associated with prominent temperature differences between small-scale urban and rural environments, assessing the impact of large-scale features on thermal stress at the local scale.

Effect of Heatwaves on Urban Sea Breeze, Heat Island Intensity, and Outdoor Thermo-Hygrometric Comfort in Rome (Italy)

“…This implies that the rural area, despite being characterised by natural land use, without buildings and/or obstacles and, hence, without UHI, records daytime maximum temperatures higher than the city centre. In contrast, during night-time, the city is warmer than the rural surroundings because of the UHI which, as it is known, limits the nocturnal cooling of the atmosphere close to the ground (Possega et al, 2022). The behaviour is similar in 2022 (Fig.…”

“…In this context, the choice of the stations is not straightforward, and the selection is performed by rearranging the constrains proposed by Possega et al (2022) for the investigation of the synergy between heatwaves and UHII in several European cities:…”

“…The SAP and PAL stations meet the above-mentioned criteria, allowing for the correct evaluation of the UHII. The altitude difference between SAP and PAL is 67 m, i.e., it is considerable although below the threshold proposed by Possega et al (2022). Nonetheless, these stations can be assumed as suitable for this study as the focus is on the summer periods, characterised by persistent high-pressure systems, so without requiring corrections in the calculation of the night-time UHII (Martin-Vide et al, 2015).…”

“…The UHII is computed in the present study adopting the simplest definition of UHI, i.e., considering only two stations, located respectively in the urban centre and in the neighbouring rural area. Although the sites were chosen following the criteria proposed by Possega et al (2022) (see Section 3.3), it should be noted that this definition does not allow to assess the spatial UHI inhomogeneity. Cecilia et al (2023) performed a detailed evaluation of the UHI in Rome considering several in-situ meteorological stations, linking the UHII to satellite measurements of imperviousness, which describes the presence of artificially sealed surface.…”

Analysis Options for Urban Green Spaces Based on Unified Urban Masks: Selected Results for European Cities

“…To quantify the urban heat island effect, meteorological point data from georeferenced stations are needed covering long periods of time-in a study by Possega et al (2022), for example, for 37 European cities over a period of 20 years were needed [50]. When selecting meteorological stations, it is necessary to choose urban and rural stations on a uniform geodatabase, which requires the definition of an urban mask.…”