Effect of heatwaves on urban sea breeze, heat island intensity, and outdoor thermo-hygrometric comfort in Rome (Italy)

The purpose of this study is to investigate the impact of heatwaves on sea breeze, urban heat island intensity, and outdoor thermo-hygrometric comfort analysing ground-based remote sensing and in-situ meteorological measurements collected in a Mediterranean coastal metropolitan area (Rome, Italy) during the summer of 2022. The dataset (01 May - 31 August 2022) has been divided into heatwaves and not-heatwaves days to highlight the distinctive features of extreme heat episodes. Furthermore, the year 2022 was compared to 2020, the latter having been characterised by temperatures in line with the last two-decades. The heatwaves do not significantly affect the onset time and the duration of sea breeze, while its cooling effect intensifies during the heatwaves. The urban heat island intensity (UHII) is evaluated by computing the hourly-averaged air temperature difference between two stations, one in the urban centre and the other in rural surroundings. Although the daily-averaged UHII does not show significant differences among the years under investigation and the subsets, marked differences can be identified in the urban heat island daily cycle. In fact, during daytime the highest temperatures are detected in rural inland areas while, at night, the city experiences higher temperatures, especially during the heatwaves. Finally, the Mediterranean outdoor comfort index suggests that a heatwave seems to be a not sufficient condition for human thermo-hygrometric stress, even if stronger discomfort is experienced during heatwaves.