On the multi-scale nature of the UHI during heatwaves - Evidence from an experimental field campaign in the city of Bologna

As global temperatures rise and urban areas expand, the impacts of Urban Heat Islands (UHI) are becoming of even greater concern. In the Padan Plain, a densely populated area in the north of Italy, the warming rate is especially high. To quantify the magnitude of the UHI in one of its largest cities, an intensive 24-hour field campaign was conducted in Bologna under heatwave conditions; with the objective of exploring the multi-scale characteristics and processes associated with the UHI during a heatwave, focusing on the interplay between thermal forcing and local effects. To fulfill this type of evaluation, a central and a more peripheral zone with different morphological characteristics and vegetation distribution were probed via portable sensor devices, with the aim of mapping the air temperature at the pedestrian level, while thermal imaging of urban surfaces was acquired to assess the behaviour of the heat sources. The results were also related to a climatological review of the city of Bologna and the whole Emilia-Romagna region, particularly regarding temperature trends, UHI and heatwaves. The data gathered during the field campaign revealed an Urban Heat Island of almost 7 °C between the city centre and the outskirts, along with an intra-city scale UHI of 2 °C between the densely built central area and the greener urban neighbourhood. Considerations on the different scales of the UHI, on the convergence velocity, urban canopy cooling trends, and relation with planar and frontal area coefficients were presented. The climatological analysis also revealed how heatwave events that were once rare are becoming the new norm. In face of this and of the implications for the growing urban population, case studies and tailored mitigation strategies for cities are now of literally vital importance.