Scale-dependent response of the Urban heat island to the European heatwave of 2018

Extreme heat continues to be a pressing challenge of the changing climate.The impacts of extreme heat manifest on two different spatio-temporal scales: (1) episodiccontinent-wide heatwaves (HW) and (2) the city-scale Urban Heat Island (UHI). As HWs arebecoming more frequent, longer, and severe, they pose serious implications of increased publichealth risks at a city scale, and have adverse impacts on agricultural and terrestrial/aquaticecosystems on the regional scale. Here we offer a fresh perspective of the HW as a forcing thatinvokes dynamic, heterogeneous, scale-dependent responses evident in inter and intra-urbanheat islets. A numerical simulation of the 2018 European heatwave including the surfaceand air temperature-based UHIs of six urban agglomerations, with a high-resolution focus onParis, serves as our case study. We find that the mean nighttime UHI intensities are reducedfor inland cities but increased for coastal cities. Our examination of the heat islets revealstwo major findings: (i) the HW homogenizes the intra-urban surface temperatures during thedaytime (reduces variance) (ii) the HW impacts are most significant on the scale of large,spatially discontiguous extreme heat islets during nighttime. These results underscore the needto move beyond the prevalent HW-mean UHI intensity characterization and toward intra-urbanheat islet analyses that aid targeted mitigation.