Different urban heat mitigation strategies are optimal day versus night and for distinct synoptic weather types

Abstract Extreme heat is projected to increase in frequency and intensity. Commonly applied infrastructure-based urban heat mitigation strategies, including greenery and reflective materials, have the potential to reduce heat exposure. However, there has been limited assessment of the cooling efficacy of these strategies as a function of synoptic weather type, neighbourhood morphology, and time of day. Using a neighbourhood-resolving mesoscale meteorological model, the urbanised Weather Research and Forecasting model, the dependence of heat mitigation cooling efficacy in Toronto, Canada during daytime versus nighttime is assessed as a function of the following: 1) weather type, using the spatial synoptic classification (SSC); and 2) local built structure and cover, using the local climate zone (LCZ) scheme. Heat mitigation efficacy is quantified by changes in three metrics: land surface temperature, air temperature, and Humidex. Large increases to rooftop albedo resulted in the greatest daytime reductions in air and surface temperatures, and Humidex. Replacing impervious surfaces with low vegetation provided the greatest nighttime reductions of temperature and Humidex. These results indicate that different heat mitigation infrastructures excel for cooling during daytime, when the highest temperatures occur, versus nighttime, when the largest heat island intensity prevails. These findings are consistent for the weather type exhibiting the highest Humidex, moist tropical, when reduction of heat exposure is most critical. The greatest daytime cooling was found during the sunny dry moderate SSC type. The largest nocturnal cooling occurred during the dry polar weather conditions, which were characterised by the lowest nighttime downwelling longwave radiation. Vegetation and albedo-based heat mitigation strategies yielded the greatest cooling when implemented in LCZs with greater impervious fraction. These results highlight the potential importance of tailoring heat mitigation strategies to prevailing synoptic weather conditions, local neighbourhood morphology, and the time of day when cooling is most needed, to maximise cooling benefits.