City-Scale Nocturnal Urban Heat Mitigation with Selectively Emitting Roofs

The canopy-layer urban heat island (UHI), whereby urban nocturnal air temperatures are observed to be higher than rural ones, is a common feature of cities. Especially in warm, dry climates, it degrades thermal comfort at night. Much research has been dedicated to urban heat mitigation strategies that are effective during the day such as reflective and green roofs and urban trees. However, because of the projected significant contribution to urban canopy-layer warming during nighttime hours that results from GHG-induced climate change and expanding built environments, it is important to explore heat mitigation strategies that target nighttime cooling. To assess potential nocturnal cooling from urban adaptation strategies, we apply high-resolution numerical modelling for contemporary and projected urban climates in select U.S. cities, using the WRF-BEP model configured to explicitly resolve the pedestrian-layer air temperature. Our simulations demonstrate that the widespread implementation on roofs of a novel material that is emissive in the atmospheric window but reflective elsewhere has the potential to cool air temperature more than do standard reflective roofs that have comparable shortwave radiative properties. Furthermore, unlike standard reflective roofs, selectively emitting high-reflectivity roofs cool pedestrian-level air temperature at night by ~2 K, demonstrating the potential of this mitigation strategy to offset urban-induced warming during the portion of the diurnal cycle when the UHI is greatest. Finally, we discuss the theoretical limits of selectively emitting roofs, which emphasize the need for additional methods for nighttime urban heat mitigation and for reduced greenhouse gas emissions.