Effects of cool roofs on air temperature within the urban canyon: A Large-Eddy Simulation study

Urban areas, home to about half of the world’s population, are increasingly adopting cool roofs to mitigate heat. These roofs reflect more sunlight and absorb less solar energy compared to conventional roofs, thereby reducing sensible heat fluxes from building rooftops. Many previous studies on the effects of cool roofs on near-surface air temperature were performed with traditional weather and climate models (e.g., the Weather Research and Forecasting or WRF model), where the near-surface air temperature is parameterized based on schemes designed for non-urban environments (such as Monin-Obukhov similarity theory) and does not represent the outdoor air temperature felt by urban residents. To quantify the effects of cool roofs on air temperature within the urban canyon, we perform building-resolving large-eddy simulations across different but idealized urban canyons that mimic different local climate zones. Our findings reveal that the cooling sensitivity, which characterizes the air temperature change (K) per unit amount of forcing (W/m²), increases with the canyon aspect ratio (AR) up to AR = 1, after which it decreases. We further compare the LES results to the simulated results by the single-layer urban canopy model in WRF. This study contributes to the understanding of how cool roofs impact within-canyon air temperatures in different urban settings.