Cooling Effect of Urban Trees on the Built Environment of Contiguous United States

Exacerbated heat stress has resulted in a series of environmental issues in urban areas. Mounting empirical evidence shows that urban trees are effective in mitigating the thermal stress in the built environment, whereas large-scale numerical simulations remain scarce. In this study, the effects of shade trees on the built environment, in terms of radiative cooling, pedestrian thermal comfort, and urban land surface energy balance, were evaluated over the contiguous United States. The projected scenario was simulated using a coupled Weather Research and Forecasting-urban modeling system, incorporating the radiative shading of urban trees only. Results show that on average the mean near-surface air temperature in urban areas decreases by 3.06 °C over the entire contiguous United States with the shading effect. Analysis of pedestrian thermal comfort shows that shade trees improve the thermal comfort level in summers, but could be detrimental in winters for cities located in temperate or subpolar climate zones. In addition, it was found that trees alter the surface energy balance by primarily enhancing the radiative cooling, leading to significant changes in the sensible heat but the ground heat comparatively intact.