Micrometeorological simulations to predict the impacts of heat mitigation strategies on pedestrian thermal comfort in a Los Angeles neighborhood

Abstract

The urban heat island impacts the thermal comfort of pedestrians in cities. In this paper, the effects of four heat mitigation strategies on micrometeorology and the thermal comfort of pedestrians were simulated for a neighborhood in eastern Los Angeles County. The strategies investigated include solar reflective “cool” roofs, vegetative “green roofs”, solar reflective cool pavements, and increased street-level trees. A series of micrometeorological simulations for an extreme heat day were carried out assuming widespread adoption of each mitigation strategy. Comparing each simulation to the control simulation assuming current land cover for the neighborhood showed that additional street-trees and cool pavements reduced 1.5m air temperature, while cool and green roofs provided cooling at heights above pedestrian level. However, cool pavements increased reflected sunlight from the ground to pedestrians near pavements in unshaded areas. This reflected radiation intensified the mean radiant temperature and consequently increased physiological equivalent temperature (PET) by 2.2°C during the day, reducing the thermal comfort of pedestrians. In locations near roadways with preexisting tree cover, cool pavements caused significant reductions in surface air temperatures and small changes in mean radiant temperature during the day, leading to decreases in PET of 1.1°C, and consequent improvements in thermal comfort. For improving thermal comfort of pedestrians during the afternoon in unshaded locations, adding street trees was found to be the most effective strategy. However, afternoon thermal comfort improvements in already shaded locations near streets were most significant for cool pavements. Green and cool roofs showed the lowest impact on the thermal comfort of pedestrians since they modify the energy balance at roof level, well above the height of pedestrians.