Disentangling the relative importance of tree-cast shadows and transpiration in mitigating urban extreme heat

Abstract This study quantifies and compares the relative importance of tree-cast shadows and transpiration in mitigating urban extreme heat through online numerical simulations using the Weather Research and Forecasting (WRF) model coupled with an urban canopy model that represents street trees and a factor-separation method. During the extreme heat event, tree-cast shadows and transpiration by street trees with a crown radius of 1 m reduce urban canyon air temperature by 0.43 ± 0.06 °C and 1.15 ± 0.09 °C in the daytime, respectively, the tree transpiration effect being much greater than the tree-cast shadow effect. The warming associated with the interception of direct shortwave radiation by street trees is greater than the cooling associated with tree-cast shadows in the daytime. In the nighttime, the mean cooling effects due to tree-cast shadows and transpiration are 0.09 ± 0.02 °C and 0.15 ± 0.03 °C, respectively, the two cooling effects being comparably small. The relative importance of the two effects is found to be similar under differing urban geometry and tree-crown size. This study provides quantitative evidence highlighting the importance of managing tree soil moisture to sustain tree-induced cooling benefits.