Numerical simulation of the effect of street trees on outdoor mean radiant temperature through decomposing pedestrian experienced thermal radiation: A case study in Guangzhou, China

Street trees have often been proposed to improve the outdoor thermal environment. However, due to the combined effects generated by street trees, the impacts of street trees on the thermal radiation field in a street canyon and mean radiant temperature (Tmrt) reduction have not been systematically analyzed in previous studies. In this paper, we developed a Tmrt decomposition model using first-order Taylor series expansion to decompose the various factors of the street trees on Tmrt reduction and quantify their individual and combined effects in Guangzhou, China. Our results showed that street trees significantly reduce Tmrt and severe heat stress events (Tmrt ≥ 60 °C). For 80% tree crown cover, the maximum decrease of Tmrt by 26.2 °C - 42.1 °C and the severe heat stress events decreased by 197 h - 500 h over the course of a year at different sidewalk locations, compared to the street canyon without trees. During the daytime, the interception of radiation (especially short-wave radiation) by street trees was the primary determinant in reducing Tmrt, with a contribution that mostly exceeded 70%. Additionally, the shading effects of street trees decreased the short-wave radiation reflected from urban surfaces, contributing about 20% to Tmrt reduction. Lastly, our results also showed the leaf area index played an important role in the Tmrt reduction. Our study quantified the individual and combined effects of each factors of reducing Tmrt by street trees, which will better understand the thermal benefits of urban street trees.