How does shade infrastructure affect outdoor thermal comfort during hot, humid summers? Evidence from Nanjing, China

Abstract

• Developed scenarios with and without trees to explore the cooling effect of trees. • Tree shade reduces PET by 5.9 to 9.4 °C. • Urban non-tree shade reduces PET by 3.5 to 9.8 °C. • Natural and engineered shade synergistically reduces PET by 9.4 to 17.1 °C. • Structural characteristics of trees have a non-linear relationship with PET. With global climate change and urbanization, the urban heat island effect and extreme heat have negatively affected the outdoor thermal comfort (OTC) of urban residents. Therefore, it is critical to investigate how the built environment affects OTC to improve urban design and promote outdoor activities. Trees increase OTC under hot, sunny conditions, and dynamic shade cast from buildings has also been found to affect OTC positively. However, limited research has been conducted to understand the synergistic cooling effects of trees and building shade on OTC. Taking Nanjing City, China, as the study area, this study investigates the differences and synergistic effects of various urban shade infrastructure on OTC. The ENVI-met model was used to simulate two scenarios with and without trees, and physiological equivalent temperature (PET) was calculated to quantify OTC. The results show that tree removal increased air temperature, wind speed, direct shortwave radiation, and decreased relative humidity, leading to an increase in PET by about 5.9 to 9.4 °C. Our shade analysis reveals that building shade can reduce PET by around 5.1 to 9.8 °C. Overall, urban shade infrastructure (mainly including trees and buildings) can reduce PET by around 9.4 to 17.1 °C synergistically. The nonlinear relationship of tree structural characteristics related to PET is further discussed to determine planning priorities and propose heat mitigation strategies to optimize physical spaces. We recommend combining dynamic shade from various urban infrastructure to improve OTC in the summer, improving urban outdoor livability.