Simulation of the effect of downtown greenery on thermal comfort in subtropical climate using PET index: a case study in Hong Kong

Understanding how greenery can improve outdoor thermal comfort is important in downtown planning. This article presents a simulation approach to investigate the cooling effect of downtown greenery on the urban microclimate, primarily air temperature and solar radiation, and therefore pedestrian thermal comfort during summertime in Hong Kong. The numerical simulation software of ENVI-met was used to generate microclimate data for a downtown development site with and without greening design scenarios. Two types of scenarios were tested, one being 50% of tree coverage and the other being 30% of grass coverage with modified building design. The simulation results were transformed into geographic information system (GIS)-supported format using self-developed computer programs. Another computer module was developed to assess pedestrian thermal comfort based on the human-biometeorological index of physiological equivalent temperature (PET) within a GIS framework. Thermal comfort maps were generated accordingly. Through visualization and analysis using the ArcGIS software, it was shown that both the tree scenario and the grass scenario can reduce the average PET of the domain by 0.4K, suggesting that strategic design of urban greenery can effectively improve the downtown urban environment and outdoor thermal comfort.