A study on the impact of shadow-cast and tree species on in-canyon and neighborhood's thermal comfort

Abstract Tree-plantingis one of the veritable tools for combating urban heat islandand improving thermal comfortin the wake of global warming and urbanization. However, trees of different species and morphological properties have variable solar attenuation capacity and consequently, thermal comfortregulation potential. Besides, the shadow-cast effect by buildings helps in reducing pedestrian radiant load and consequently improves thermal comfortespecially in high-density cities even though ventilation is reduced. Therefore, a holistic and contextual understanding of tree-plantingand shadow-casting can help in designing climate-proof cities. In this study, we employed the ENVI-met model to better understand the interaction between these two forms of shading( treesand buildings) on the pedestrians' thermal comfortin Hong Kong and the influence of one over the other. The impact of different urban densitieson the thermal comfortimprovement potential by eight (8) common tree-species in Hong Kong was specifically studied. Results show that shallow canyonsare susceptible to worse thermal condition when compared to their deeper counterparts with similar aspect ratio value. Of all tree configuration parameters, leaf area index, tree height and trunk height are most influential in improving and aggravating daytime and nighttime thermal comfort, respectively. We also found that trees' effectiveness in improving daytime thermal comfortreduces with increasing urban densityand vice-versa for nighttime. For the reference of planners and landscape architects, this study recommends tall trees of low canopy density with high trunk in deeper canyonsand vice-versa for shallow canyonsand open-areas.