Local impact of trees on thermal comfort of pedestrians in streets

Urban thermal comfort can be improved by trees. A better understanding of the effects of trees on urban climate in streets can provide better guidelines on the optimized use of vegetation in cities to moderate temperatures. In this study, we explore and quantify both improvement and deterioration of urban thermal comfort due to street trees, modeled based on a common street tree in Montreal, by varying planting patterns, street orientation and wind direction. The impact of trees on outdoor thermal comfort is studied at street level and at balcony level for multistoried residential buildings over 24-hour cycles. The study is performed with a suite of urban climate models based on OpenFOAM, considering turbulent and buoyant air flow, heat and moisture transport in porous media and short- and long-wave radiative exchanges. Results are presented in terms of Universal Thermal Climate Index (UTCI). To quantify zones and periods of impact of trees on thermal comfort during day and night, we introduce several new comfort indices: the cooling and heating indices, that describe the level of thermal comfort improvement or deterioration, respectively, and the cooling and heating areas, indicating which areas of the street are affected. We show that tree canopies can be strong assets during the day, as trees intercept solar radiation and provide shading. However, as a counterpoint, they may cause deterioration of thermal comfort during night, stretching sometimes into the morning. Street orientation is found to have a larger impact on thermal comfort than wind direction, while wind flow parallel to the street leads to more comfortable conditions than perpendicular wind. We identify street orientations and tree planting patterns that enhance thermal comfort.