Understanding cooling potential of urban trees in a typical North America neighborhood

Urban areas are experiencing rising temperatures leading to the exploration of mitigation solutions to cool cities. Common solutions rely on trees. However, trees can both enhance and deteriorate pedestrian thermal comfort in different areas of a neighborhood and at different times of the day. The objective of this paper is to document and quantify the overall contribution of trees to the pedestrian comfort in an entire neighborhood, with a special attention to cooling. Various vegetation scenarios are applied for a typical North American urban neighborhood, inspired by a neighborhood in Montreal, during a heat-wave period. The study focuses on 6 central urban lots including alleys, streets and a large boulevard, forming an area of 250 m x 300 m, surrounded by 14 urban lots. We use all-physics computational modeling to assess the multifaceted effects of trees at urban scale, using a custom CFD-based coupled solver developed by the authors, urbanMicroclimateFoam, based on OpenFOAM. This urban microclimate model sequentially solves turbulent air flow, heat and moisture transport in porous media, and radiative exchanges. The Universal Thermal Climate Index (UTCI) is used to document outdoor thermal comfort. The analysis reveals that trees in ventilation corridors, i.e. streets aligned with the primary wind, reduce average pedestrian comfort in the neighborhood by blocking wind despite providing shade. Conversely, trees in private gardens or in crosswind corridors can enhance both local thermal comfort and overall average pedestrian comfort throughout the neighborhood, thereby improving walkability. The paper highlights the non-local effects of trees over the entire neighborhood.