How to plant trees on an elevated road to improve thermal comfort in a street canyon

This study investigates the impact of trees planted on an elevated road on airflow and thermal comfort in a street canyon using a computational fluid dynamics (CFD) model, incorporating parameterization schemes for tree drag, shading, and evapotranspiration. To systematically assess the effects of trees on in-canyon airflows and temperatures, varying tree heights and locations were evaluated for three solar altitudes: morning, noon, and afternoon. The elevated road reduced surrounding wind speeds and increased air temperatures due to surface heating. When trees were planted on the elevated road, their drag and cooling effects further decreased wind speeds and temperatures. Specifically, trees taller than 6 m reduced average temperatures by -1.7 °C on the elevated road and -0.6 °C beneath it in pedestrian areas. Thermal comfort was assessed using the Universal Thermal Climate Index (UTCI). Shrubs under 1 meter showed high cooling efficiency but limited shading, whereas trees taller than 4 m improved UTCI by 2–8 °C. Central tree planting on the elevated road provided the most significant UTCI improvement, while planting on both sides resulted in higher UTCI in the center. These findings offer guidance for optimizing vegetation placement to enhance thermal comfort and promote sustainable urban environments.