Effects of tree plantings with varying street aspect ratios on the thermal environment using a mechanistic urban canopy model

Urban tree planting is an effective strategy to reduce temperature locally and eventually mitigate Urban Heat Island. Although its cooling effects have been widely reported, understanding of coupling effects of street morphologies and tree characteristics on thermal environment is insufficient, especially in high-density cities. A parametric study is conducted to investigate effects of leaf area index (LAI = 1.0,2.0,3.0,4.0,5.0,6.0) and tree crown radius (rtree = 2.0,3.4,4.4,5.0 m) on thermal environment inside streets with various aspect ratios (street height/width, H/W = 1.0-5.0). A mechanistic urban canopy model Urban Tethys-Chloris (UT&C) is selected and integrated with a new semi-empirical wind parameterization based on drag coefficients and frontal area densities of buildings and trees. Modified UT&C is validated by a unique and controllable scaled outdoor experiment. The research outputs can be summarized as follows: (1) Calculated air temperature (Ta), wind speed (V), and thermal comfort indices (Mean Radiant Temperature (Tmrt) and Universal Thermal Climate Index (UTCI)) are in good agreement with results from scaled outdoor experiments, while slight overestimation of relative humidity (RH) occurs in tree cases; (2) Increasing LAI and rtree both bring stronger cooling effects by up to 20.2 degrees C (Tmrt) and 4.0 degrees C (UTCI); (3) Narrower streets weaken trees' cooling effects and increasing LAI and rtree could be less efficient for higher H/W than lower H/W; (4) LAI = 4.0 and rtree = 5.0 m are optimal for wide streets (H/W = 1.0-3.0) to provide maximum UTCI cooling (about 4.0 degrees C), while LAI = 4.0 and rtree = 3.4 m are suggested for narrow streets (H/W = 3.0-5.0). Our study provides useful information on evidence-based urban tree selection strategies for urban planning and design.