Simulation study on the impact of tree-configuration, planting pattern and wind condition on street-canyon's micro-climate and thermal comfort

Many previous studies have investigated the role of urban greening (especially trees) on thermal comfort condition of open spaces while its influence in street canyons has, however, received relatively lower research attention. In this study, numerical experiments involving street canyons of varying aspect ratio (AR_B) with embedded trees of varying aspect ratio (AR_T), leaf area index (LAI), leaf area density (LAD) distribution and trunk height under different wind conditions were conducted using a micro-meteorological model, ENVI-met. Employing physiological equivalent temperature (PET), the in-canyon thermal comfort was characterized. Results show variable magnitude of PET reduction (δPET) with trees of different vertical LAD distribution even though they are of similar LAI. While larger tree-covered area (TCA) ensures improved thermal comfort, the magnitude varies with tree-planting pattern. Between the tree-planting patterns in TCA = 0.6, δPET with double-rows is higher than centre tree-planting. Between eastern and western side tree-planting (TCA = 0.3), the former is more temporally effective than the latter. Furthermore, strong oblique wind condition ("without trees") was found to be more efficient in improving thermal comfort than trees under calm perpendicular wind, reinstating the importance of desirable ventilation in achieving a thermally comfortable environment. However, vegetation offers improved PET reduction under calmer perpendicular than stronger oblique wind condition. Finally, recommendations on applications of our key findings were presented.