Urban tree design approaches for mitigating daytime urban heat island effects in a high-density urban environment

• Morphology-based tree planning methods are evaluated by measurement and simulation.
• Cooling on road surface is 18 °C and 15 °C under high and low SVF, respectively.
• Comfortable Tmrt (34 °C) is provided by trees in heavily built areas with low SVF.
• Combining trees with wind path doubles cooling in temperature and sensible heat.

Hong Kong suffers from an intense urban heat island (UHI) effect of up to 4 °C as a result of compact urban form and highly urbanized land cover. Enhancing the cooling efficiency of urban greenery is essential for improving the microclimate in high-density cities. This paper aims to delineate design strategies for urban greenery to maximize thermal benefits and mitigate the daytime UHI effect. Two site-specific design strategies for tree planting in the urban environment are proposed. The sky view factor (SVF)-based design approach and the wind-path design approach are evaluated in the neighbourhood scale in two climate-sensitive areas with different urban morphologies. Observed data and simulation results indicated that the cooling effect of urban trees is highly associated with SVF. Air temperature reduction (a 1.5 °C reduction) is the most profound for the high SVF scenario, whereas substantial radiation shading (Tmrt reduced to 34 °C) is detected in areas with medium-low SVFs. The modelling study also showed that the cooling of air temperature and sensible heat were twice as high for vegetation arranged in wind corridors than those for leeward areas. The study demonstrated that tree planting in conjunction with proper planning is an effective measure to mitigate daytime UHI.