Influence of Greenery in Cooling the Urban Atmosphere and Surfaces in Compact Old Residential Building Blocks: A Building Morphology Approach

Outdoor thermal environment at great extent determines the possibility and vitality of outdoor social activities and communication. The hot air and heated surfaces in tropical/subtropical areas during summer period would compromise the outdoor thermal environmentt in the city. This study aims to investigate the effect of street trees in cooling the air and surfaces in traditional high-dense residential area in Hong Kong. Sham Shui Po is a traditional residential district in Hong Kong with relatively regular building arrays oriented in different orientations. With simplified morphology, this paper aims to investigate the influence of building morphology and tree configuration on greenery cooling. In the simulation, the testing parameters including sky view factor (SVF) and orientation of street canyon, and the leaf area density (LAD) of the tree crowns. Based on the findings from parametric study, case study adopting the existing building morphology was also conducted to offer further understanding in greenery cooling. This study is expected to offer site-specific greening design suggestions for tropical/subtropical cities with compact building layouts. The simulation result shows that cooling efficiency varies with tree crown density. With the average of the LAD profile being 0.5 or above, substantial cooling in urban atmosphere and surfaces can be achieved in compact residential neighborhoods. And with denser tree crowns of LAD averaged at 1.0 or above, the mean radiant temperature would be reduced to a comfortable level of 33 degree in hot summer.The study also revealed with the high summer solar angle at noon, the cooling outcome under different orientations would be more homogenous with higher SVF; and for lower SVF condition, the cooling effect differs along streets sections and varies with street orientation. For subtropical cities, it is an effective measure to shade the street-crossing areas in high-dense neighborhoods to avoid overheat problem.