Cooling Effects of Urban Greenery at Three Scales

Abstract

Study intent Greenery in a high-rise urban environment provides accessible amenity space and moderated microclimate for the local community. At the urban-to-local scale, urban greenery contributes to heat island mitigation; at the microscale, landscape elements moderate pedestrian-level thermal comfort; and at the building scale, nearby trees and facade greening or, vertical greening system (VGS), mediate building heat gain and improve indoor comfort and indoor energy performance without occupying valuable urban land. This chapter synthesizes a series of case-based investigations on the thermal environmental performance of urban greenery at three scales. The first case attempted to understand the local urban effects on the cooling performance of greenery under a simplified LCZ scheme. The second case compared two housing block design strategies to reduce summertime heat gain in outdoor space, i.e., high-albedo pavement and vegetation, particularly trees, through field measurement and numerical simulation. The third case experimentally studied the energy and thermal performance of a double-skin green facade system on an office building. Results and discussion The local urban density and land cover properties have notable effect on the microscale cooling rates of greenery. Tree canopy shading and overall foliage density, qualified by the tree view factor and green plot ratio, respectively, were capable of partly explaining the variation in air temperature reduction. To optimize cooling performance, residential greenery should be integrated into the local green network system. Regarding the two strategies, i.e., albedo and greenery, the second case study reveals that increasing greenery cover, especially tree cover, improves thermal comfort from mid-day till early night. In contrast, increasing ground surface albedo overall worsens the thermal comfort during the day with a marginal benefit at night. At the building scale, the experiment reveals that the double-skin green facades on both south and north facade create a distinctive microclimate in the cavity. The exterior wall surface temperature reduction of the southern facade reached maximally 9 ℃, whereas the interior surface reduced maximally 2 ℃. The resultant indoor thermal and energy improvements by the VGS were evaluated accordingly.