Urban tree design approaches for mitigating daytime urban heat island effects in a high-density urban environment
Introduction
High population density and rapid urban growth increase the vulnerability of cities to climate change [1]. As one of the world's high-density cities, Hong Kong suffers from a severe urban heat island (UHI) effect. According to Siu and Hart [2], the annual UHI intensity in Hong Kong ranges from 2 °C to 4 °C. With the effect of climate change and high-density development, increasing heat stress would trigger health problems for the habitants [3]. A study of Hong Kong reported a 2% increase rate in heat-related mortality associated with a 1 °C increase of air temperature when the threshold of 28 °C is reached [4]. Moreover, thermal discomfort and heat stress would be more fatal to the elderly and those with chronic illness [5], [6].
Urban greenery has been proposed as an effective measure to mitigate UHI in the city and improve the urban microclimate. Intra-urban measurement at the macro scale showed a 4 °C difference in the air temperature between the urban centre and a well planted area in the city [7]. Armson et al. [8] also showed that greenery could effectively cool urban surfaces by 20 °C, and that shading by trees could reduce the global temperature by 5 °C to 7 °C, thus providing a more comfortable microclimate in cities. The environmental effects of vegetation depend on foliage density [9] and the leaf area index (LAI)1 and leaf area density (LAD)2 proved to be important metrics in determining cooling outcomes [10]. A study on hot humid climates demonstrated that with maximum LAD being 1.0 m2/m3 and LAI value around 5, air temperature and surface temperature were reduced by 1.3 °C and 14.7 °C, respectively [11]. A modelling study for hot dry climates showed that 84% of free horizontal direct radiation was intercepted by tree canopy with a maximum LAD of 1.8 m2/m3 and LAI value of 3 [12]. The green coverage ratio in an urban area also significantly affected the air temperature distribution and UHI intensity [13]. A microclimate study of the building block scale revealed that a 10% increase in the green/built area ratio would create 0.8 °C cooling [14]. Observational studies of urban greenery further showed that urban parks would be 1 °C cooler than non-vegetated sites [15]. About 20% to 30% coverage of greenery has been proposed as a mitigation measure for newly built projects to improve the living environment in Hong Kong [16]. Achieving this standard greenery coverage in many urban areas is challenging because of intense urban land use and compact urban morphology. As cities continue to develop, their increasing density may result in environmental issues [17]. In this regard, strategies for optimizing greenery design with regard to the built environment and local climatic conditions can enhance thermal benefits of vegetation in urban areas [18], [19]. Hence, further research on context-based planning and design strategies for greenery in high-density cities is necessary [15]. The present study evaluated two site-specific design approaches for urban trees on mitigating daytime UHI in a compact urban environment. The findings could quantify the cooling effect of different greenery design strategies at the site-level. Urban planners and designers can employ this knowledge to maximize the use of greenery design to improve daytime thermal comfort and mitigate the UHI effect in high-density urban environments.
Section snippets
Sky view factor (SVF) as a thermal indicator
SVF is an important factor in morphology-related urban microclimate studies. Mills [20] reported that solar exposure and SVF are two key factors that determine the daily heat balance of building structures. Holst and Mayer [21] also investigated the thermal conditions in street canyons and observed a linear relationship between the mean radiant temperature (Tmrt) and SVF. Giridharan et al. [22] further reported that a 1% reduction in SVF would reduce the daytime UHI intensity by 1–4% under
Area studied and sensitivity test
This study considers two morphology-oriented design strategies for tree planting in high-density areas of Hong Kong. A SVF-based approach is proposed for urban areas with an irregular building layout and different building heights, while a wind-path approach is recommended for areas with regular building block arrays and a prevailing summer wind direction. Sensitivity test and modelling study are conducted to evaluate these two design approaches. Hong Kong is located at 22°16′42″ N 114°09′32″ E
Sensitivity test
Table 2 presents the measured data from sites of low and high SVF during the early-afternoon period in summer. Ali-Toudert et al. [53] pointed out that the vegetation effect on Tmrt was more sensitive to building geometry than the effect on Ta. The observed data shows that the effect of trees on Ts and Tmrt exhibits a SVF-related pattern. Ts of the road area shaded by tree is reduced by 18.7 °C under 0.8 SVF and 15.9 °C under 0.2 SVF, comparing to the exposed point. Similarly, cooling in Tmrt is
Summary
In this study, optimized tree planning strategies for daytime UHI mitigation in high-density subtropical cities are investigated. Morphology-based planning approaches for tree planting are evaluated in two climate-sensitive waterfront areas of Hong Kong. The study demonstrates that small parks in the high SVF areas with highly localized tree planting significantly cools down the air temperature and mitigates daytime UHI effect in the urban centre. On the other hand, mid-size green space and
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