Abstract
Shading is one of the most effective strategies to mitigate urban local-scale heat stress during summer. Therefore, this study investigates the effects of shading caused by buildings and trees via exhaustive field measurement research on urban outdoor 3D radiant environment and human thermal comfort. We analyzed the characteristics of micrometeorology and human thermal comfort at shaded areas, and compared the difference between building and tree shading effects as well as that between shaded and sunlit sites. The results demonstrate that mean radiant temperature Tmrt (mean reduction values of 28.1°C for tree shading and 28.8°C for building shading) decreased considerably more than air temperature Ta (mean reduction values of 1.9°C for tree shading and 1.2°C for building shading) owing to shading; furthermore, the reduction effect of shading on UTCI synthesized the variation in the above two parameters. Within the shaded areas, short-wave radiant components (mean standardized values of 0.104 for tree shading and 0.087 for building shading) decreased considerably more than long-wave radiant components (mean standardized values of 0.848 for tree shading and 0.851 for building shading) owing to shading; the proportion of long-wave radiant flux densities absorbed by the reference standing person was high, leading to a relatively high long-wave mean radiant temperature, and R2 between long-wave mean radiant temperature and air temperature exceeded 0.8. Moreover, the directional sky view factor (SVF) was utilized in this study, and it showed significant positive correlation with short-wave radiant flux densities, but no statistically evident correlation with long-wave radiant flux densities. Meanwhile, Tmrt was most relevant with SVFS⟶ with R2 of 0.9756. Furthermore, UTCI rose two categories at the sunlit areas compared with that at the shaded areas. In contrast, Ta and Tmrt played the first positive role in UTCI at shaded and sunlit areas, respectively.
Highlights
Urbanization is accelerating worldwide and is affecting urban living and transforming human society in various manners [1]
E outdoor thermal environment is significantly affected by the design of the built and vegetation environment [3], and it affects the thermal comfort experienced by people; people’s perception for the thermal environment as a result influences their usage of outdoor spaces [4, 5]
Unlike studies that were conducted in warm or hot areas, the localscale ambient temperature in severely cold regions is not as high as in other climates, but the heat stress is still high during summer because of the high radiant flux densities outdoors. is indicates that the outdoor radiant environment may affect human thermal comfort differently in severely cold regions than in other regions. erefore, research subjects with the regional features of this study provide a necessary supplement to expand the research locations and results for related fields of study
Summary
Urbanization is accelerating worldwide and is affecting urban living and transforming human society in various manners [1]. Exhaustive measurements and an investigation that targeted different practical effects of building and tree shading forms on the outdoor thermal and 3D radiant environment and human thermal comfort during summer were conducted in a severely cold region. E research involved the subdivision and proportion of the composition of radiant flux densities that related to shortand long-wave radiation; the reduction influence of the two shading methods on detailed directional short- and longwave radiant flux densities from 3D environment and the roles of short- and long-wave radiant flux densities in Tmrt; the radiant environment features and different wavelength radiant flux densities functions at the shaded areas; and the different shading effects on human thermal comfort. Unlike studies that were conducted in warm or hot areas, the localscale ambient temperature in severely cold regions is not as high as in other climates, but the heat stress is still high during summer because of the high radiant flux densities outdoors. E main objectives of this study are as follows: (1) to explore the shading effects on micrometeorological parameters, on the detailed directional 3D radiant components, and on human thermal comfort; (2) to investigate the radiant characteristics under shaded conditions; (3) to propose and apply directional SVF and evaluate its relationship with 3D radiant flux densities; and (4) to probe human thermal comfort differences and the impact of micrometeorological parameters on thermal comfort under shaded and sunlit conditions. is study offers a practical research method and quantified data targeting summer conditions for developing general and specific strategies of rational urban planning
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