Abstract
To quantify the ecosystem services of trees in urban environments, it is necessary to assess received direct solar radiation of each tree. While the Sky View Factor (SVF) is suitable for assessing the total incoming short- and longwave radiation fluxes, its information is limited to specific points in space. For a spatial analysis, it is necessary to sample the area for SVF. A new geometrical method, Area View Factor (AVF), for the calculation of sunlit areas is proposed. AVF is the ratio of the unhidden, projected surface of an object to the whole projected surface of an object in a complex environment. Hereby, a virtual, orthographic camera is oriented in accordance to the sun’s position in the 3D model domain. The method is implemented in the microscale model SkyHelios, utilizing efficient rendering techniques to assess AVF of all urban trees in parallel. The method was applied to Rieselfeld in Freiburg, Germany. The assessed sunlit area is compared to the SVF at the top of each tree and solar altitude angle, revealing a strong relationship between sunlit areas to solar altitude angles. This study shows that AVF is an efficient methodology to assess received direct radiation of urban trees. Based on AVF, it is possible to identify urban areas with shaded and sunlit trees, but it can also be applied to other objects in complex environments. Therefore, AVF is applicable for urban architecture or energetic research questions.
Highlights
The urban morphology describing the composition, layout, and form of a city has great impact on the radiation absorption of urban trees [1,2,3]
The mean Area View Factor (AVF) between a typical spring and summer day are compared (Section 3.1) and the mean AVF is linked to the solar altitude angle (Section 3.2)
The spatial, geometrical assessment of sun-lit and shaded areas of urban entities based on aligned orthographic views with respect to the position of the sun, is an additional method for modeling the urban radiation regime
Summary
The urban morphology describing the composition, layout, and form of a city has great impact on the radiation absorption of urban trees [1,2,3]. Tree physiological processes (e.g., transpiration and photosynthesis), which are controlled by these radiation fluxes, provide microclimatic regulations as ecosystem services through cooling by shading and transpiration [10,11,12,13,14]. These processes modify the local meteorological conditions of the microclimate (e.g., increased humidity and decreased air temperature [15]) and human thermal bioclimate [16,17]. Quantifying the impact of ecosystem services on the urban microclimate as well as the assessment of ecological and financial benefits [18,19] is required for improved and optimized urban planning, architecture, human thermal comfort and health
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