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The influence of anisotropic diffuse shortwave radiation on mean radiant temperature in outdoor urban environments

Journal article
Authors Nils Wallenberg
Fredrik Lindberg
Björn Holmer
Sofia Thorsson
Published in Urban Climate
Volume 31
ISSN 2212-0955
Publication year 2020
Published at Department of Earth Sciences
Language en
Links https://doi.org/10.1016/j.uclim.202...
Subject categories Meteorology and Atmospheric Sciences, Climate Research, Physical Geography

Abstract

During clear weather conditions the main part of the shortwave irradiance derives from sun direct-beam radiation. However, part of the shortwave radiation also originates from all-sky diffuse radiation (D), with a large part being circumsolar in origin. Many radiation models considers the sky as isotropic when estimating D. Here we implement an anisotropic model for D into the SOLWEIG model to examine the spatial patterns of D in a built-up environment, as well as its influence on mean radiant temperature (Tmrt), a variable essential for estimating outdoor human thermal comfort. Comparisons between the anisotropic and the isotropic models indicates that the D in the isotropic model is overestimated in shaded areas and underestimated in areas close to sunlit walls. This is explained by the circumsolar origin of D during clear and semi-cloudy conditions and solar altitude. These over- and underestimations, consecutively, have implications for Tmrt, which can differ by up to 3 °C. The deviations in D thus signify the importance of using an anisotropic model when estimating D and Tmrt, especially since the areas with the highest reported radiant loads receive even more radiation considering an anisotropic diffuse sky, i.e. the hottest areas are even hotter than previously reported.

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