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How to transform the standing man from a box to a cylinder – a modified methodology to calculate mean radiant temperature in field studies and models

Conference contribution
Authors Björn Holmer
Fredrik Lindberg
D.P. Rayner
Sofia Thorsson
Published in ICUC9 – 9 th International Conference on Urban Climate jointly with 12th Symposium on the Urban Environment. 20-24 July, Toulouse, France
Publication year 2015
Published at Department of Earth Sciences
Language en
Links www.meteo.fr/cic/meetings/2015/ICUC...
Subject categories Climate Research, Meteorology and Atmospheric Sciences, Physical Geography

Abstract

Mean radiant temperature (Tmrt) has shown to be an important meteorological variable in studies of human comfort and health. The Tmrt is calculated as the surface temperature of a standing man approximated as a cylinder emitting the same amount of longwave radiation as all short- and longwave radiation fluxes received from the surrounding four cardinal points and down- and upwards. The calculation was introduced by Höppe in 1992 and has then been used both in models (e.g. SOLWEIG) and field studies. However, the formula by Höppe describes in fact a man shaped like a box and not a cylinder, which has resulted in some peculiar features noticed in studies of Tmrt such as a secondary daytime minimum and an influence of the orientation of the field equipment. A methodology to change the box man to a cylindrical man is proposed. It will remove the peculiarities that have been observed in earlier studies. The methodology is based on the partition of the observed shortwave fluxes in direct and diffuse radiation. The minimum shortwave radiation of the four cardinal points is used as diffuse radiation since it is monitored by a sensor that is not sunlit. By subtraction of this quantity the horizontal direct fluxes are obtained. Calculation of the resultant flux of the sunlit sensors and adjustment for solar angle gives the direct shortwave radiation. The surface of the standing man (as a cylinder) perpendicular to the direct radiation must be determined and the direct shortwave radiation received by the standing man can be calculated. Then the sum of the shortwave fluxes can be calculated. The diffuse and longwave fluxes can be calculated according to the Höppe formula since they differ little with direction. In the SOLWEIG model the direct shortwave radiation is used as an input. Thus the calculation according to the new methodology is easy to apply, only the solar position needs to be added. The new methodology is tested by model calculations with SOLWEIG and field studies in both high-latitude Gothenburg, Sweden and low-latitude Ouagadougou, Burkina Faso. The secondary minimum disappears. In Gothenburg at a site with SVF=0.95 the noon depression of Tmrt by the Höppe formula was about 2 °C and there was an overestimation of 1.5-1.7 °C two-three hours before and after noon.differences in summer. In Ouagadougou data from an open site (SVF=0.83) in the dry season the differences were slightly smaller. Sites with lower SVF and much reflected direct shortwave radiation differed less from the Tmrt obtained with the Höppe formula.

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