Dependence of Mean Radiant Temperature on 3D Radiant Flux Densities: The Example of Urban Quarters in a Mid-Size Central European City during Summer Heat

Due to regional climate change in Central Europe, the significance of human-biometeorology is increasing for urban planning as neither structures nor people of Central European cities are adapted to the severe heat in summer, which will routinely occur in Central Europe starting in mid-century. The perception of outdoor heat by citizens can be quantified by thermo-physiological assessment indices like the physiologically equivalent temperature PET. In the daytime, the mean radiant temperature MRT represents a key variable for the assessment indices. It depends on the short- and long-wave radiant flux densities from the 3D environment, which is sometimes relatively heterogeneous within urban quarters. They can be simulated by radiation models for the urban environment like ENVI-met, RayMan or SOLWEIG. Another option is their direct measurement by use of a suitable setup. This excludes the application of a globe thermometer.

Based on results of experimental investigations in different quarters of Freiburg, a mid-size city in Southwest Germany, which were conducted on typical Central European summer weather from 2007 to 2010, the radiant flux densities from the 3D environment are analyzed with respect to their contribution to MRT outdoors in the daytime. The modern human-biometeorological approach to assess the human perception of heat, which can be regarded as state-of-the-art in urban human-biometeorology, relates to a standardized human-biometeorological reference person characterized by its standing position. Therefore, the horizontal radiant flux densities have a higher significance for the mean radiant temperature than the vertical ones. This provides the opportunity for urban planning to manipulate physical characteristics of vertical walls in order to obtain lower MRT values during daytime heat. Results of simulations by use of the ENVI-met model performed for a regular E-W street canyon (H/W = 1), however, show an increase of MRT with higher albedo values of the vertical walls due to the increasing short-wave radiant flux density reflected from the vertical walls.

Dependent on the sky view factor SVF, the contribution of the long-wave radiant flux densities to MRT in the daytime is distinctly higher as compared to the short-wave radiant flux densities. It varies between 70% for sunny sites and 95% for shaded sites. Due to the short-wave radiant flux density from the upper hemisphere, SVF for the southern part of the upper hemisphere turns out to be more suited for the analysis of shading effects than SVF for the whole upper hemisphere. The analysis of all experimental results in terms of mean values for the period 10 to 16 CET shows that a reduction of SVF for the southern part of the upper hemisphere by 10% through shading of direct solar radiation by tree canopies leads to a lowering of near-surface air temperature by 0.2 °C, MRT by 3.8 °C and PET by 1.4 °C.