Importance of 3-D radiant flux densities for outdoor human thermal comfort on clear-sky summer days in Freiburg, Southwest Germany

This study concerns the role of short- and long-wave radiant flux densities from different directions in complex urban settings for human thermal comfort on clear-sky summer days. The aims of the investigation are to quantify the importance of the sky view factor as an urban design-dependent variable for the 3-D radiant flux densities absorbed by the standardized human-biometeorological reference person and to analyze the varying impact of the absorbed 3-D short- and long-wave radiant flux densities on the mean radiant temperature (Tmrt$T_{\text{mrt}}$), near-surface air temperature (Ta$T_{\text{a}}$) and physiologically equivalent temperature (PET).The results obtained by measuring campaigns and numerical simulations point to the different importance of the absorbed 3-D radiant flux densities for human thermal comfort characterized by Ta$T_{\text{a}}$, Tmrt$T_{\text{mrt}}$ and PET. The magnitude of Tmrt$T_{\text{mrt}}$ is mainly determined by the total of the absorbed 3-D long-wave radiant flux densities. However, the fluctuations of Tmrt$T_{\text{mrt}}$ are mainly governed by the total of the absorbed 3-D short-wave radiant flux densities. Their variance can be well explained by the variance of the sky view factor related to the southern part of the upper half space. Taking account of the different impact of the 3-D radiant flux densities, Tmrt$T_{\text{mrt}}$ can be quite well estimated by a multiple regression using the total of the absorbed 3-D short-wave radiant flux densities and the absorbed long-wave radiant flux density from the lower half space as independent variables. PET can be well estimated by a multiple regression showing Tmrt$T_{\text{mrt}}$ and Ta$T_{\text{a}}$ as independent variables. On a hot summer day, the increase of the albedo of vertical building walls within a simple E-W oriented street canyon leads to a decrease of the surface temperature of the S-facing wall, but to an increase of Tmrt$T_{\text{mrt}}$ and PET at the S-facing sidewalk.