Validation of the mean radiant temperature simulated by the RayMan software in urban environments

The RayMan software is worldwide applied in investigations on different issues in human-biometeorology. However, only the simulated mean radiant temperature (T (mrt)) has been validated so far in a few case studies. They are based on T (mrt) values, which were experimentally determined in urban environments by use of a globe thermometer or applying the six-directional method. This study analyses previous T (mrt) validations in a comparative manner. Their results are extended by a recent validation of T (mrt) in an urban micro-environment in Freiburg (southwest Germany), which can be regarded as relatively heterogeneous due to different shading intensities by tree crowns. In addition, a validation of the physiologically equivalent temperature (PET) simulated by RayMan is conducted for the first time. The validations are based on experimentally determined T (mrt) and PET values, which were calculated from measured meteorological variables in the daytime of a clear-sky summer day. In total, the validation results show that RayMan is capable of simulating T (mrt) satisfactorily under relatively homogeneous site conditions. However, the inaccuracy of simulated T (mrt) is increasing with lower sun elevation and growing heterogeneity of the simulation site. As T (mrt) represents the meteorological variable that mostly governs PET in the daytime of clear-sky summer days, the accuracy of simulated T (mrt) is mainly responsible for the accuracy of simulated PET. The T (mrt) validations result in some recommendations, which concern an update of physical principles applied in the RayMan software to simulate the short- and long-wave radiant flux densities, especially from vertical building walls and tree crowns.