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.