Sensitivity studies on the calculation of the radiation balance of urban surfaces: I. Shortwave radiation

A rigorous model is developed to calculate the shortwave radiation incident on building surfaces, based on radiation enclosure theory. When compared with field measurements, the model predictions are found to be associated with errors of approximately 10 W m^–2, with underestimations of 5–10 W m^–2 being caused on some walls by neglect of specular reflections. The effects of various simplifying assumptions are tested. The assumption of isotropic sky radiation and the neglect of multiple reflections are found to lead to errors of 15 W m^–2 or less; the neglect of horizon obstructions and the use of albedos taken from the literature instead of measured values can result in errors of 50 W m^–2 or more. Although these magnitudes of errors are site-specific, it is concluded that the use of simplifications in the modelling of shortwave radiation incident on urban surfaces must be approached with care.