Quantification of Uncertainties of Radiative Transfer Calculation in Urban Canopy Models

Urban canopy models simplify urban morphology and physical processes such as radiative transfer to calculate the urban surface energy balance as a lower boundary condition for atmospheric models at low computational cost. The present study uses a reference model of urban radiative transfer based on the Monte Carlo method, which solves the radiative transfer equation by taking into account the complex geometry of buildings and vegetation. Procedurally-generated urban morphologies similar to the local climate zones (LCZ) are studied to cover the variety of urban forms that exist globally. The uncertainties arising from the simplification of the urban morphology as an infinitely-long street canyon or a regular array of square blocks are quantified. In addition, uncertainties due to the neglect of specular or spectral material reflectivities and the involved atmosphere are investigated. It is found that for all LCZ, the street canyon and block morphologies lead to a systematic overestimation (underestimation) of the fraction of solar radiation absorbed by the walls (ground). The neglect of pitched roofs has a strong influence on the simulated urban solar radiation budget for low solar elevation angles. Neglecting the spectral reflectivity of urban materials does not lead to relevant uncertainties in the broadband radiative fluxes. Specularly reflecting windows only change the urban solar radiation budget for a central business district morphology with a high glazing ratio. The participating atmosphere can strongly influence the urban terrestrial radiation budget, especially for high-rise districts. Future urban canopy models should therefore improve the realism of the urban morphology, and consider a participating atmosphere for the calculation of terrestrial radiation.