Coupling the urban canopy model TEB (SURFEXv9.0) with the radiation model SPARTACUS-Urbanv0.6.1 for more realistic urban radiative exchange calculation

Abstract. The urban canopy model TEB is coupled with the radiation model SPARTACUS-Urban to improve both the urban geometry simplification and radiative transfer calculation. SPARTACUS-Urban assumes that the probability density function of wall-to-wall and ground-to-wall distances follows a decreasing exponential. This matches better the distributions in real cities compared to the infinitely-long street canyon employed by the classical TEB. SPARTACUS-Urban solves the radiative transfer equation using the discrete ordinate method. This allows to take into account physical processes like the interaction of radiation with air in the urban canopy layer, spectral dependency of urban material reflectivities, or specular reflections. Such processes would be more difficult to account for with the radiosity method used by the classical TEB. With SPARTACUS-Urban, the mean radiant temperature, a crucial parameter for outdoor human thermal comfort, can be calculated using the radiative fluxes in vertical and horizontal direction incident on a human body in the urban environment. TEB-SPARTACUS is validated by comparing the solar and terrestrial urban radiation budget observables with those simulated by the Monte-Carlo-based HTRDR-Urban reference model for procedurally-generated urban districts mimicking the Local Climate Zones. An improvement is found for almost all radiative observables and urban morphologies for direct solar, diffuse solar, and terrestrial infrared radiation. TEB-SPARTACUS might therefore lead to more realistic results for building energy consumption, outdoor human thermal comfort, or the urban heat island effect.