Evaluation of vertically resolved longwave radiation in SPARTACUS-Surface 0.7.3 and the sensitivity to urban surface temperatures

Abstract. Cities materials and urban form impact radiative exchanges, and hence both surface and air temperatures. Here, the ‘SPARTACUS’ multi-layer approach to modelling longwave radiation in urban areas (SPARTACUS-Urban) is evaluated using the explicit DART (Discrete Anisotropic Radiative Transfer) model. SPARTACUS-Urban describes realistic 3D urban geometry statistically, rather than assuming an infinite street canyon. Longwave flux profiles are compared across an August day for a 2 km x 2 km domain in central London. Simulations are conducted with multiple temperature configurations, including realistic temperature profiles derived from thermal camera observations. The SPARTACUS-Urban model performs well (cf. DART) when all facets are prescribed a single temperature, with normalised bias errors (nBE) < 2.5 % for longwave downwelling at the surface, and < 0.5 % for the upwelling longwave at the top of the canopy. Errors are larger (nBE < 8 %) for the net longwave fluxes from walls and roofs. Using more realistic surface temperatures, which vary depending on whether a surface is sunlit, the nBE in upwelling longwave increases to ~2 %. Errors in roof and wall net longwave fluxes increase through the day, but still nBE are 8–11 %. This increase in nBE occurs because SPARTACUS-Urban represents vertical variation of surface temperature but not horizontal variations within a domain. We conclude that SPARTACUS-Urban accurately predicts longwave fluxes, requiring less computational time cf. DART, but with larger errors when surface temperatures vary because of being sunlit and/or shaded. SPARTACUS-Urban could enhance multi-layer urban energy balance schemes prediction of within-canopy temperatures and fluxes.