Abstract/Summary

Cities materials and urban form impact radiative exchanges, and 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 downwelling fluxes, and < 0.5% for top-of-canopy upwelling fluxes. Errors are larger (nBE < 8%) for net longwave fluxes from walls and roofs. Using more realistic surface temperatures, varying depending on surface shading, the nBE in upwelling longwave increases to ~2%. Errors in roof and wall net longwave fluxes increase through the day, but nBE are still 8–11%. This increase in nBE occurs because SPARTACUS-Urban represents vertical, but not horizontal, surface temperature variation within a domain. Additionally, SPARTACUS-Urban outperforms the Harman single-layer canyon approach, particularly in the longwave interception by roofs. We conclude that SPARTACUS-Urban accurately predicts longwave fluxes, requiring less computational time cf. DART, but with larger errors when surface temperatures vary due to shading. SPARTACUS-Urban could enhance multi-layer urban energy balance schemes prediction of within-canopy temperatures and fluxes.