Preprints
https://doi.org/10.5194/egusphere-2022-1002
https://doi.org/10.5194/egusphere-2022-1002
 
09 Jan 2023
09 Jan 2023
Status: this preprint is open for discussion.

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

Megan Alice Stretton1, William Morrison1,2, Robin Hogan1,3, and Sue Grimmond1 Megan Alice Stretton et al.
  • 1Department of Meteorology, University of Reading, Reading, UK
  • 2Chair of Environmental Meteorology, Faculty of Environment and Natural Resources, University of Freiburg, Freiburg, Germany
  • 3European Centre for Medium-Range Weather Forecasts, Reading, UK

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.

Megan Alice Stretton et al.

Status: open (until 06 Mar 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Megan Alice Stretton et al.

Model code and software

SPARTACUS-Surface Robin Hogan https://github.com/ecmwf/spartacus-surface

DART Jean Philippe Gastellu-Etchegorry https://dart.omp.eu/

Megan Alice Stretton et al.

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Short summary
Cities materials and form impact radiative fluxes. The SPARTACUS-Urban multi-layer approach to modelling longwave radiation, describing realistic 3D geometry statistically, is evaluated using the explicit DART (Discrete Anisotropic Radiative Transfer) model. Temperatures configurations used are derived from thermal camera observations. SPARTACUS-Urban accurately predicts longwave fluxes, with low computational time cf. DART, but has larger errors when sunlit/shaded surface temperatures are used.