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Radiative transfer over resolved topographic features for high‐resolution weather prediction
Author(s) -
Manners J.,
Vosper S. B.,
Roberts N.
Publication year - 2011
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.956
Subject(s) - terrain , radiative transfer , numerical weather prediction , sky , grid , radiation , environmental science , meteorology , atmospheric radiative transfer codes , remote sensing , geology , physics , geodesy , optics , geography , cartography
Abstract Regional numerical weather prediction models are now routinely run at near kilometre‐scale resolutions where surface features are well resolved and mean grid‐box slopes are highly significant. The resolved topography has an important effect on the radiative transfer at the surface which is not generally represented using fast two‐stream radiation codes. This paper presents a parametrisation of surface radiative transfer for resolved topography suitable for use with a two‐stream code. For the short‐wave bands, the first‐order effects are the interaction of direct solar radiation with surface slopes and shading by surrounding terrain. In the infrared (long‐wave bands), the first‐order effect is the transfer of radiation between visible points on the terrain making use of the sky‐view factor for a sloping surface and the enhanced area of the slope. Energy is conserved over an extended region where the mean flux into the surface will be independent of resolution. These effects are included in the radiation scheme of the Met Office Unified Model and case‐studies are run over the UK using horizontal grid resolutions from 100 m to 1.5 km. The scheme adds fine detail to the forecast which is shown to have a significant impact on forecast skill. Under clear‐sky conditions, short‐wave surface effects can lead to temperature differences of up to 2.5 K, and long‐wave effects to differences of up to 1 K. The magnitude of these effects are expected to be much greater for more complex terrain. A consistent extension to the scheme is outlined to take account of second‐order effects using short‐wave diffuse albedo and long‐wave surface emissivity. These lead to a net change in the surface flux due to multiple surface reflections and should be considered in conjunction with a treatment of sub‐grid terrain complexity. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.