Radiative transfer over resolved topographic features for high‐resolution weather prediction

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.

[1]  R. Vogt,et al.  Surface radiation budget in an Alpine valley , 2003 .

[2]  Michael Lehning,et al.  Radiosity Approach for the Shortwave Surface Radiation Balance in Complex Terrain , 2009 .

[3]  J. Dozier,et al.  Rapid calculation of terrain parameters for radiation modeling from digital elevation data , 1990 .

[4]  Jeff Dozier,et al.  Topographic distribution of clear‐sky radiation over the Konza Prairie, Kansas , 1990 .

[5]  Dieter Scherer,et al.  A Grid- and Subgrid-Scale Radiation Parameterization of Topographic Effects for Mesoscale Weather Forecast Models , 2005 .

[6]  Richard Essery,et al.  Scaling and parametrization of clear-sky solar radiation over complex topography , 2007 .

[7]  C. Whiteman,et al.  Topographic Effects on the Surface Radiation Balance in and around Arizona’s Meteor Crater , 2010 .

[8]  R. Dubayah.  Estimating net solar radiation using Landsat Thematic Mapper and digital elevation data , 1992 .

[9]  Jean-Claude Thelen,et al.  Two fast radiative transfer methods to improve the temporal sampling of clouds in numerical weather prediction and climate models , 2009 .

[10]  L. Hole,et al.  Implementation of slope irradiance in Mesoscale Model version 5 and its effect on temperature and wind fields during the breakup of a temperature inversion , 2003 .

[11]  R. Dubayah,et al.  Modeling Topographic Solar Radiation Using GOES Data , 1997 .

[12]  Peter Clark,et al.  COLPEX: Field and Numerical Studies over a Region of Small Hills , 2011 .

[13]  N. Roberts,et al.  Scale-Selective Verification of Rainfall Accumulations from High-Resolution Forecasts of Convective Events , 2008 .

[14]  Hannu Savijärvi,et al.  Parametrization of orographic effects on surface radiation in HIRLAM , 2007 .

[15]  Rachel Spronken-Smith,et al.  Spatial Variability of Surface Radiation Fluxes in Mountainous Terrain , 2003 .

[16]  A. Slingo,et al.  Studies with a flexible new radiation code. I: Choosing a configuration for a large-scale model , 1996 .

[17]  Po-Hsiung Lin,et al.  Parameterization of topographic effect on surface solar radiation , 2010 .

[18]  Alex Hall,et al.  Application of three-dimensional solar radiative transfer to mountains , 2006 .