Radiative transfer over resolved topographic features in a high resolution weather prediction model

The traditional approach for radiation schemes in a numerical weather prediction model is to use a plane parallel approximation with a flat surface. At high resolutions this can lead to errors in the surface radiation budget. The orientation of the surface slope with respect to the sun and the effect of surrounding terrain can have a significant effect on the amount of radiation received at the surface. The interaction of radiation with the complex terrain leads to a transfer of energy between surface features which poses a problem for the independent column approximation. It is possible to include these effects without explicitly modelling the transfer of energy by making use of the "sky-view factor" from a given grid-box, but this must be done carefully to maintain conservation of energy.

The Met Office UK regional forecast model uses a resolution of 1.5km and includes a parametrisation of the effect of surface slope on the direct shortwave radiation. This parametrisation is now being extended to include the effects of sky-view factor and surface slope on the net longwave radiation. Here the extra surface emission due to the increase in surface area is balanced by the use of a sky-view factor convolved with the angle of incidence on the sloping surface. This leads to conservation of energy over an extended region (assuming constant temperature) which may be demonstrated by solving analytically for a 2D semi-circular aperture.

Here the scheme will be presented along with results of its impact in the 1.5km forecast model.