Correction and downscaling of NWP wind speed forecasts

NWP models typically parametrize the effects of unresolved orography, often through use of an effective (orographic) roughness. Whilst this parametrization realistically models the orographic drag on the synoptic-scale flow, it creates two problems for the assimilation of wind observations from high ground. First, the artificially increased surface stress causes a reduction in the predicted wind speed at the standard wind observing height of 10 m, and second, the speed-up over the unresolved summits is not modelled. A method is described for reconciling observed and modelled wind speeds. The method is based on the linear theory of neutral boundary-layer flow over hills and includes a resolution of both the problems described above. The method is applied to both the assimilation of observations and the creation of an improved 10 m wind analysis. The method has been on trial in the Met Office's nowcasting system; significant improvements are demonstrated, particularly during strong wind events. The simplified model presented here is not claimed to represent the full complexities of the boundary layer, but nevertheless produces computationally cheap, low-level wind forecasts, which are a significant improvement on the existing output from the Unified Model. (C) Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd.