The U.K. Meteorological Office (UKMO) has, for nearly a decade, provided operational short range forecasts via a Mesoscale model for aggregated areas/grid-boxes of the order of 15km x 15km. This resolution is still too coarse to accurately forecast conditions that are dominated by strong local forcing e.g. overnight fog or frost, and the projected future increases in computer power suggest it will be a decade before truly site-specific forecasts will be achievable from 3D models. Therefore a need was identified to develop a high vertical resolution local or site-specific 1D model that could capture the flow features unique to a particular location situated anywhere within a Mesoscale grid-box. This site-specific model contains the same physical parametrizations as the UKMO Unified Model with improvements to the treatment of the surface exchange and soil. The large-scale flow and forcing are defined by coupling to the operational Mesoscale model, yet its local flow and forcing is provided by interaction with the surface characteristics (heterogeneity) found within the upwind fetch of the chosen site. A first version of the model has been running since October 1996 and these changes have already provided a significant improvement in site-specific forecast skill over the Mesoscale model.
The parametrization of surface exchange within each grid-box of the UKMO Unified Model has been via a single surface scheme where surface heterogeneity was described by the generation of effective surface characteristics. These produce the correct spatially averaged or grid-box surface fluxes whilst retaining the similarity forms applicable to homogeneous terrain. This paper describes and attempts to validate the replacement of the single surface scheme based upon effective parameters with a multi or `tiled' surface exchange scheme methods employed to deduce the local flow and forcing within the Site-Specific model. The `tile' approach calculates surface fluxes in the surface layer independently on a land-use patch-by-patch basis over land. Its main benefit over effective parameters is regions, such as partly forested areas, where the stability differs between land-use patches (tiles).Validation of the above scheme will be presented via intercomparison with observational data from a field campaign conducted in a partially forested area of Eastern England (Sherwood Forest) during early summer 1994. These observations consist of surface based mean and flux measurements over trees and grass plus profile mean and flux measurements up to a height of 600m from a tethered balloon system.