The U.K. Meteorological Office (UKMO) has been developing a high
vertical resolution local or site-specific 1D model to capture
flow features unique to a particular location situated anywhere
within a Mesoscale model grid-box. The motivation for this
development is that currently the UKMO Mesoscale model provides
operational short range forecasts for aggregated
areas/grid-boxes of the order of 15km x 15km. This resolution is
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. The site-specific model contains the same
physical parametrizations as the UKMO Unified Model with
improvements to the treatment of the surface exchange via the
use of a `tiled' approach 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.
This paper briefly describes and attempts to validate the
methods employed to deduce the local flow and forcing within the
Site-Specific model via correct diagnosis of upwind land surface
characteristics. In particular the coupling of a tile surface
exchange scheme with a Gaussian source area model (SAM). The
motivation for this coupling stems from the assumption made
by UKMO surface exchange schemes, both the single surface scheme
based upon effective parameters and the multi-surface or `tiled'
scheme, that all upwind surfaces contribute equally to the
area-average flux representative at a specific site. The
SAM, based upon the local static-stability, diagnoses a
distance weighting function in order to deduce the upwind fetch
relevant for the aggregation of surface fluxes within the tile
scheme. This is in addition to calculating the relative
contribution of each land-use type within the fetch to the
aggregation process. 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.