Geoscience Reference
In-Depth Information
explicitly when their scale is larger than the grid or resolution of the model. For
those phenomena with a scale smaller than the model's grid, it is necessary to set
physical parameterizations. The resolution of a model is determined by the size of
the considered geographical area, the lead time of the prediction, the relevance of
physical assumptions made and the available computer resources. This means that
different types of models are used, depending on the objective. For example, the
hydrostatic approximation is no longer valid on a small scale. Atmospheric
processes that have non-hydrostatic effects include surface and atmospheric heat and
moisture fluxes, turbulence, convection, evaporation, and condensation. For features
less than 10-20 km, meteorological models must incorporate these non-hydrostatic
processes.
Figure 4.3.
Spatial and temporal scales of the main meteorological phenomena
As an example, the resolution of operational weather forecast models is typically
several tens of kilometers, whereas the resolution of a limited-area model is only a
few kilometers, and the resolution of a global climate model, used e.g. to simulate
the climate of one century, ranges from 200 to 300 km.
The geographical information that is to be used will therefore have to be adapted
to these different resolutions. A value that is representative of a particular average
on the model's grid as well as a value that describes the sub-grid variations will also
have to be provided.
4.3. Brief overview of numerical weather forecast models
A numerical weather forecast model is a complex system comprising different
phases and tools:
