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Fig. 13.6
Vertical disposition of variables in the ECMWF grid point model. (After Simmons et al.,
1989.)
In this case, however, the vertical average of is required since, as indicated in
Fig. 13.6, is staggered vertically with respect to u, v, T . The complete finite
difference equations for this model, including the required special treatment of
polar regions, are quite complex and will not be given here. They are discussed in
Simmons et al. (1989).
The vertical finite differences in this model are defined so that the model can
be run in either the σ coordinates discussed in Section 10.3.1 or in a hybrid coor-
dinate system in which the coordinate varies from pure sigma coordinates in the
troposphere to isobaric coordinates in the stratosphere.
13.6.2
Spectral Models
ECMWF and most other operational forecast centers utilize spectral models as their
primary global forecast models. Grid point models are generally employed for fine
scale limited area models and may become more common for very high resolution
global models because the number of operations increases linearly with the number
of grid points, but increases as n log n in a spectral model (where n is the number
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