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The model equations are discretized in space
and time and solved numerically by a semi-Lagrangian
advection scheme. It ensures stability and accuracy, while using as
large time-steps as possible to progress the computation of the
forecast within an acceptable time.
For the horizontal representation a
dual representation of spectral components and grid points is used.
All fields are described in grid point space. Due to the
convergence of the meridians, computational time can be saved by
applying a “reduced Gaussian grid”. This keeps the
east-west separation between points almost constant by gradually
decreasing the number of grid points towards the poles at every
latitude in the extra-tropics. For the convenience of computing
horizontal derivatives and to facilitate the time-stepping scheme,
a spectral representation, based on a series expansion of
spherical harmonics, is used for a subset of the prognostic
variables.
The vertical resolution is finest in
geometrical height in the planetary boundary layer and coarsest
near the model top. The “σ-levels” follow the
earth’s surface in the lower-most troposphere, where the
Earth’s orography displays large variations. In the
upper stratosphere and lower mesosphere they are surfaces of
constant pressure with a smooth transition in between.
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