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Horizontal resolution

In the horizontal, a dual representation of spectral components and grid points is used.  All fields are described in grid point space.  The grid is not completely uniform due to the convergence of the meridians towards the poles, and a Reduced Gaussian Octahedral Grid (Fig2.1.3) is now employed where used.  This means the separation between grid points is kept almost constant by gradually decreasing the number of grid points towards the poles at all extratropical latitudes. In  In effect, within each quadrant, two grid boxes (triangles) are removed as one steps away from the equator to the next latitude row, two grid boxes (triangles) are removed.  This equates to a reduction of one grid point per quadrant per latitude row (Fig2.1.3).  This grid point configuration results in a saving in computational time.   For  For the convenience of computing horizontal derivatives and to facilitate simplify the time-stepping scheme, a A spectral representation, based on a series expansion of spherical harmonics, is used for a subset of the prognostic variables, namely, surface pressure, temperature and winds, . moist variables and cloud variables are never transformed to spectral space.

A spectral representation, based on a series expansion of spherical harmonics, is used for a subset of prognostic variables (surface pressure, temperature and winds).  Moisture and cloud variables are not transformed into spectral space.

The HRES IFS model uses a Gaussian grid (O1280) which has 1280 latitude lines between pole and equator with the number of grid points on each latitude line rising from 20 near the poles to 5136 near the equator giving a resolution of about 9km.

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