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Horizontal resolution

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 (Fig2Fig2111.1.3A) is 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 effect, within each quadrant, two grid boxes (triangles) are removed as one steps away from the equator to the next latitude row.  This equates to a reduction of one grid point per quadrant per latitude row.  This grid point configuration results in a saving in computational time.

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The heights approximate to geopotential heights, but are referenced to the surface pressure (not mean sea level).  For correct geopotential height with respect to mean sea level the height of the orography must be added.

Fig2Fig2111.1.3A: The Reduced Gaussian Octahedral Grid used by IFS.  Broadly, it is derived from a projection from an enclosing octahedron onto the earth.  A reasonably consistent grid spacing (dx) is maintained, even towards the poles. The apex of the octahedron is truncated.  Colours on the globe show the resolution of the grid. 

Fig2Fig2111.1.4AB: The 137 level configuration used in ENS and Extended Range configurations.   Sigma levels are terrain-following at lower levels and become constant pressure levels for the upper troposphere and above.

Fig2Fig2111.1.4BC: The 91 level configuration used in the Seasonal configuration.   Sigma levels are terrain-following at lower levels and become constant pressure levels for the upper troposphere and above.

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