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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.

Fig2111.AFig2.1.1.1-1: 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. 

Fig2111.BFig2.1.1.1-2: 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.

Fig2111.Fig2.1.1.1-C: 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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