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Many prognostic variables are evaluated and calculated on the grid.  However, a subset of prognostic variables (surface pressure, temperature, winds and moisture) are calculated using a spectral representation.  This is for the convenience of computing horizontal derivatives and to assist effectiveness in the time-stepping scheme.  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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The vertical resolution varies with geometric height.  The vertical resolution is greatest (most fine) in the planetary boundary layer while more coarse near the model top.  The “σ-levels” follow the earth’s surface in the lower layers of the troposphere, where the Earth’s orography has large variations, but in the upper stratosphere and lower mesosphere they are surfaces of constant pressure.  There is a smooth transition from “σ-levels” to pressure levels between lower and upper levels.

The HRES*,  ENS and Extended Range ENS IFS models have 137 levels in the vertical; the four lowest levels are at 10m, 31m, 54m, 79m (geometric altitude).  The Seasonal IFS model has 91 levels in the vertical; the four lowest levels are at 10m, 34m ,68m, 112m.  These are approximate geopotential heights, but are referenced to the surface pressure (not MSL).  For correct geopotential height (with respect to MSL) the height of the orography must be added.

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Fig2.1.4A: The 137 level configuration used in HRES*, ENS, and Extended Range configurations of the IFS.   Sigma levels are terrain-following at lower levels and become constant pressure levels for the upper troposphere and above.

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