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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 IFS medium range ensemble and HRES use HRES use 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.  Both the 10 day and 15 day ensembles use this grid. 

The IFS extended range ensemble uses a Gaussian grid (O320) which has 320 latitude lines between pole and equator with the number of grid points on each latitude line rising from 20 near the poles to 1296 near the equator giving a resolution of about 36km.

The IFS seasonal ensemble (SEAS5) uses a Gaussian grid (O320) which has 320 latitude lines between pole and equator with the number of grid points on each latitude line rising from 20 near the poles to 1296 near the equator giving a resolution of about 36km.

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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 IFS IFS HRES, ENS and Extended Range ENS models have 137 levels in the vertical; the four lowest levels are at 10m, 31m, 54m, 79m above the model surface.
• The IFS Seasonal model has 91 levels in the vertical; the four lowest levels are at 10m, 34m ,68m, 112m above the model surface.

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Fig2.1.1.1-2: The 137 level configuration used in HRES, ENS and Extended Range configurations.   Sigma levels are terrain-following at lower levels and become constant pressure levels for the upper troposphere and above.

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