Vertical coordinate
IFS uses a hybrid sigma coordinate vertical coordinate, η, in which the lowest layers of the model are pure so-called 'sigma' levels, whilst the topmost model levels are pure pressure levels. The hybrid scheme provides for a smooth transition between the two. 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. Pure sigma levels follow the surface topography in the lower layers of the troposphere and ensure no levels intersect the surface.
A characteristic of the hybrid scheme is that the spacing between levels is less over steep orography than at the surface.
The vertical coordinate, η, is monotonic function of the pressure, p, and surface pressure, p s, such that:
η(0,p s) = 0 and η(p s, p s) = 1
The atmosphere in IFS is divided into layers. The pressure at the interfaces between these layers are known as 'half-levels' and are defined by:
p k+½ = A k+½ + p s.B k+½
where p s is the surface pressure.
The A k+1/2 and B k+1/2 coefficients are constants that determine the vertical coordinate. They are stored in the GRIB header of all the fields output from IFS and can be found in the tables listed below for each of the supported vertical configurations of IFS.
The 'full-level' pressure, p k, associated with each model layer (i.e. the middle of the layer) is defined as:
p k = 0.5 ( p k-½ + p k+½ )
The IFS prognostic variables are represented by their values at the 'full-level' pressures p k.
Vertical discretization
IFS uses a vertical finite-element scheme for improved accuracy and reducing numerical noise from the vertical discretization. In the finite element scheme, all variables (including pressure) are stored on the full model levels.
For more details of the vertical scheme, please refer to the references at the bottom of this page.
Vertical configurations
IFS supports a number of vertical configurations, though the mostly commonly used are the 60, 91 and 137 level configurations. Note that the top model pressure level varies between these configurations.
| Model level definition | Correspondence | L91 schematic illustration |
|---|---|---|
| L137 model levels | L91 to L137 levels |
|
| L91 model levels | L60 to L91 levels | |
| L62 model levels | L40 to L62 levels | |
| L60 model levels | ||
| L50 model levels | ||
| L40 model levels | ||
| L31 model levels | ||
| L19 model levels |
References
See IFS Documentation Part III - Chapter 2.
Simmons A.J., and D.M. Burridge, 1981, An energy and angular momentum conversing vertical finite difference scheme and hybrid vertical coordinates, Mon. Wea. Rev., 109, 758-766.
Untch, A. and Hortal, M., 2004. A finite-element scheme for the vertical discretization of the semi-Lagrangian version of the ECMWF forecast model. Q. J. R. Meteorol. Soc., 130, 1505–1530.
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