Descriptions of WAM representations

Technical documentation

Model Level definitions

Gaussian grids

ECMWF versions of GRIB Table 2

Deficiencies of the archive

Discretisation of soil layers

Model id numbers

WAM representations

See also

Technical Site Map

The numerics of WAM are limited by the well-known CFL criterion which imposes a restriction on the size of the integration time step based on the reciprocal of the smallest spatial step. In regular spherical coordinates, both step increments are constant in longitude and latitude, for that reason, the zonal spatial step can become rather small (in actual distance) as one approaches the poles. To alleviate this restriction, a new type of lat-long grid was developed. It maintains a constant latitudinal increment but adjusts the size of the longitudinal increment in such a way that the actual distance between grid points is almost constant. Such a grid is commonly known as reduced or irregular lat-long grid.

This grid option was implemented in WAM. It involves the computation of the number of grid points used along each latitude circle. This number is obtained by multiplying the number of points at the equator (determined as before by the longitudinal step which defines the grid) with the cosine of the latitude and rounding it off in such a way that the same parity is kept as the equatorial one (to insure that the grid points are aligned along the 0 and 180 degree meridians. This number of points per latitude is used to determine the actual grid spacing and is given in the grib headers

Operational grid:

The global deterministic model currently uses the 0.5 degree irregular lat-long grid. This means that the grid points are set up such that they are spaced by 0.5 degree in latitudes or roughly 55km and due to the definition of the irregular grid (see above), they are spaced by roughly the same distance in longitudes.

Before the introduction of the 0.5 degree grid, the 1.5 regular grid was used. This means that the grid points were set up such that they were spaced by 1.5 degree in latitudes and longitudes

The current limited area model uses a 0.25 degree irregular grid and therefore the grid spacing is about 28 km. Its former version, also known as the Mediterranean model used a 0.25 regular grid.

The current EPS wave model uses a 1.0 irregular lat long grid.

18.10.2002