The ECMWF model uses two different numerical representations of global fields:

• A spectral method, based on a spherical harmonics expansion, truncated at a particular wave number, e.g. T1279 to identify truncation at wave number 1279. This representation is used in the model to compute some horizontal derivatives and for the implicit computations.
• The reduced Gaussian grid. This is based on the so-called "Gaussian" grid, which is regular in longitude and almost regular in latitude (Hortal and Simmons, 1991). The standard Gaussian grid has the disadvantage that the east-west distance between the grid points decreases polewards. To avoid some numerical problems around the poles and to save computing time, a reduced Gaussian grid was introduced in 1991, which reduces the number of grid points along the shorter latitude lines near the poles, so as to keep the east-west separation between points on different latitudes almost constant. This grid is used in the model for the computation of the advection terms and the physics tendencies.

Some of the fields are archived in spectral space (temperature, geopotential and wind) to save storage space. Other fields are archived as grid-point fields (moisture, cloud variables and surface fields). The MARS system can convert from spectral to grid-point space.

The grid point resolution and spectral truncation in the model are related. The table below shows the correspondence between spectral, reduced Gaussian and latitude/longitude resolution for some ECMWF products.

Other Gaussian grids used at ECMWF are described here.