What is the connection between the spectral truncation and the Gaussian grids - Metview FAQ

The IFS model uses a spherical harmonic expansion of fields, truncated at a particular wave number. For example T1279 identifies truncation at wave number 1279.  Each spectral truncation is related to a regular Gaussian grid, which is regular in longitude and almost regular in latitude. Gaussian grids are described by the number of latitude bands between the Equator and the poles. For example the N640 regular Gaussian grid has 1280=2*640 gridpoints in latitude and it corresponds to the T1279 spectral truncation. There is also the reduced Gaussian grid, which uses the same latitudes as the regular Gaussian grid but 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.

The table below shows the correspondence between spectral, Gaussian and latitude/longitude resolution for some ECMWF products.

¹for reduced Gaussian grids this is only valid for latitude