Since the data is already in spherical harmonics it is only necessary to truncate and then convert the fields to grid point space. Hybrid Levels Hybrid, or Model, levels are a type of vertical level which is a combination of the two standard notations, Pressure and Sigma levels.
Pressure levels represent the data on constant pressure surfaces, eg.
Sigma levels represent the data on a surface which is some factor multiplied by the surface pressure, eg. 0.63, and as such are terrain following fields. Hybrid levels have a sigma and a pressure component either one of which may be 0. At low altitudes, the levels tend to have a strong sigma component to follow the geography and avoid below ground effects. At high altitudes, the sigma component often goes to 0, leaving pure pressure levels, since the geography effects are limited or non-existent at those altitudes. At truncations less than T106, a taper is applied to the spherical harmonics to reduce the ringing effects around geography.