Vertical Coordinates » Hybrid coordinates

Hybrid vertical coordinate systems combine two coordinate system in a seamless manner, between terrain following coordinates near the surface to constant geopotential surfaces in the upper atmosphere. The levels and related equations shown are for the ACCESS model, where z_i is the geopotential height above mean sea level of model level i, z_T is the geopotential height at the top of the model, z_S is the terrain height above mean sea level, η_i is the "eta" value of model level i and ηInterface is the "rho"-level η value at i = Interface.

Models like the ECMWF use sigma and pressure levels.