some atmospheric models use a hybrid sigma-pressure coordinate scheme, combining sigma-denominated layers at the bottom (following terrain) with isobaric (pressure-denominated) layers aloft. the isobaric upper layers are generally more numerically tractable (since flatter), and specifically more tractable for radiative transfer calculations (important for assimilating satellite radiance observations). some models (e.g., the 2009 nam) have a pure sigma domain at the bottom and a fixed transition level, above which all layers are exactly isobaric. other models (e.g., gfs) gradually transition from sigma to isobaric.