From Glossary of Meteorology
The quantity gz + cpT measured on an isentropic surface where g is the acceleration of gravity, z the height of the isentropic surface, cp the specific heat of air at constant pressure, and T the Kelvin temperature; or it is sometimes defined as the same quantity divided by the Coriolis parameter.
This is the streamfunction for the geostrophic wind on the isentropic surface.
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