Eta vertical coordinate

eta vertical coordinate

A vertical coordinate for atmospheric models defined with a steplike representation of topography, having mountains formed of grid boxes of the model.

The vertical coordinate surfaces are quasi-horizontal, intersecting model mountains or forming their nearly horizontal upper sides. It is defined by

where p is pressure; the subscripts T and S stand for the top and the ground surface values of the model atmosphere, respectively; z is geometric height; and p_ref(z) is a suitably defined reference pressure as a function of z. The ground surface heights z_S are permitted to take only a discrete set of values, those of elevations at which the interfaces of the chosen model layers are located when pressure is equal to the reference pressure p_ref(z). The "Eta Model" using the eta coordinate became the primary short-range operational forecasting model of the U.S. National Weather Service in 1993.
See sigma vertical coordinate, isentropic vertical coordinate, hybrid vertical coordinate, vertical coordinate system.

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