# Eta vertical coordinate

From AMS Glossary

## 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.