The Eta () coordinate

The Eta coordinate

The Eta vertical coordinate is used in Canadian numerical models. A demonstration of this coordinate is shown as red lines in the diagram below. To simplify the diagram, only 8 Eta levels are shown. In reality, our Canadian models consider many more of these levels during their calculations.

The black section at the bottom represents the topography of the landscape while the pressure in the atmospheric column is given along the vertical axis.

As shown by the red curves below, the Eta coordinate ranges in value from 1 at the surface of the earth and decreases to a value of 0.0 at the top of the atmosphere as defined by the computer model. 

The Eta coordinate is defined by the following equation:

,

where p is the pressure at a particular level in the atmosphere, is the surface pressure below p, and is the pressure at the top of the atmosphere above p. The top of the atmosphere is defined in current Canadian GEM models to be 10 hPa.   

Top of page