[CF-metadata] Vertical Coordinate Systems

From: Brian Eaton <eaton>
Date: Sun, 27 Jul 2003 18:03:47 -0600

Hi Jonathan and Juerg,

I agree with Jonathan that we should not reformulate p0 as ptop in the
atmosphere_hybrid_sigma_pressure_coordinate. I don't think we should
reformulate any of the existing coordinates in appendix C. While there is
a certain appeal to a consistent general formulation for all the
coordinates, that is not a goal of these conventions. Usability is a goal
and it is easier for data writers to use formulas that they're familiar
with. That is why we provide two formulations for
atmosphere_hybrid_sigma_pressure_coordinate. There are large groups that
use each of the formulations given, so we agreed to disagree on which
formulation was "better" and included them both.

Juerg, I'm confused by your last example which contains both a height
coordinate variable and a height auxillary coordinate variable.

> Consider for example a model where the height of the model levels is a
> function of x and y. Thus the "generalised" height is a 3-dimensional
> auxillary coordinate variable, and not a coordinate variable.
>
> Is this what you would suggest?
>
> netcdf arp_general {
> dimensions:
> xc = 16 ;
> yc = 2 ;
> zlay = 60 ;
> time = 2 ;
> one = 1 ;
> variables:
> float xc(xc) ;
> xc:axis = "X" ;
> xc:long_name = "x-coordinate" ;
> xc:units = "m" ;
> float yc(yc) ;
> yc:axis = "Y" ;
> yc:long_name = "y-coordinate" ;
> yc:units = "m" ;
> float zlay(zlay) ;
> zlay:axis = "Z" ;
> zlay:standard_name = "height" ;
> zlay:units = "m" ;
> zlay:positive = "up" ;
> float time(time) ;
> time:axis = "T" ;
> time:units = "seconds" ;
> float U(time, zlay, yc, xc) ;
> U:long_name = "u wind component" ;
> U:units = "m s-1" ;
> U:coordinates = "Z" ;
> float Z(zlay, yc, xc) ;
> Z:standard_name = "height"
> }

A CF-compliant application would identify zlay as the vertical coordinate
variable since it has the positive attribute. But if you want the
auxillary coordinate variable Z to be recognized as containing height
coordinates, then zlay should not have the positive attribute and Z should
have both the positive attribute and the units of "m" as Jonathan
suggested.

Brian
Received on Sun Jul 27 2003 - 18:03:47 BST