[CF-metadata] Vertical Coordinate Systems

From: Jonathan Gregory <j.m.gregory>
Date: Thu, 24 Jul 2003 18:42:42 +0100

Dear Juerg

> > I don't think p0 is the same
> > as ptop, though. p0 is simply a reference pressure which allows a(k) to be
> > dimensionless.
>
> I think p0 is very closely related to ptop. If one sets
> a(k) = 1-sigma(k),
> b(k) = sigma(k),
> then one sees immediately that the atmosphere_sigma_coordinate
> is a special case of the atmosphere_hybrid_sigma_pressure_coordinate.

Formally that is true, but in practice this isn't really the case. a is
zero near the surface, but it does not increase to 1 at the top. p0 is of
the order of the surface pressure e.g. 1e5 Pa. It is not similar to ptop.

> 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 {
...
> 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"
> }
>
> So the visualization package would have to analyze all
> variables mentioned in the coordinates attribute and
> see if there is one with the standard_name = "height"?

Yes. That doesn't seem hard to me. It is the same as inspecting the ordinary
coordinate variables to find the vertical coordinate. Since it is a vertical
coord var, Z should have a positive attribute, because it doesn't have units
of pressure. It should also have units="m".

Best wishes

Jonathan
Received on Thu Jul 24 2003 - 11:42:42 BST