[CF-metadata] Ocean sigma coordinate

Rich Signell rsignell at usgs.gov
Tue Oct 14 14:52:00 MDT 2008


There are at least 4 different ways that I see to deal with your
non-standard sigma issue:

1. You could subtract 1 from the values of the sigma variable you
currently store in your NetCDF file.

2. If it's important to keep your old non-compliant sigma variable,
you could just add a new CF-compliant sigma variable to your NetCDF
file.   Then add the "standard_name = ocean_sigma_coordinate" to this
new variable (and assign it values that are one less than in your old

3. If your goal is just to have your vertical coordinate understood by
NetCDF-Java (compatible with Unidata's CDM), you could use the
"explicit_field" and provide the 4D field for z.
See details at:

4. Propose a new dimensionless coordinate called
"ocean_sigma_coordinate2" to the CF standards committee.

Good luck,

-Rich Signell

On Fri, Oct 10, 2008 at 1:28 PM, Stephane TAROT
<Stephane.Tarot at ifremer.fr> wrote:
> Dear all,
> For a french project of coastal oceanography (Previmer :
> http://www.previmer.org/en), I have to define
> a netcdf file format for our gridded products (in fact, that format exists,
> but needs to be improved). Of
> course, I want it to be compliant with CF1.3.
> I have a problem with some data with dimensionless vertical coordinates. The
> model (called MARS) is
> using an ocean sigma coordinate which defnition is :
> sigma = (z + depth)/(eta + depth)
> This is different from the definition of the CF ocean_sigme_coordinate
> (appendix D.6) :
> z = eta + sigma(depth + eta)   or   sigma=(z - eta)/(eta + depth)
> In our case, at the sea surface (z=eta) sigma=1, and at the sea floor
> (z=-depth) sigma=0.
> In the CF case, at the sea surface (z=eta) sigma=0, and at the sea floor
> (z=-depth) sigma=-1.
> How can I deal with that ?
> Thanks for your help
> Stephane Tarot
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Dr. Richard P. Signell   (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598

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