[CF-metadata] Need for a new vertical coordinate definition:"Ocean generalized sigma coordinate"

[Rich Signell]

Steve Hankin wrote:

>As Rich says, getting this "right" is very important to the HYCOM modeling
>community and others.  Outputting the Z field as a 4-dimensional variable has nasty
>consequences in file sizes, access demands, etc.
>  
>
Don't you need to specify Z as a 4D variable in a model like HYCOM, 
where the vertical coordinate depends on density information that may 
not even be saved in the output file?    This isn't that nasty, is it?  
The  Z field just becomes the same size as other 4D fields like 
potential temperature, salinity, etc.

>My gut sense is that we're conflating two issues here.  One is the idea of
>time-dependent sigma levels -- as are used in HYCOM.  I have not been aware that we
>were abusing the CF conventions by allowing the sigma levels to be time-dependent.
>Doing so seemed to be completely in the spirit of CF and netCDF.  (I haven't gone
>back just now to review the actual words from the CF document.)  No mods are
>necessary; the time dimension is self-explanatory.
>  
>
This seems to be agreeing with Brian's earlier statement that saving 4D 
fields of Z are allowed already, enabled by the "coordinates" 
attribute.   Can one of you guys give an example of how this would be 
done?  I have only seen the "coordinates" attribute used to specify 
two-dimensional longitude and latitude fields.  

>The second idea is of "generalized sigma coordinates", where sigma levels are a
>fraction of (depth+eta).  If one asserts (true?, false?) that such coordinate
>systems represent a significant class of models, then there should be a serious
>discussion of whether CF should support them.  It **sounds** as if support would
>require only the addition of a single attribute,
>
>    sigma: fraction_of = depth
>or
>    sigma: fraction_of = depthPlusFreeSurface
>  
>
All sigma models I am aware of represent layers as fractions of the 
total_water_depth = depth + eta, never just the depth.

-Rich

-- 
Richard P. Signell           rsignell at usgs.gov
U.S. Geological Survey       Phone: (508) 457-2229
384 Woods Hole Road          Fax:   (508) 457-2310
Woods Hole, MA  02543-1598