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
Received on Wed Dec 22 2004 - 12:28:16 GMT
