hybrid S-Z coordinates?

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esatel

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Aug 15, 2012, 7:15:29 PM8/15/12
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Hi all,
Does anyone have a suggestion as to how to represent hybrid coordinates (S-Z, with Z in the bottom) in the new standard as in SELFE? I have seen posting that included this as a topic but I did not see any resolution of the issue.
 
Thanks,
Eli

Bert Jagers

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Aug 16, 2012, 3:01:56 AM8/16/12
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Hi Eli,

The new standard uses the same conventions for the vertical coordinate
as the standard CF-conventions.
This means that you have basically two options:
1) Explicitly specify the vertical coordinates for all points and
layers, e.g. z(nnodes,nlayers).
2) Use a CF formula for the vertical coordinates; do you mean S or
sigma? If you mean sigma then the ocean_sigma_z_coordinate does what you
need. If you mean S then you need to contact the CF community to define
a convention for ocean_s_z_coordinate.

Cheers,

Bert
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Bert Jagers

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Aug 16, 2012, 3:12:50 AM8/16/12
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Hi all,

Triggered by Eli's question I just noticed that the
ocean_sigma_z_coordinate requires that the layers are numbered positive
down (since the layers k<=nlayers are defined to be on top; see the
section on
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/apd.html). How
common is this numbering convention actually? We have a long lasting
in-house discussion about layer numbering order. We have in-house model
codes that

a) count layers positive increasing down (k = 1 at surface, k = kmax
near bed) - sigma layer code
b) count layers positive increasing up (k = 1 bottom most layer, k =
kmax topmost layer) - z layer code

and I remember that other codes may

c) count layers negative increasing up (k = -kmax near bed, k = -1 at
surface)

- For simulation engines: what is the convention are you using in your
code? And what do you write to NetCDF file?
- For post-processing tools: does it matter for you what convention is used?

Best regards,

Bert

esatel

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Aug 16, 2012, 5:40:30 PM8/16/12
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Thanks, This makes sense.

Rich Signell

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Aug 20, 2012, 12:06:50 PM8/20/12
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Folks,
Just be aware that not all the CF-compliant vertical coordinates are
currently coded into some of the libraries that take advantage of CF
-- in particular, I think the latest NetCDF-Java only implements the
ocean_sigma_coordinate and ocean_s_coordinate for ocean coordinates,
so the sigma-over-z may need to be added to the code if you are trying
to use NetCDF-Java dependent toolboxes like NCTOOLBOX. The good
news is that this is pretty easy, and Unidata can usually get stuff
added quickly to the next release....

-Rich
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Rich Signell

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Aug 20, 2012, 1:02:15 PM8/20/12
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From my experience, the layer convention for stretched vertical
coordinate models seems pretty evenly split:

FVCOM, HYCOM, POM: 1st layer is surface
ROMS, SELFE: 1st layer is bottom

-Rich
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