Date: Thu, 23 Aug 2001 15:31:25 +0200
From: Rich Signell 
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To: Brian Eaton 
CC: Charlie Barron , Paul Martin , Patrick Haley , Hernan Arango , Tal Ezer , "netcdfgroup@unidata.ucar.edu" 
Subject: Re: CF conventions for common ocean models
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Congrats to the CF Convention team!

As an oceanographic modeler who has been a NetCDF advocate for a long
time but been burned by the lack of netcdf conventions among similar
models,  I was very pleased to see the CF conventions.  If adopted,
these should finally allow us to share analysis and visualization tools
efficiently with a variety of different atmospheric and oceanographic
models. 
  
For the dimensionless vertical coordinate description, those of us
who are using ocean models that could be handled by CF need to get our
"standard names" and formulas down and submitted now so that tools 
can be built to work with our model outputs.  Of course, this assumes
that we have written our netcdf output files using the CF conventions
as well, but we will have plenty of motivation after the great tools
are built.   ;-)

Here is a stab at the vertical coordinate expressions for
some of the models I am familiar with.  The choice of the "standard_name"
names I just picked out of the air. 


1) The Princeton Ocean model is probably one of the most popular ocean
models, and
uses a sigma coordinate system with extends from the free surface of
the ocean (which varies in lon,lat and *time*) to the sea floor.  So here
the vertical coordinate actually changes with time.  Denoting the time
index as "n", the vertical coordinate info might be defined as:

standard_name="POM_sigma_free_surface" 

definition:  z(i,j,k,n) = eta(i,j,n) + sigma(k) * (depth(i,j) + eta(i,j,n))


2) The Rutgers/UCLA ROMS model uses an "S-coordinate", which is similar
to POM, but with some additional parameters to exert control over the
distribution
of layers so that each layer need not be a fixed percentage of the water
column:

standard_name="ROMS s-coordinate" 

definition: z(i,j,k,n) = eta(i,j,n) * (1 + sigma(k)) +
                         depth_c * sigma(k) +
                         (depth(i,j) - depth_c)* C(s)

                 where C(s) = 
                         (1-b)*sinh(a*sigma(k))/sinh(a) + 
                         b*(tanh(a*(sigma(k)+0.5))/(2*tanh(0.5*a))-0.5)

 (Note: depth_c, a, and b are constants that control the stretching)


3) The Navy NCOM model uses a hybrid sigma over z coordinate, so that above
a
certain depth "depth_c" there are "nsigma" layers, and then below "depth_c"
there are "nz" z layers.

standard_name= "NCOM_sigma_over_z" 

definition: 

 for k <= nsigma:

  z(i,j,k,n) = eta(i,j,n)*sigma(k)*(min(depth_c,depth(i,j))+eta(i,j,n))
 

 for k > nsigma:

  z(i,j,k,n) = zlev(k)


4) The Harvard Ocean Prediction System uses a double sigma coordinate system 
with a rigid lid

standard_name = "HOPS double-sigma"

for k <= k_c

z(i,j,k)= sigma(k)*f(i,j)

for k > k_c

z(i,j,k)= f(i,j) + (sigma(k)-1)*(depth(i,j) - f(i,j))

where f(i,j)= 0.5*(z1+ z2) + 0.5*(z1-z2)* tanh(2*a/(z1-z2)*(h(i,j)-href))

and "z1","z2","a" and "href" are constants
that define the characteristics of the double sigma system.


These are just a few that I know about, but it seems it would be wise for
the modeling community to
get as many "standard_names" for vertical coorindate dimensions defined
early, before the first official release of CF!

Thanks,

Rich 

--
Dr. Richard P. Signell                 |  signell@saclantc.nato.int
NATO/SACLANT Undersea Research Centre  |    Tel: (+39) 0187 527 381
Viale San Bartolomeo 400               |    Fax: (+39) 0187 527 331
19138 La Spezia, ITALY  --> From USA/CANADA, use: APO AE 09613-5000



> 
> Announcing the beta release of the "NetCDF Climate and Forecast (CF)
> Metadata Conventions."
> 
> Over the past 2 years there has been an effort to merge the GDT and NCAR
> CSM conventions for climate and forecast netCDF metadata.  Both of these
> conventions were designed as extensions to the COARDS conventions.  The
> merged convention is simpler and emphasizes both COARDS conformance and
> backwards compatibility.
> 
> The new convention is linked to Unidata's netCDF conventions page at
> http://www.unidata.ucar.edu/packages/netcdf/conventions.html.  The
> following is the abstract from the convention document:
> 
>   This document describes the CF conventions for climate and forecast
>   metadata designed to promote the processing and sharing of files created
>   with the netCDF Application Programmer Interface [NetCDF]. The
>   conventions define metadata that provide a definitive description of what
>   the data in each variable represents, and of the spatial and temporal
>   properties of the data. This enables users of data from different sources
>   to decide which quantities are comparable, and facilitates building
>   applications with powerful extraction, regridding, and display
>   capabilities.
> 
>   The CF conventions generalize and extend the COARDS conventions
>   [COARDS]. The extensions include metadata that provides a precise
>   definition of each variable via specification of a standard name,
>   describes the vertical locations corresponding to dimensionless vertical
>   coordinate values, and provides the spatial coordinates of
>   non-rectilinear gridded data. Since climate and forecast data are often
>   not simply representative of points in space/time, other extensions
>   provide for the description of coordinate intervals, multidimensional
>   cells and climatological time coordinates, and indicate how a data value
>   is representative of an interval or cell. This standard also relaxes the
>   COARDS constraints on dimension order and specifies methods for reducing
>   the size of datasets.
> 
> We invite comments from the community on this beta release.
> 
> Brian Eaton, NCAR
> Jonathan Gregory, Hadley Centre, UK Met Office
> Bob Drach, PCMDI, LLNL
> Karl Taylor, PCMDI, LLNL
> Steve Hankin, PMEL, NOAA

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