[CF-metadata] CF compliant tripolar grid representation

Jim Biard jbiard at cicsnc.org
Thu Mar 30 10:42:01 MDT 2017


If I'm not mistaken, we would need to propose a new grid_mapping to be 
added to the Conventions that would define a Tripolar Coordinate 
Reference System, along with any attributes that don't currently exist 
that are needed to complete the definition. I did a search for a 
standard tripolar CRS in proj4 or epsg, and was unable to find one. Is 
it possible to make such a definition?

Regarding the standard names for your X and Y coordinate variables, I 
think you could use "projection_x/y_coordinate" once a grid_mapping has 
been defined. Of course you could always leave the attribute off, since 
a standard_name attribute is not a requirement.

If making a new grid_mapping is not feasible, you could request standard 
names along the lines of mesh_grid_i_index and mesh_grid_j_index. These 
standard names would (on reading their definitions) make it clear that 
the measurements are on a mesh grid for which there is no CRS. At least 
that's what comes to mind at the moment.

Grace and peace,


On 3/30/17 11:52 AM, Sebastien Villaume wrote:
> Hello all,
> I am looking for the best approach to describe in a CF compliant way the tripolar grids usually used in NEMO configurations.
> Basically, the difference with a usual bipolar grid (north pole-south pole) is that the north pole is split into 2 poles moved over Canada and Russia (to have distortions/singularities not over the ocean). A good visual representation can be found here:
> http://www.geomar.de/typo3temp/pics/globe_grid2_14_b8edb639ae.png
> everything south of the green line (40degN) is identical to a regular grid, but everything north of it is computed using a technique described here:
> Madec, G. and M. Imbard, 1996 : A global ocean mesh to overcome the north pole singularity. Clim. Dyn., 12, 381–388.
> The usual NEMO output of the grid looks like this:
>      float longitude(y, x) ;
>          longitude:standard_name = "longitude" ;
>          longitude:units = "degrees_east" ;
>          longitude:long_name = "longitude" ;
>      float latitude(y, x) ;
>          latitude:standard_name = "latitude" ;
>          latitude:units = "degrees_north" ;
>          latitude:long_name = "latitude" ;
> Basically both latitudes and longitudes need to be specified for each grid point, hence lat and lon are 2D arrays. This is not a problem itself but I would like to give more information through maybe grid_mapping or crs so it is clear that the grid is tripolar. This is useful information if one want to project/interpolate this back to a more regular representation.
> Looking at the CF conventions, I can see that grids can be fairly nicely documented but nothing for tripolar grids.
> Is there some documentation/guidelines on how to derive a proper grid_mapping/crs with valid attributes for tripolar grids?
> I would also like to add to my netcdf file a way to better describe axes:
>      double y(y) ;
>          y:units = "1" ;
>          y:long_name = "j-index of mesh grid" ;
>          y:standard_name = ??? ;
>      double x(x) ;
>          x:units = "1" ;
>          x:long_name = "i-index of mesh grid" ;
>          x:standard_name = ??? ;
> what would be the standard name of these?
> Thanks,
> ____________________________________
> Dr. Sébastien Villaume
> Analyst
> ECMWF Shinfield Park,
> Reading RG2 9AX, UK
> +44 7825 521592
> sebastien.villaume at ecmwf.int
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