S?bastien,
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,
Jim
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. S?bastien Villaume
> Analyst
> ECMWF Shinfield Park,
> Reading RG2 9AX, UK
> +44 7825 521592
> sebastien.villaume at ecmwf.int
> ____________________________________
> _______________________________________________
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Received on Thu Mar 30 2017 - 10:42:01 BST
