[CF-metadata] CF compliant tripolar grid representation

Sebastien Villaume sebastien.villaume at ecmwf.int
Thu Mar 30 09:52:56 MDT 2017

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:


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?



Dr. Sébastien Villaume 
ECMWF Shinfield Park, 
Reading RG2 9AX, UK 
+44 7825 521592 
sebastien.villaume at ecmwf.int 

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