[CF-metadata] CF compliant tripolar grid representation

Karl Taylor taylor13 at llnl.gov
Thu Mar 30 14:27:27 MDT 2017


Hi Sebastien,

More than one group stored output on a tripolar grid in CMIP5.  I'm 
pretty sure they did it in a CF-conforming way.  I know at least some of 
the GFDL model output was reported on a tripolar grid, as described at 
http://nomads.gfdl.noaa.gov/CM2.X/oceangrid.html (or search on "tripolar 
grid" for additional links).  You could look to their example, and see 
if you think it is done correctly.

I don't think extensions or modifications to CF are needed for tripolar 
grids.

best regards,
Karl

On 3/30/17 9:42 AM, Jim Biard wrote:
>
> Sébastien,
>
> 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. Sébastien Villaume
>> Analyst
>> ECMWF Shinfield Park,
>> Reading RG2 9AX, UK
>> +44 7825 521592
>> sebastien.villaume at ecmwf.int  
>> ____________________________________
>> _______________________________________________
>> CF-metadata mailing list
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
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