[CF-metadata] CF compliant tripolar grid representation

Jim Biard jbiard at cicsnc.org
Thu Mar 30 15:16:38 MDT 2017


Karl,

I think Sébastien raises some valid questions regarding appropriate 
standard names and best form for specifying that the coordinate system 
is a tripolar grid.

The projection_x_coordinate and projection_y_coordinate definitions 
state " The relationship between the projection coordinates andlatitude 
<http://mmisw.org/cfsn/#/latitude>andlongitude 
<http://mmisw.org/cfsn/#/longitude>is described by thegrid_mapping 
<http://mmisw.org/cfsn/#/search/grid_mapping>." If you can't specify a 
grid_mapping, then these standard names don't really apply, do they? And 
perhaps, if there's no way to construct a CRS for a tripolar grid, these 
standard names shouldn't be used.

I agree that you can specify all the parts (2D lat and lon grids, 1D i 
and j "coordinates") without using a standard name on the 1D coordinate 
variables and without ever specifying a grid_mapping, and that such an 
arrangement is CF-compliant. But the connection of the 1D coordinate 
variables with the lat and lon grids is then left to information 
provided in comment attributes or non-standard attributes created by the 
authors. To put this grid on the same footing as, say, a Lambert 
Conformal Conic CRS, don't you need to have a grid_mapping or some other 
equivalent standard mechanism?

Grace and peace,

Jim

On 3/30/17 4:27 PM, Karl Taylor wrote:
> 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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Facebook <http://www.facebook.com/cicsnc> 	*Jim Biard*
*Research Scholar*
Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
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NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
/formerly NOAA’s National Climatic Data Center/
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