[CF-metadata] CF compliant tripolar grid representation

Sebastien Villaume sebastien.villaume at ecmwf.int
Fri Mar 31 15:37:32 MDT 2017

Hi all,

I have checked both IPSL and CNRM CMIP5 datasets. It is indeed NEMO datasets and it is probably a ORCA tripolar grid in both cases. I write "probably" because it is not clear and conclusive without plotting the datasets: lat and lon are 2D fields, the datasets define 2 extra 1D coordinates "i" and "j" to be used as mesh indices (but without a proper standard name). The datasets also have bounds for lat and lon, defined as "lat_vertices" and "lon_vertices" which I think is one solution to describe the tripolar grid. I would prefer something more standardized and documented so that one can quickly identify from the metadata that it is a tripolar grid (defining the resolution, where are the poles, how it is derived, etc.)

I would like to propose for addition standard names to support the mesh indices/coordinates: 

"mesh_grid_i/j_index" suggested by Jim 
"x/y_coordinate_index" suggested by Jonathan

I let the experts in standard names decide which pair suits best the present case.

Regarding tripolar grids characteristics, I did some research and came to the conclusion that "Murray tripolar grids" are not identical to "ORCA/NEMO tripolar grids". This is true even without considering characteristics like the grid resolution, the location of the poles or where the latitude boundary is placed between the modified and unmodified parts. 

The Murray tripolar grid (used by GFDL) has its "north" poles on the boundary as shown here: https://www.gfdl.noaa.gov/wp-content/uploads/pix/user_images/mw/bipolar.gif

The ORCA/NEMO tripolar grids have the "north" poles within the modified regions but not on the boundary as shown in my original post: http://www.geomar.de/typo3temp/pics/globe_grid2_14_b8edb639ae.png 

This complicates things...


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

----- Original Message -----
From: "James Orr" <James.Orr at lsce.ipsl.fr>
To: "Karl Taylor" <taylor13 at llnl.gov>
Cc: cf-metadata at cgd.ucar.edu
Sent: Thursday, 30 March, 2017 23:01:54
Subject: Re: [CF-metadata] CF compliant tripolar grid representation

The IPSL and CNRM cimate models that participated in CMIP5 both used the NEMO 
model (ORCA2 and ORCA1 configurations) with tripolar grids.  Both provided 
output the was CF compliant.


On Thu, 30 Mar 2017, 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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