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.villa...@ecmwf.int
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