[CF-metadata] CF compliant tripolar grid representation

Hedley, Mark mark.hedley at metoffice.gov.uk
Mon Apr 3 03:28:05 MDT 2017


Hello All,

I'd like to pick up on an earlier comment from Jim:

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?

I don't think this is the correct approach

In my opinion, the tri-polar grid is described with respect to a Geographic Coordinate Reference System: typically the one used to co-locate the observations for assimilation, by spatial coordinates.
The 'Grid' is not a projection and it is not a coordinate reference system: it is the description of a model grid.
In data files I have seen, each spatial location is defined by a location in latitude, longitude and depth, with respect to a suitable geodetic datum.


I agree with your more recent comment Jim:

I'm wondering if x and y have too strong an association to projected coordinate
 systems. I also like u/v, but that may be too strongly associated for some people
 with vector components (wind, for example).

I think that describing grid indices should be carefully distinguished from spatial coordinates.  Put a different way, I don't think a grid index can be georeferenceable.

I think that a good deal of care not to confuse the grid indices with any interpretation of 'grid_mapping' relations is required here.
I don't think that a CF grid mapping should be used to connect any description of model index space with geographic space in these cases.

Sebastien states:


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

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

The mixing of the terms coordinate and index gives me pause for thought.  What information is being encoded here?

A key question I have is about the expectation for values of these indices, under operations such as sub-setting.  I have seen many files which do not have coordinate variables for the x-like and y-like dimensions, the only horizontal spatial metadata is contained in auxiliary coordinates.
Clearly I can perform index operations on these arrays, but I don't consider the index values important and I don't preserve them.

Sebastien:
Is it the case that you would like to ensure that model index space values are preserved, for example when removing a regional subset from a tri-polar ocean model?
Would you like to be able to encode a result where it is clear that a regional subset of 50 <= x < 150, 70 <= x < 120  has been taken from a larger extent model?

If standard names are provided to encode such information, I would advocate clear descriptive text stating that there is no mathematical relationship between such index coordinates (i still don't like mixing these terms) and projection coordinates or geographic coordinates

Sebastien states:

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 appreciate the desire to have a standardised approach to defining such a model grid.  I would not advocate trying to use grid mapping variables
and relationships for this, I think this could do more harm than good.
I don't have a better suggestion to hand, I'm sad to say.

I am not raising principled objections to this conversation or the direction of travel; I am raising waryness and caution about introducing further confusion or implying stronger relationships than can be provided.

all the best
mark

________________________________
From: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] on behalf of Jim Biard [jbiard at cicsnc.org]
Sent: 31 March 2017 23:26
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] CF compliant tripolar grid representation


Hi.

I like the more generic x/y_coordinate_index name, but I'm wondering if x and y have too strong an association to projected coordinate systems. I also like u/v, but that may be too strongly associated for some people with vector components (wind, for example). What do the rest of you think? Here are some names that come to mind. Feel free to suggest something better!

  *   mesh_grid_i_index, mesh_grid_j_index
  *   grid_i_index, grid_j_index
  *   grid_i_coordinate, grid_j_coordinate
  *   x_coordinate_index, y_coordinate_index
  *   index_x_coordinate, index_y_coordinate (this ordering matches the projection_x/y_coordinate naming)
  *   u_coordinate, v_coordinate
  *   i_coordinate, j_coordinate
  *   grid_row_coordinate, grid_column_coordinate
  *   row_coordinate, column_coordinate

The more I look at these, the more I like the last two.

As for a definitions, how about something like this variation on the ones for the projection_x/y_coordinate?

column_coordinate: "column" indicates the fastest-changing dimension of a two-dimensional grid, when this is not associated with a spatial coordinate dimension such as longitude or projected X, positive with increasing column. The column coordinate, possibly in conjunction with the row coordinate, serves as a parametric driver mapping abstract grid positions to spatial coordinates such as latitude and longitude.

row_coordinate: "row" indicates the the slowest-changing dimension of a 2-dimensional grid, when this is not associated with a spatial coordinate dimension such as latitude or projected Y, positive with increasing row. The row and column coordinates serve as a parametric driver mapping abstract grid positions to spatial coordinates such as latitude and longitude.

Grace and peace,

Jim

On 3/31/17 5:37 PM, Sebastien Villaume wrote:

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
or
"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
Analyst
ECMWF Shinfield Park,
Reading RG2 9AX, UK
+44 7825 521592
sebastien.villaume at ecmwf.int<mailto:sebastien.villaume at ecmwf.int>
____________________________________

----- Original Message -----
From: "James Orr" <James.Orr at lsce.ipsl.fr><mailto:James.Orr at lsce.ipsl.fr>
To: "Karl Taylor" <taylor13 at llnl.gov><mailto:taylor13 at llnl.gov>
Cc: cf-metadata at cgd.ucar.edu<mailto: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.

James

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<mailto:sebastien.villaume at ecmwf.int>  ____________________________________
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formerly NOAA’s National Climatic Data Center
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