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    <p>Karl,</p>
    <p>I think Sébastien raises some valid questions regarding
      appropriate standard names and best form for specifying that the
      coordinate system is a tripolar grid. <br>
    </p>
    The projection_x_coordinate and projection_y_coordinate definitions
    state "
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    <span style="color: rgb(51, 51, 51); font-family: "Helvetica
      Neue", Helvetica, Arial, sans-serif; font-size: 14px;
      font-style: normal; font-variant-ligatures: normal;
      font-variant-caps: normal; font-weight: normal; letter-spacing:
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      word-spacing: 0px; -webkit-text-stroke-width: 0px;
      background-color: rgb(245, 245, 245); text-decoration-style:
      initial; text-decoration-color: initial; display: inline
      !important; float: none;">The relationship between the projection
      coordinates and<span class="Apple-converted-space"> </span></span><a
      class="stdname" href="http://mmisw.org/cfsn/#/latitude"
      style="box-sizing: border-box; background: 0px 0px rgb(245, 245,
      245); color: steelblue; text-decoration: none; font-family:
      "Helvetica Neue", Helvetica, Arial, sans-serif;
      font-size: 14px; font-style: normal; font-variant-ligatures:
      normal; font-variant-caps: normal; font-weight: normal;
      letter-spacing: normal; orphans: 2; text-align: start;
      text-indent: 0px; text-transform: none; white-space: normal;
      widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px;">latitude</a><span
      style="color: rgb(51, 51, 51); font-family: "Helvetica
      Neue", Helvetica, Arial, sans-serif; font-size: 14px;
      font-style: normal; font-variant-ligatures: normal;
      font-variant-caps: normal; font-weight: normal; letter-spacing:
      normal; orphans: 2; text-align: start; text-indent: 0px;
      text-transform: none; white-space: normal; widows: 2;
      word-spacing: 0px; -webkit-text-stroke-width: 0px;
      background-color: rgb(245, 245, 245); text-decoration-style:
      initial; text-decoration-color: initial; display: inline
      !important; float: none;"><span class="Apple-converted-space"> </span>and<span
        class="Apple-converted-space"> </span></span><a class="stdname"
      href="http://mmisw.org/cfsn/#/longitude" style="box-sizing:
      border-box; background: 0px 0px rgb(245, 245, 245); color:
      steelblue; text-decoration: none; font-family: "Helvetica
      Neue", Helvetica, Arial, sans-serif; font-size: 14px;
      font-style: normal; font-variant-ligatures: normal;
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      text-transform: none; white-space: normal; widows: 2;
      word-spacing: 0px; -webkit-text-stroke-width: 0px;">longitude</a><span
      style="color: rgb(51, 51, 51); font-family: "Helvetica
      Neue", Helvetica, Arial, sans-serif; font-size: 14px;
      font-style: normal; font-variant-ligatures: normal;
      font-variant-caps: normal; font-weight: normal; letter-spacing:
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      text-transform: none; white-space: normal; widows: 2;
      word-spacing: 0px; -webkit-text-stroke-width: 0px;
      background-color: rgb(245, 245, 245); text-decoration-style:
      initial; text-decoration-color: initial; display: inline
      !important; float: none;"><span class="Apple-converted-space"> </span>is
      described by the<span class="Apple-converted-space"> </span></span><a
      class="search" href="http://mmisw.org/cfsn/#/search/grid_mapping"
      style="box-sizing: border-box; background: 0px 0px rgb(245, 245,
      245); color: seagreen; text-decoration: none; font-family:
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      font-size: 14px; font-style: normal; font-variant-ligatures:
      normal; font-variant-caps: normal; font-weight: normal;
      letter-spacing: normal; orphans: 2; text-align: start;
      text-indent: 0px; text-transform: none; white-space: normal;
      widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px;">grid_mapping</a><span
      style="color: rgb(51, 51, 51); font-family: "Helvetica
      Neue", Helvetica, Arial, sans-serif; font-size: 14px;
      font-style: normal; font-variant-ligatures: normal;
      font-variant-caps: normal; font-weight: normal; letter-spacing:
      normal; orphans: 2; text-align: start; text-indent: 0px;
      text-transform: none; white-space: normal; widows: 2;
      word-spacing: 0px; -webkit-text-stroke-width: 0px;
      background-color: rgb(245, 245, 245); text-decoration-style:
      initial; text-decoration-color: initial; display: inline
      !important; float: none;">."</span> 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.<br>
    <br>
    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?<br>
    <br>
    Grace and peace,<br>
    <br>
    Jim<br>
    <br>
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      !important; float: none;"></span>
    <div class="moz-cite-prefix">On 3/30/17 4:27 PM, Karl Taylor wrote:<br>
    </div>
    <blockquote cite="mid:d95cff64-de31-e8a3-ee00-d307fcd411ea@llnl.gov"
      type="cite">
      <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
      Hi Sebastien,<br>
      <br>
      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 <a moz-do-not-send="true"
        class="moz-txt-link-freetext"
        href="http://nomads.gfdl.noaa.gov/CM2.X/oceangrid.html">http://nomads.gfdl.noaa.gov/CM2.X/oceangrid.html</a>
      (or search on "tripolar grid" for additional links).  You could
      look to their example, and see if you think it is done correctly.<br>
      <br>
      I don't think extensions or modifications to CF are needed for
      tripolar grids.<br>
      <br>
      best regards,<br>
      Karl<br>
      <br>
      <div class="moz-cite-prefix">On 3/30/17 9:42 AM, Jim Biard wrote:<br>
      </div>
      <blockquote
        cite="mid:a865d5c2-e595-d40e-5084-743b2ca94e87@cicsnc.org"
        type="cite">
        <meta http-equiv="Content-Type" content="text/html;
          charset=utf-8">
        <p>Sébastien,</p>
        <p>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?<br>
        </p>
        <p>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.<br>
        </p>
        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.<br>
        <br>
        Grace and peace,<br>
        <br>
        Jim<br>
        <br>
        <div class="moz-cite-prefix">On 3/30/17 11:52 AM, Sebastien
          Villaume wrote:<br>
        </div>
        <blockquote
          cite="mid:1002596496.27548921.1490889176744.JavaMail.zimbra@ecmwf.int"
          type="cite">
          <pre wrap="">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:

<a moz-do-not-send="true" class="moz-txt-link-freetext" href="http://www.geomar.de/typo3temp/pics/globe_grid2_14_b8edb639ae.png">http://www.geomar.de/typo3temp/pics/globe_grid2_14_b8edb639ae.png</a> 

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 
<a moz-do-not-send="true" class="moz-txt-link-abbreviated" href="mailto:sebastien.villaume@ecmwf.int">sebastien.villaume@ecmwf.int</a> 
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<a moz-do-not-send="true" class="moz-txt-link-freetext" href="http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata">http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata</a>
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      <pre wrap="">_______________________________________________
CF-metadata mailing list
<a class="moz-txt-link-abbreviated" href="mailto:CF-metadata@cgd.ucar.edu">CF-metadata@cgd.ucar.edu</a>
<a class="moz-txt-link-freetext" href="http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata">http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata</a>
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                            Visit us on
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                        <span style="font-size: 11px; "><span
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                            <b>Jim Biard</b>
                            <br>
                            <b>Research Scholar</b>
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                            <a href="http://cicsnc.org/" style="color:
                              rgb(38, 58, 143); text-decoration: none;
                              font-weight: bold; ">
                              Cooperative Institute for Climate and
                              Satellites NC
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                            <a href="http://ncsu.edu/" style="color:
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                              North Carolina State University
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                              style="color: rgb(38, 58, 143);
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                              NOAA National Centers for Environmental
                              Information
                            </a>
                            <br>
                            <i>formerly NOAA’s National Climatic Data
                              Center</i>
                            <br>
                            151 Patton Ave, Asheville, NC 28801
                            <br>
                            e: <a href="mailto:jbiard@cicsnc.org"
                              style="color: rgb(38, 58, 143); ">jbiard@cicsnc.org</a>
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