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NCL Graphics: Tripole Grids (ORCA, CICE)

Non-uniform grids and triangular mesh conversion

The ORCA grid used for many of the examples on this page is a non-uniform grid. It was given to us by Christophe Cassou from the Centre National de la Recherche Scientifique (CNRS/CERFACS) in Toulouse. This grid is common in Europe, particularly in France; the IPSL centre ("Institut Pierre-Simon Laplace model") developed it. More information on the grid can be obtained here.

It is better to convert this grid to a triangular mesh rather than interpolate it to a uniform grid before contouring. Note that only contouring is available with triangular mesh conversion at this time.

If the input array to a gsn_csm graphical interface is one- dimensional, and also has one-dimensional lat/lon arrays of the same length, NCL automatically uses triangular mesh conversion. The lat/lon information must be provided by setting sfXArray and sfYArray.

For more information on non-uniform grids that NCL can handle, see the document "Non-uniform grids that NCL can contour."


orca_1.ncl: Note that the trGridType resource is set to "TriangularMesh" in this example, so that it will use a triangulation algorithm to contour the mesh.

The lat/lon information must be provided by setting the resources sfXArray and sfYArray.

This example compares the results between the default cnFillMode value of "AreaFill" and "RasterFill".

ORCA Grid using CellFill

orca_2.ncl: This example is very similar to the previous example, except that it shows what happens when you set cnFillMode value of "CellFill", and then how to turn on the cell edges and missing value cell edges with the resources cnCellFillEdgeColor and cnCellFillMissingValEdgeColor.

CICE Tripole Grid

ice_4.ncl: This 3 frame plot demonstrates a solution to the gap that appears along a line between the two northern poles for data represented on the CICE T-fold Tripole grid. Thanks to Petteri Uotila of CSIRO Marine & Atmospheric Research in Aspendale, Victoria, Australia for providing this solution. The first frame demonstrates the problem when the data is plotted normally using the T-fold grid coordinates. The second frame draws the same data but switches to the U-fold grid coordinates that are also present in the file. This rendering eliminates the gap but slightly misplaces the data in the coordinate space. The third frame shows the solution obtained by adding the top row of the U-fold grid to the T-fold grid and simply repeating the top row data in a row added to the data array.