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To remap data from the SE grid to other grids, ACME relies on ESMF mapping utilities. ESMF's conservative remapping algorithm can currently only map cell centered data. For every point in the mesh, ESMF treats that point as a cell average value, and requires that we specify its cell boundaries as a polygon. These polygons are the dual grid to the SE subcell grid. For conservative remapping, the areas of these polygons must match the Gauss-Lobatto weight of each Gauss-Lobatto node. If we construct this dual grid in the usual way, by connecting the centers of all the subcells shown in the subcell grid, the areas of the cells will in general not match the Gauss-Lobatto weights. We thus have to perform an iteration, similar to spring dynamics, tweaking the polygons until the areas are correct. We have a couple of algorithms to do this, none of which are perfectbut the resulting polygons can be a little odd. The result of one of these algorithms is shown below:
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. To get the algorithm to converge, chevrons have been inserted in some of the polygons, making them slightly non-convex.
If we dont use chevrons, the resulting polygons are more distorted:
TempestRemap
We not that the TempestRemap package (github link) understands both SE and FV grids and can produce map files dirctly from the CAM-SE Native grid. These mapping files are usually more accurate since they take advantage of the SE discretization. If ACME adopts TempestRemap mapping files, we will not need to use the SE dual grid shown above.
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