ACME model output (on unstructured grids) and observational data (on a variety of grids) are often remapped in a post-processing step to structured lat/lon analysis grids for use and visualization by analysis tools. There are numerous small problems with these analysis grids employed for remapping by ACME (and CESM) prior to 20150901. These flaws or limitations propagate into the weights and/or grids output by the weight-generation utility. Flawed weights produce undesirable outcomes (loss of precision, gaps) when converting from source to destination grids. All tested regridders correctly apply the weights they are supplied, and migrating to improved grids (and to mapfiles generated from those grids, e.g., by ESMF_RegridWeightGen or TempestRemap) can automatically improve both the numerical accuracy and the data and metadata completeness and consistency of the files produced by the regridding procedure. None of the problems described below affect the accuracy of the model results on the native grid. The affected grids include many FV (plain and staggered) and Gaussian grids known to be used for ACME analysis, mapfiles produced from those grids, and all mapfiles employing bilinear interpolation. The new grids improve the accuracy of diagnostics and the aesthetics of plots produced from regridded files.
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We analyzed the global mean net TOA solar radiation for January, 1979 from an ne30 simulation on the native grid and then with old and mapfiles. Tests were performed with commands of the form "ncwa -w area -v FSNT in.nc out.nc" and "ncwa -w gw -v FSNT in.nc out.nc" where in.nc = famipc5_ne30_v0.3_00003.cam.h0.1979-01.nc for the native grid and a regridded version of that for the other mapfiles. The native grid analysis is independent of mapfile and is show shown for claritycomparison. Analysis grids produced with conservative regridding should exactly reproduce the native grid results. The "area" and "gw" rows show this is only true for a new map produced with the ESMF_RegridWeightGen (ERWG) "–user_area" switch enabled (indicated by the "_ua" in the mapfile name). The reasons for this are subtle, and are ultimately due to the approximation that the grid vertices are always joined by great circle arcs when though in reality, for regular lat/lon grids, arcs of constant longitude are great circles, while arcs of constant latitude are not.
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