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 CAM-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.
...
5. For historical reasons, mapfiles generated by ESMF_RegridWeightGen (up to and including version 6.3.0rp1) using bilinear interpolation do not include complete output grid information. In particular, they lack gridcell area, because the ESMF team thought area would not be desired by users employing non-conservative mapping. However, it is helpful to include area so long as users understand that interpolative maps are non-conservative. The new mapfiles include gridcell area for bilinear interpolation maps. NCO now diagnoses and adds the gridcell area to files regridded with bilinear mapfiles.
6. Weight-generators assume that great circle arcs connect grid vertices, a reasonable assumption for unstructured grids. This simplifies computing the weight of an arbitrarily shaped mesh on a sphere. However, lat/lon analysis grid vertices are connected by small circle arcs in latitude (arcs of constant latitude) and by great circles in longitude (meridians). It is difficult (there are no simple analytic formulae) to map between the two systems in the general case. This limitation in state-of-the-art remapping software (ESMF_RegridWeightGen and TempestRemap) can lead to unexpected behavior (see Examples below). Unlike the five issues above, this issue has not yet been solved.
...
...