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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2. SCRIP introduced, and CESM and ACME inherited, coordinate storage in double precision (yay!). Unfortunately, every Gaussian grid that I have examined (T42, T62, and T85) from the CESM/CSEG grid repository has grid center latitudes (= sine of the Gaussian quadrature points) accurate to no greater than more than eight digits. This problem also appears in files in the SCRIP distribution, and in all grid files produced by NCL that I have examined. The solution is to base latitudes on quadrature points (i.e., Legendre solutions) computed to full double precision. NCO generates SCRIP-format Gaussian grids accurate to sixteen digits, the best that double precision can reach.
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