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.
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3. All SCRIP and CESM-maintained Gaussian grids that I have examined (T42, T62, and T85) appear to infer gridcell interfaces as midpoints between Gaussian quadrature points/angles. Software then infers defines the gridcell areas from the inferred gridcell interfaces. The problem is that interfaces defined by the midpoint rule will always produce areas inconsistent with those implied by the quadrature weights. This causes a mismatch between "area"- and "gw"- weighted statistics. NCO now uses Newton-Raphson iteration (instead of the quadrature midpoints) to determine the gridcell interface location that exactly matches areas determined by the (now fully double-precision) Gaussian weights. The Newton-Raphson iteration moves interfaces by, typically, a few tenths of a degree (for moderate resolution Gaussian grids) from their previous locations as quadrature midpoints. With these grids, "area"- and "gw"-weighted statistics can be consistent and agree to double-precision (see examples below).
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