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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The new grids and mapfiles address these problems 1-5, which have always existed in ACME and its predecessors (CESM, CCSM, CCM), and therefore cannot be too severe. The numerical flaws explained above can be thought of as fuzziness at the level of a few tenths of a degree in geo-referencing regridded data to the native model grid. These location errors produce only small (<< 1%) errors in regional or global statistics. So, why migrate? One aim is that diagnostics and observational evaluations with regridded data (often much more intuitive to visually evaluate than native SE grids) produce the same answers (to double precision whenever possible) as statistics computed on the native model grid. Without migration, agreement between statistics computed on the two grids (native and regridded statistics analysis) beyond single precision (and often worse for regional statistics) is a matter of luck and coincidence, not determinism and reproducibility. As ACME grids shift to ~1/4 degree and finer, it becomes even more important to exploit the full double precision accuracy that software can guarantee when supplied with accurate grids.
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We generated new FV-type analysis grids with NCO (version 4.5.2 or later) and commands shown in the tables below. These grids have replaced the flawed grids on the ACME inputdata server: https://acme-svn2.ornl.gov/acme-repo/acme/mapping/grids. Gaussian grids were never not originally distributed within ACME and so the improved versions are not currently by the ACME grid/map server, though they could be found in the coupler directories. The improved Gaussian grids are now also stored on ACME's inputdata server . However, Gaussian grids have been used extensively by the in case they are of use to the ACME/CESM community. Their replacements are The improved grids are also directly available on rhea (/lustre/atlas/proj-shared/cli115/zender/grids) and yellowstone (/glade/p/work/zender/grids).
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