This page is under construction...explanation and links will be added in the coming days and weeks...feedback welcome!
ACME model output on unstructured grids is often remapped in a post-processing step to structured lat/lon analysis grids for use by most of our analysis tools.
There are numerous problems with these analysis grids employed for remapping by ACME (and CESM) prior to 20150901. These small flaws or limitations propagate into the weights produced by the weight-generation utility. The flawed weights produce undesirable outcomes (loss of precision, gaps) when converting from source to destination maps. 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 TempestTempestRemap) automatically improves 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 within 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.
Known Issues: We identified five issues (four numerical flaws and one empty field) in the pre-migration (pre-20150901) gridfiles and mapfiles.
1. ACME adopted flawed FV-style gridfiles that omitted a small strip of longitude to the east of Greenwich. This problem was identified independently by Charles Doutriaux and myself. For FV 129x256, this amounted to 0.2% of global area, that might appear as a gap or blank strip when plotted. Maps based on the flawed grids somehow reapportion area so that total area is conserved (4*pi sr), yet this necessarily redistributes weights from their true positions. This causes a mismatch between "area"- and "gw"- weighted statistics. The solution is to generate grids that center Greenwich in the first zonal gridcell, and to base maps on those grids. With these grids, the gap disappears and "area"- and "gw"-weighted statistics agree to double-precision.
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The new grids and mapfiles address these problems, 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 native and regridded statistics beyond single precision 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.
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 distributed within ACME and so the improved versions are not currently stored on ACME's inputdata server. However, Gaussian grids have been used extensively by the CESM community. Their replacements are available on rhea (/lustre/atlas/proj-shared/cli115/zender/grids) and yellowstone (/glade/p/work/zender/grids).
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