This page is under construction...explanation and links will be added in the coming days and weeks...feedback welcome!
There are numerous problems with grids employed for remapping by ACME (and CESM) prior to 20150901. These problems arise from small flaws or limitations in the geometry of grids supplied to the utility (typically ESMF_RegridWeightGen) that generates the weights that regridders apply to convert between the source and 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 Tempest) 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 all the many FV (plain and staggered grids) , and Gaussian grids for spectral modelsknown to be used within ACME, 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.
The Known Issues: We identified five issues identified (four numerical flaws and one empty field) in the pre-migration (pre-20150901) toolchain and fixed by migration are:gridfiles and mapfiles.
1. ACME adopted flawed FV (regular in latitude, except at the poles) grids 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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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.
Why Migrate?
The new grids and mapfiles address these problems, which have always existed in ACME and its predecessors (CESM, CCSM, CCM). The , 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 georeferencing 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.
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| Old (flawed) Gridfiles | New (corrected) | Generation: | |
|---|---|---|---|
| 129x256_SCRIP.20150901.nc257x512 | ncks -O -D 7 --rgr grd_ttl='FV-scalar grid 129x256' --rgr grid=${DATA}/grids/129x256_SCRIP. | 13051020150901.nc --rgr lat_nbr=129 --rgr lon_nbr=256 --rgr lat_typ=fv --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc | |
| 257x512_SCRIP.20150901130510.nc 801x1600 | 257x512_SCRIP.13051020150901.nc801x1600 | ncks -O -D 7 --rgr grd_ttl='FV-scalar grid 257x512' --rgr grid=${DATA}/grids/257x512_SCRIP.20150901.nc |
Flawed ACME Mapfiles:
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| --rgr lat_nbr=257 --rgr lon_nbr=512 --rgr lat_typ=fv --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc | ||
ncks -O -D 7 --rgr grd_ttl='FV-scalar grid 801x1600' --rgr grid=${DATA}/grids/801x1600_SCRIP.20150901.nc --rgr lat_nbr=801 --rgr lon_nbr=1600 --rgr lat_typ=fv --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc |
| Old (flawed) Mapfiles | New (corrected) | Generation: | |
|---|---|---|---|
| map_ne30np4_to_fv129x256_aave.150418.nc | map_ne30np4_to_fv257x512fv129x256_bilinaave.15041820150901.nc | ||
| map_ne120np4ne30np4_to_fv257x512_aavebilin.150418.nc | ne120np4fv801x1600150418 |
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Corrected Other Gridfiles:
180x360_SCRIP.20150820.nc
90x180_SCRIP.20150820.nc
t42_SCRIP.20150901.nc
t62_SCRIP.20150901.nc
t85_SCRIP.20150901.nc
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These keywords allow one to generate grids commonly used by ACME, CESM, and other models. New FV grids have replaced the flawed grids on the ACME inputdata server: https://acme-svn2.ornl.gov/acme-repo/acme/mapping/grids. The newer FV grids were generated by NCO (version 4.5.2 or later) with:ncks -O -D 7 --rgr grd_ttl='FV-scalar grid 129x256' --rgr grid=${DATA}/grids/129x256_SCRIP.20150901.nc --rgr lat_nbr=129 --rgr lon_nbr=256 --rgr lat_typ=fv --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc
ncks -O -D 7 --rgr grd_ttl='FV-scalar grid 257x512' --rgr grid=${DATA}/grids/257x512_SCRIP.20150901.nc --rgr lat_nbr=257 --rgr lon_nbr=512 --rgr lat_typ=fv --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc
ncks -O -D 7 --rgr grd_ttl='FV-scalar grid 801x1600' --rgr grid=${DATA}/grids/801x1600_SCRIP.20150901.nc --rgr lat_nbr=801 --rgr lon_nbr=1600 --rgr lat_typ=fv --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc
Gaussian grids were never distributed within ACME and the improved versions are not on ACME's inputdata server, either. 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). The newer Gaussian grids were generated by NCO (version 4.5.2 or later) with:
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