This page is under construction...explanation and links will be added in the coming days weeks...
There are numerous problems with grids employed for remapping by ACME (and CESM) prior to 20150901. These problems arise from 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 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 FV (plain and staggered grids), Gaussian grids for spectral models, 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 five issues identified and fixed are:
1. ACME uses flawed FV grids that omit a small strip of longitude to the east of Greenwich. For FV 129x256, this amounts to 0.2% of global area. The 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. Fixed FV grids and maps (with suffix .20150724.nc) are in
/lustre/atlas/proj-shared/cli115/zender/[grids,maps] on rhea
With these grids, "area"- and "gw"-weighted statistics agree to double-precision. This problem was identified independently by Charles Doutriaux and myself. Together with the Gaussian grid problems described below, this shows that ACME (and CESM) should migrate to more accurate structured 2D grids.
2. SCRIP introduced, and CESM and ACME inherited, coordinate storage in double precision (yay!). Unfortunately, every Gaussian grid I have examined has grid center latitudes (= sine of the Gaussian quadrature points) accurate to no greater than eight digits. This goes all the way back to grand-daddy SCRIP. NCO now generates SCRIP-format Gaussian grids accurate to sixteen digits, the best that double precision can reach.
3. The SCRIP/CESM/ACME Gaussian grids that I have examined (T42, T62, and T85) infer gridcell interfaces as midpoints between Gaussian quadrature points/angles. Software then infers the gridcell areas from gridcell interfaces. The single-precision quadrature weights are inconsistent with area determined by the the midpoint rule for interfaces. NCO now uses Newton-Raphson iteration (instead of the quadrature midpoints) to determine the gridcell interface location that exactly matches the area determined by the (now double-precision) Gaussian weights. With these grids, "area"- and "gw"-weighted statistics agree to double-precision. This procedure moves interfaces by, typically, a few tenths of a degree (for moderate resolution spectral grids) from their previous locations as quadrature midpoints.
4. Some if not all staggered FV grids historically produced by NCL and fed to ESMF_RegridWeightGen utilize the continuous form of the weighting function (i.e., cosine(lat)) evaluated on the discretized grid, rather than the exact discretized weight function (i.e., the difference of the sine of the latitudes bounding the zone). In fact the staggered/offset FV grid used for dynamics variables must have the same functional form for weights as the FV grid itself, as they are simply offsets of eachother. Without this correction, statistics of variables computed on the dynamics grid may be misdiagnosed (fxm: last sentence not yet verified).
5. For historical reasons, mapfiles generated by ESMF_RegridWeightGen using bilinear interpolation do not include complete output grid information. In particular, they lack gridcell area, probably because the presence of a diagnosed area could be misconstrued to imply a conservation property of the mapping which, by their nature, interpolative schemes lack. 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.
The new grids and mapfiles address these problems, which have always existed in ACME and its predecessors (CESM, CCSM, CCM)...