This page is under construction...explanation and links will be added in the coming days weeks...
There are numerous problems with grids employed 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 the are supplied, and migrating to improved grids (and to the mapfiles generated from those grids by ESMF_RegridWeightGen or Tempest) improves both the numerical accuracy and the data and metadata completeness and consistency of 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. These improved grids improve the accuracy of diagnostics and the aesthetics of plots produced from regridded files.
The four 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. This may cause the behavior you noticed (if you are looking at FV grids). 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 should agree to double-precision. This problem was identified independently by Charles Doutriaux and myself. Together with the Gaussian grid problems described below, it this shows that ACME (and CESM) should migrate to more accurate structured 2D grids.
2. SCRIP introduced, and CESM and ACME inherited, a format of storing all coordinates 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 ~8 digits. This goes all the way back to grandaddy SCRIP. NCO now generates SCRIP-format Gaussian grids accurate to 16 digits.
3. The Gaussian grids employed by SCRIP/CESM/ACME that I have examined (T42, T62, and T85) infer gridcell interfaces as midpoints between the Gaussian quadrature points/angles. Gridcell areas are inferred from the area between gridcell interfaces. The ~single-precision quadrature weights are inconsistent with area determined by the the midpoint rule for interfaces. This may be the problem you noticed (if you are working on your own with Gaussian grids). 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 do agree to double-precision.
4. It is thought that staggered FV grids 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 (which depends on the difference of the sine of the latitudes). This leads to a problem similar to the problem with the interface location on the Gaussian grid.
The new grids and mapfiles address these problems, which have always existed in ACME and its predecessors (CESM, CCSM, CCM)...