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 in the pre-migration (pre-20150901) toolchain and fixed by migration are:
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.
2. SCRIP introduced, and CESM and ACME inherited, coordinate storage in double precision (yay!). Unfortunately, every Gaussian grid that I have examined has grid center latitudes (= sine of the Gaussian quadrature points) accurate to no greater than eight digits. This problem appears in files in the SCRIP distribution, and in all files produced by NCL that I have examined. The solution is to base latitudes on quadrature points (i.e., Legendre solutions) computed to full double precision. NCO now generates SCRIP-format Gaussian grids accurate to sixteen digits, the best that double precision can reach.
3. All SCRIP and CESM-maintained 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 problem is that interfaces defined by the midpoint rule will always produce areas inconsistent with those implied by the quadrature weights. This causes a mismatch between "area"- and "gw"- weighted statistics. NCO now uses Newton-Raphson iteration (instead of the quadrature midpoints) to determine the gridcell interface location that exactly matches areas determined by the (now double-precision) Gaussian weights. The Newton-Raphson iteration moves interfaces by, typically, a few tenths of a degree (for moderate resolution Gaussian grids) from their previous locations as quadrature midpoints. With these grids, "area"- and "gw"-weighted statistics are consistent and agree to double-precision.
4. Some regular lat/lon grids historically produced and used in the CESM community 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). And some of these regular lat/lon grids are used as staggered/offset grids for FV dynamics variables (e.g., U, V). All regular lat/lon grids must have the same functional form for weights as the FV grid itself, since they are simply offsets of eachother. Without this correction, statistics of variables computed on the FV dynamics grid may be misdiagnosed (fxm: last sentence not yet verified). Users of all regular grids, including staggered FV grids, should ensure their grids were produced with the correct weighting function. One way to do this is to verify the "area" variable, if present, exactly matches the area between the gridcell interfaces. If unsure, consider generating a new regular grid with the instructions below, or ask me and I will be happy to generate one for you and keep it in a standard location where others may access it.
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.
The new grids and mapfiles address these problems, which have always existed in ACME and its predecessors (CESM, CCSM, CCM)...