This page is under construction...explanation and links will be added in the coming days and weeks...feedback welcome!
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Known Issues: We identified five issues (four numerical flaws and one empty field) in the pre-migration (pre-20150901) gridfiles and mapfiles. The sixth issue is with the weight-generators themselves.
1. ACME adopted flawed FV-scalar 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 proper solution here is to correct the grids to avoid gaps, in this case, to restore the longitude strip to the west of 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 can agree to double-precision (see examples below).
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4. Some equi-angular 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 equi-angular lat/lon grids are used as staggered/offset grids for FV dynamics variables (e.g., U, V). All equi-angular 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 equi-angular 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 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.
6. Great circle arcs vs. lat/lon arcs, latitude-weights, and --user_areas...fxmWeight-generators assume that great circle arcs connect grid vertices, a reasonable assumption for unstructured grids. This simplifies computing the weight of an arbitrarily shaped mesh on a sphere. However, lat/lon analysis grid vertices are connected by small circle arcs in latitude (arcs of constant latitude) and by great circles in longitude (meridians). It is difficult (there are no simple analytic formulae) to map between the two systems in the general case. This limitation in state-of-the-art remapping software (ESMF_RegridWeightGen and TempestRemap) can lead to unexpected behavior (see Examples below). Unlike the five issues above, this issue has not yet been solved.
Why Migrate?
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.
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