Page Comparison

Ignore the last column (with --user_area = "_ua" mapfiles) for now. The alert reader will see that the first two rows of both tables are identical, i.e., weighting by "area" produces identical answers whether or not one migrates to the new mapfiles. This surprised us because issues 1, 2, and 3 cause described above are that the old grid to have has a gap (for FV maps) and non-precise weights with mis-positioned centers and interfaces (for Gaussian maps). How can global-mean area-weighted answers from the flawed maps agree to double-precision with the updated maps? There are two reasons for this. First, ESMF_RegridWeightGenERWG, by default, constructs its own areas for all grids it receives. Here it somehow decides that the grids it receives are global (even though the FV grids are missing a longitude strip), and it builds its own internal representation of these grids with total surface = 4*pi sr. Second, it imposes the normalization requirement for first-order conservative remapping, meaning that it guarantees global integrals on the source and destination grids agree. In other words, it adjusts the output values of the field (FSNT, in this case) such that the integral of those values times its internally-diagnosed area-weights equals the input global integral. Some local values of FSNT in the output file are therefore scaled by an unrealistic factor, and this is non-obvious from looking at only the global integral.

Next we see that the "gw"-weighted answers change when migrating, and that the new answer is not correct (i.e., does not agree to double-precision) with the native grid either. Here "gw" is the name of the variable holding the latitude-weights (which may or may not be Gaussian weights) for the output grid. So we will call the contents of "gw" the latitude-weights. They are diagnosed from the user-specified gridcell interfaces on the output grid. Because the improved grid-files change the interfaces (for both FV and Gaussian grids), the latitude-weighted answers change in both cases. The latitude-weighted answers are (still) incorrect with the new map-files (i.e., the middle column) because those weights are applied to the field-values (e.g., FSNT) consistent with the internally diagnosed area, and that area embodies the assumption that gridcell vertices are connected by great-circle arcs (whereas small-circles not great-circles connect points with the same latitude in FV and Gaussian grids). In other words, the latitude-weights are correct but ERWG has adjusted the fields to be consistent with its internal notion of area, which is based on great-circles and is therefore incorrect for rectangular lat/lon grids.