This page is under construction...explanation and links will be added in the coming days and weeks...feedback welcome!
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1. ACME adopted flawed FV-style 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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| Old (flawed) Gridfile | New (corrected) | FV Grid Generation: |
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| 129x256_SCRIP.20150901.nc | ncks --rgr grd_ttl='FV-scalar grid 129x256' --rgr grid=${DATA}/grids/129x256_SCRIP.20150901.nc --rgr lat_nbr=129 --rgr lon_nbr=256 --rgr lat_typ=fv cap --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc | |
| 257x512_SCRIP.130510.nc | 257x512_SCRIP.20150901.nc | ncks --rgr grd_ttl='FV-scalar grid 257x512' --rgr grid=${DATA}/grids/257x512_SCRIP.20150901.nc --rgr lat_nbr=257 --rgr lon_nbr=512 --rgr lat_typ=fv cap --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc |
ncks --rgr grd_ttl='FV-scalar grid 801x1600' --rgr grid=${DATA}/grids/801x1600_SCRIP.20150901.nc --rgr lat_nbr=801 --rgr lon_nbr=1600 --rgr lat_typ=fv cap --rgr lon_typ=grn_ctr ~zender/nco/data/in.nc ~/foo.nc |
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The lat_typ options for global grids are "uni" for Uniform-angle (currently same as "eqa" for Equi-angular in latitude), "cap" for Cap (same as "fv" for Finite Volume scalar grid in Lin-Rood representation), and "gss" for Gaussian. An equiangular A uniform (or equi-angular) in-latitude grid has constant spacing in latitude, and may have any number of latitudes. There is point No points are centered at the poles in an Equia Uniform-angular latitude grid. An FV Cap grid is nearly identical to an Equia Uniform-angular latitude grid, with some important data and metadata differences at the poles. Cap grids (aka FV-scalar grids) are rectangular representations of equi Uniform-angular (in latitude ) grids that have a "cap" point at each pole. In order to add-up to the same latitude across the cap as in the rest of the zones, the polar cells in the FV Cap rectangular representation on disk must span half the width of the other latitudes. Moreover, that is why the center and outer interface of the polar points in the FV Cap grid are both labeled as 90 degrees. Gaussian grids must have an even number of latitudes, and therefore never have points centered on the Equator or at the poles. The lon_typ options for global grids are "grn_ctr" and "180_ctr" for the first gridcell centered at Greenwich or 180 degrees, respecitvely. And "grn_wst" and "180_wst" for Greenwich or 180 degress lying on the western edge of the first gridcell.
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