E3SM component models employ multiple grids that require regridding to exchange information during coupling and for off-line analyses. High-quality regridding weights are necessary to exploit E3SM's grid versatility, and are traditionally stored in map-files. This page documents the internal structure of map-files, and describes how to assess the quality of their weights. This page assumes some familiarity in regridding (see Regridding E3SM Data with ncremap), though no prior exposure to the internal structure of map-files..
As of NCO version 4.9.0 (December, 2019), invoking --chk_map causes ncks to evaluate the quality of regridding weights in the map-file provided as input-file. This option works with map-files (not grid-files) in ESMF/CMIP6-compliant format (i.e., a sparse matrix variable named S and coordinates [xy][ab]_[cv]. When invoked with the additional --area_wgt option, the evaluation statistics are area-weighted and thus exactly represent the global-mean/min/max/mebs/rms/sdn biases expected when regridding a globally uniform field. This tool makes it easier to objectively assess weight-generation algorithms, and will hopefully assist in their improvement. Thanks to Mark Taylor of Saturday Night Live (SNL) and Paul Ullrich of UC Davis for this suggestion and early prototypes.
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