This page is devoted to instruction in ncremap. It describes steps necessary to create grids, and to regrid datasets between different grids with ncremap. Some of the simpler regridding options supported by ncclimo are also described at Generate, Regrid, and Split Climatologies (climo files) with ncclimo. This page describes those features in more detail, and other, more boutique features often useful for custom regridding solutions.
The Zen of Regridding
Most modern climate/weather-related research requires a regridding step in its workflow. The plethora of geometric and spectral grids on which model and observational data are stored ensures that regridding is usually necessary to scientific insight, especially the focused and variable resolution studies that E3SM models conduct. Why does such a common procedure seem so complex? Because a mind-boggling number of options are required to support advanced regridding features that many users never need. To defer that complexity, this HOWTO begins with solutions to the prototypical regridding problem, without mentioning any other options. It demonstrates how to solve that problem simply, including the minimal software installation required. Once the basic regridding vocabulary has been introduced, we solve the prototype problem when one or more inputs are "missing", or need to be created. The HOWTO ends with descriptions of different regridding modes and workflows that use features customized to particular models, observational datasets, and formats. The overall organization, including TBD sections (suggest others, or vote for prioritizing, below), is:
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ncremap -m map.nc dat_src.nc dat_rgr.nc # Integral-preserving treatment of missing data is used by default
ncremap --preserve=integral -m map.nc dat_src.nc dat_rgr.nc # Explicitly specify integral-preserving treatment (requires NCO 4.9.2+)
ncremap --preserve=mean -m map.nc dat_src.nc dat_rgr.nc # Explicitly specify mean-preserving treatment (requires NCO 4.9.2+)
By default, ncremap implements the integral-preserving, conservative approach because it has useful properties, is simpler to understand, and requires no additional parameters. However, this approach will often produce unrealistic gridpoint values (e.g., ocean temperatures < 100 K and that is not a typo) values along coastlines or near data gaps where state variables are regridded to/from small fractions of a gridcell. The mean-preserving approach solves this problem, yet incurs others, like non-conservation. The mean-preserving approach (aka re-normalization) ensures that the output values are physically consistent and do not stick-out like sore thumbs in a plot, although the integral of their value times area is not conserved.
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TempestRemap (TR) is the chief alternative to ERWG and NCO for regridding weight-generation. TR will replace ERWG as the default on-line weight generator in E3SMv2. Tempest algorithms, written by @paulullrich, have many numerical advantages described in papers and at https://acme-climate.atlassian.net/wiki/spaces/Docs/pages/178848194/Transition + to + TempestRemap + for + Atmosphere + grids. Verify that ncremap can access your Tempest installation as described in the above section on "Software Requirements" before trying the examples below. Once installed, TR can be as easy to use as ERWG or NCO with FV grid-files, e.g.,
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This command, the same as shown in the "Create Map-file from Known Grid-files" section above, except using alg_typ='tempest', is a jumping-off point for understanding Tempest features and quirks. First, simply note that the ncremap interfaces for ERWG, NCO, and TR weight-generators are the same even though the underlying ERWG, NCO, and TR applications have different APIs. Second, Tempest accepts SCRIP-format grids (as shown) and Exodus-format grid-files, also stored in netCDF though typically with a '.g' suffix, e.g., ne30.g as described on the at Transition -to-Tempest page just mentionedto TempestRemap for Atmosphere grids. Exodus grid-files contain grid "connectivity" and other information required to optimally treat advanced grids like SE. Hence this also works
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ncremap -a se2fv_flx -s ne30.g -g 129x256_SCRIP.20150901.nc -m map_ne30np4_to_fv129x256_tempest.YYYYMMDD.nc
The exact switches that NCO invokes for a specific TR algorithm like se2fv_flx can be discovered in multiple ways: First, all TR options that NCO employs are recommended on the Transition to TempestRemap for Atmosphere grids; Second, the NCO Users Guide documents TR options at http://nco.sf.net/nco.html#tr; Third, the options --dbg_lvl=1 or --dbg_lvl=2 cause ncremap to print the sub-commands it executes, including the TR commands with options. Experiment, intercompare, and find the algorithm that works best for your purposes. Advanced users may be interested in the quantitative evaluations of the quality of the weights in the map-file provided by the map-checker (ncks --chk_map map.nc) described below.
As mentioned in the at Transition -to-Tempest pageto TempestRemap for Atmosphere grids, the Tempest overlap-mesh generator expects to be sent the two grid-files in the order smaller then larger (ERWG and NCO have no corresponding restriction). For example, Tempest considers the global ocean to be a smaller domain than the global atmosphere since it covers less area (due to masked points). Hence ncremap must be told when the source grid-file covers a larger domain than the destination. Do this with the "--a2o" or "--l2s" switch (for "atmosphere-to-ocean" or "large-to-small", respectively):
ncremap --l2s -a se2fv_flx -s atm_se_grd.nc -g ocn_fv_grd.nc -m map.nc # Source larger than destination
ncremap -a fv2se_flx -s ocn_fv_grd.nc -g atm_se_grd.nc -m map.nc # Source smaller than destination
ncremap -a se2fv_flx -s atm_se_grd.nc -g atm_fv_grd.nc -m map.nc # Source same size as destination
Finally, the Tempest-transition page Transition to TempestRemap for Atmosphere grids describes eight specific global FV<->SE mappings that optimize different combinations of accuracy, conservation, and monotonicity desirable for remapping flux-variables (flx), state-variables (stt), and an alternative (alt) mapping. A plethora of boutique options and switches control the Tempest weight-generation algorithms for these six cases. To simplify their invocation, ncremap names these eight algorithms fv2se_stt, fv2se_flx, fv2se_alt, fv2fv_flx, fv2fv_stt, se2fv_stt, se2fv_flx, se2fv_alt, and se2se. E3SM maps with these algorithms have adopted the suffixes "mono" (for both se2fv_flx and fv2se_alt), "highorder" (se2fv_stt, fv2se_stt), "intbilin" (se2fv_alt), and "monotr" (fv2se_flx). The relevant Tempest map-files can be generated with
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