This page is devoted to instruction in NCO’s regridding operator, 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:
...
The simplest regridding procedure applies an existing map-file to your data, as in the above example. E3SM map-files are publicly available at https://web.lcrc.anl.gov/public/e3sm/inputdata/cpl/gridmaps/. At most DOE High Performance Computing (HPC) centers these can also be found in my (@czender's) directory, ~zender/data/maps. If the desired map-file cannot be found, then you must create it. Creating a map-file requires a complete specification of both source and destination grids (meshes). The files that contain these grid specifications are called "grid-files". Many E3SM grid-files are available at publicly available within model-specific directories of the previous location, e.g., https://web.lcrc.anl.gov/public/e3sm/inputdata/cpl/gridmapsocn/mpas-o/oEC60to30v3/ . At most DOE High Performance Computing (HPC) centers these can also be found in my (@czender's) directory as , ~zender/data/grids. Take a minute now to look there for the prototype problem grid-files, i.e., for FV 129x256 and ne30np4 grid-files.
...
The map-files above are named with alg_typ=nco because the ncremap default interpolation algorithm is the first-order conservative NCO algorithm (NB: before NCO 4.9.1 the default algorithm was ESMF bilin). To re-create the aave map in the first example, invoke ncremap with -a esmfaave (the newest v3 naming convention) or -a conserve (same algorithm, different name in v1, v2):
| Code Block |
|---|
ncremap -a conserve -s ne30np4_pentagons.091226.nc -g 129x256_SCRIP.20150901.nc -m map_ne30np4_to_fv129x256_aave.YYYYMMDD.nc # EAMv1
ncremap -a conserve -s ne30pg2.nc -g cmip6_180x360_scrip.20181001.nc -m map_ne30pg2_to_cmip6_180x360_aave.YYYYMMDD.nc # EAMv2
ncremap -a esmfaave -s ne30pg2.nc -g cmip6_180x360_scrip.20181001.nc -m map_ne30pg2_to_cmip6_180x360_esmfaave.YYYYMMDD.nc # EAMv3 |
This takes a few minutes, so save custom-generated map-files for future use. Computing weights to generate map-files is much more computationally expensive and time-consuming than regridding, i.e., than applying the weights in the map-file to the data. We will gloss over most options that weight-generators can take into consideration, because their default values often work well. One option worth knowing now is -a. The invocation synonyms for -a are --alg_typ, --algorithm, and --regrid_algorithm. These are listed in the square brackets in the self-help message that ncremap prints when it is invoked without argument, or with --help:
| Code Block |
|---|
zender@firn:~$ ncremap ... -a alg_typ Algorithm for weight generation (default nco_conncoaave) [alg_typ, algorithm, regrid_algorithm] ESMF algorithms: esmfbilin,bilinear|esmfaave,aave,conserve|conserve2nd|nearestdtos|neareststod|patch NCO algorithms: ncoaave,nco,nco_con (1st order conservative algorithm similar to "conserve") Tempest|nco_idw,nco_dwe (inverse-distance-weighted interpolation/extrapolation) Tempest (and MOAB-Tempest) algorithms: traave,fv2fv_flx|trbilin|trfv2|trintbilin|tempest|fv2fv_flx|fv2fv_stt|fv2se_flx|fv2se_stt|fv2se_alt|se2fv_flx|se2fv_stt|se2fv_alt|se2se|tempest ... |
One At least one valid option argument for each supported interpolation type is shown separated by vertical bars. The arguments shown have multiple synonyms, separated by commas, that are equivalent. For example, -a conserveesmfaave is equivalent to -a aave and to --alg_typ=conservativeconserve. Use the longer option form for clarity and precision, and the shorter form for conciseness. The full list of synonyms, and the complete documentation, is at http://nco.sf.net/nco.html#alg_typ. The NCO algorithm nco_conserve ncoaave is the default algorithm. Commonly-used algorithms that invoke ERWG are bilinear esmfbilin and conservative esmfaave. TR options are discussed below. Peruse the list of options now, though defer a thorough investigation until you reach the "Intermediate Regridding" section.
...