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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The combination of these three data manipulations defines MPAS-mode.
Advanced Regridding II: Regional
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Unstructured Output (RRG-mode)
EAM (and CAM-SE) will produce regional output if requested to with the finclNlonlat namelist parameter. Output for a single region can be higher temporal resolution than the host global simulation. This facilitates detailed yet economical regional process studies. Regional output files are in a special format that we call RRG (for "regional regridding"). An RRG file may contain any number of rectangular regions. The coordinates and variables for one region do not interfere with other (possibly overlapping) regions because all variables and dimensions are named with a per-region suffix string, e.g., lat_128e_to_134e_9s_to_16s. ncremap can easily regrid RRG 2D logically rectangular output from an FV-dycore because ncremap can infer (as discussed above) the regional grid from any well-annotated regional FV data file. Regridding unstructured regional SE grid data, however, is more complex because SE gridcells are essentially weights without vertices (as and SE unstructured grids without cell vertices and unstructured grid weight-generators are not yet flexible enough to generate the regional to output only regional (as opposed to global) grids with weights. To summarize, regridding RRG data leads to three SE-specific difficulties (#1-3 below) and two difficulties (#4-5) shared with FV RRG files:
1. RRG files contain only regional gridcell center locations, not weights
2. Global SE grids have well-defined weights not vertices for each gridpoint
3. Grid generation software (ESMF and TempestRemap) only create global not regional SE grid files
4. Non-standard variable names and dimension names
5. Regional files can contain multiple regions
ncremap's RRG mode resolves these issues to allow trouble-free regridding of SE RRG files. The user must provide two additional input arguments, '--dat_glb=dat_glb' and '--grd_glb=grd_glb' to point to a global SE dataset and grid, respectively, of the same resolution as the model that generated the RRG datasets. Hence a typical RRG regridding invocation is:
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ncremap --dat_glb=dat_glb.nc --grd_glb=grd_glb.nc -g grd_rgn.nc dat_rgn.nc dat_rgr.nc |
Here grd_rgn is a regional destination grid-file, dat_rgn is the RRG file to regrid, and dat_rgr is the regridded output. Typically grd_rgn is a uniform rectangular grid covering the same region as the RRG file. Generate this as described in the last example in the section above on "Manual Grid-file Generation". grd_glb is the standard dual-grid grid-file for the SE resolution of the simulation, e.g., ne30np4_pentagons.091226.nc. ncremap regrids the global data file dat_glb to the global dual-grid in order to produce a intermediate global file annotated with gridcell vertices. Then it hyperslabs the lat/lon coordinates (and vertices) from the regional domain to use with regridding the RRG file. A grd_glb file with only one 2D field suffices (and is fastest) for producing the information needed by the RRG procedure. One can prepare an optimal dat_glb file by subsetting any 2D variable (e.g., ncks -v FSNT in.nc dat_glb.nc) from a full global SE output dataset.
ncremap RRG mode supports two additional options to override parameters set internally. First, the per-region suffix string may be set with '--rnm_sng=rnm_sng'. RRG mode will, by default, regrid the first region it finds in an RRG file. Explicitly set the desired region with rnm_sng for files with multiple regions, e.g., "--rnm_sng= ". Second, the bounding-box of the region may be explicitly set with '--bb_wesn=lon_wst,lon_est,lat_sth,lat_nrt'. The normal parsing of the bounding-box string from the suffix string may fail in (as yet undiscovered) corner cases, and the "--bb_wesn" option provides a workaround. The bounding-box string must include the entire RRG region, specified in WESN order. The two override options may be used independently or together, as in:
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RRG-mode supports most normal ncremap options, including input and output methods and regridding algorithms.
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