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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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At a minimum, install a recent version NCO on your executable $PATH with the corresponding library on your $LD_LIBRARY_PATH. NCO installation instructions are here. Unless you have reason to do otherwise, we recommend installing NCO through the Conda package. The Conda NCO package automagically installs two other important regridding tools, the ESMF_RegridWeightGen (aka ERWG) executable and the TempestRemap (aka TR) executables. Execute 'ncremap --config' to verify you have a working installation:

zender@aerosol:~$ ncremap --config
ncremap, the NCO regridder and grid, map- , and gridweight-generator, version 4.79.6-alpha033-alpha02 "Fuji Rolls"
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Config: ncremap script located in directory /Users/zender/bin
Config: NCO binaries located in directory /Users/zender/bin, linked to netCDF library version 4.7.3
Config: No hardcoded machine-dependent path/module overrides. (If desired, turn-on NCO hardcoded paths at supported national labs with "export NCO_PATH_OVERRIDE=Yes").
Config: External (non-NCO) program availability:
Config: ESMF weight-generation command ESMF_RegridWeightGen version 7.1.0r found as /opt/local/bin/ESMF_RegridWeightGen
Config: MOAB-Tempest weight-generation command mbtempest not found
Config: MPAS depth coordinate addition command add_depth.py found as /Users/zender/bin/add_depth.py
Config: TempestRemap weight-generation command GenerateOfflineMap found as /usr/local/bin/GenerateOfflineMap

Only NCO is required for many ncremap operations including conservative weight-generation. To create a new map-file with weights generated by ERWG or TR you will need many operations including applying existing regridding weights (aka, regridding) and/or generating grids, maps, or conservative weights with the NCO algorithms. Generating new weights (and map-files) with ERWG or TR requires that you install those packages (both of which come with the NCO Conda package). MOAB MOAB-Tempest (MBTR) is not yet available in a pre-packaged format and must currently be built from scratch (MBTR is expected to come as a Conda package sometime in 2020). MBTR is only required for power-users. Make sure ncremap reports a sufficiently working status as above before proceeding further.

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The regridding problem most commonly faced is converting output from a standard resolution model simulation to equivalent data on a different grid for visualization or intercomparison with other data. The EAM v1 model low-resolution simulations are performed and output on the ne30np4 SE (spectral element) grid, aka the "source grid". Data on this source grid have only one horizontal dimension (i.e., 1D) which makes them difficult to visualize. The recommended (fxm: link) 2D grid for analysis of these simulations is the 129x256 FV (finite-volume) grid, aka the "destination grid". The single most important capability of a regridder is the intelligent application of weights that transform data on the input grid to the desired output grid. These weights are stored in a "map-file", a single file that contains all the necessary weights and grid information necessary. Most While most regridding problems revolve around creating the appropriate map-file. This prototype problem is well-trod ground, this prototype problem assumes that has already been done, so the appropriate map-file (map.nc) already exists (fxm: link) and ncremap can immediately transform the input dataset (dat_src.nc) to the output (regridded) dataset (dat_rgr.nc):

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