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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One valid option argument for each supported interpolation type is shown separated by vertical bars. The arguments shown have multiple synonyms that are equivalent. For example, "-a conserve" is equivalent to "-a aave" and to "--alg_typ=conservative". 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" is the default algorithm. Commonly-used algorithms that invoke ERWG are "bilinear" and "conservative". TR options are discussed below. Peruse the list of options now, though defer a thorough investigation until you reach the "Intermediate Regridding" section.
Prototypical Regridding III: Infer Grid-file from Data-file
Thus far we have explained how to apply a map-file to data, and how, if necessary, to generate a map-file from known grids. What if there is no map-file and the source or the destination grid-files (or both) are unavailable? Often, one knows the basic information about a grid (e.g., resolution) but lacks the grid-file that contains the complete information for that grid geometry. In such cases, one must create the grid-file via one of two methods. First, one can let ncremap attempt to infer the grid-file from a data file known to be on the desired grid. This procedure is called "inferral" and is fairly painless. Second, one can feed ncremap all the required parameters (for rectangular grids only) and it will generate a grid-file. This requires a precise specification of the grid geometry, and will be covered the sub-section on "Manual Grid-file Generation".
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Background and distributed node parallelism (as described above in the the Parallelism section) of MWF-mode are possible though not yet implemented. Please let us know if this feature is desired.
Advanced Regridding V: CMIP6 Timeseries
This section describes the recommended procedures to construct and regrid E3SM timeseries data to CMIP6 specifications. Most models provide data to CMIP6 in timeseries format, meaning one variable-per-file with multiple years per file. These timeseries must be regridded to at least one of the CMIP6 standard grids. The E3SM project chose to supply its v1 experiments to CMIP6 archived on rectangular, uniform (i.e., equiangular in latitude and longitude), one-degree (for standard-resolution) and quarter-degree (for high-resolution) grids. Generating these timeseries from experiments as lengthy as 500 model years, formatted to CMIP6 specifications, requires many non-standard options to both ncclimo (to construct the timeseries) and to ncremap (to regrid timeseries), and is a natural capstone exercise in using both together. This section is arranged in reverse order where first we present the final actual commands, followed by the descriptions, meanings, and reasons for particular options.
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