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:
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This solution is deceptively simple because it conceals the choices, paths, and options required to create the appropriate map.nc for all situations. We will discuss creating map.nc later after showing more powerful and parallel ways to solve the prototype problem. The solution above only works for users savvy enough to know how to find appropriate pre-built map-files. E3SM mapMap-files used by the E3SM model are available at https://web.lcrc.anl.gov/public/e3sm/inputdata/cpl/gridmaps/ . Additional map-files useful in post-processing are available at https://web.lcrc.anl.gov/public/e3sm/diagnostics/maps/. Many commonly used maps and grids can also be found in my (@czender's) directories as ~zender/data/[maps,grids] at most DOE High Performance Computing (HPC) centers. Take a minute now to look at these locations.
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The simplest regridding procedure applies an existing map-file to your data, as in the above example . E3SM (public servers of pre-existing map-files are publicly available at https://web.lcrc.anl.gov/public/e3sm/inputdata/cpl/gridmaps/also linked to above). At most DOE High Performance Computing (HPC) centers these and others can also be found in my (Charlie Zender '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 publicly available within model-specific directories of the previous location, e.g., https://web.lcrc.anl.gov/public/e3sm/inputdata/ocn/mpas-o/oEC60to30v3/ . Many grids useful for post-processing are publicly served from https://web.lcrc.anl.gov/public/e3sm/diagnostics/grids/. At most DOE High Performance Computing (HPC) centers these can also be found in my (@czender's) directory, ~zender/data/grids. Take a minute now to look there for the prototype problem grid-files, ie.eg., for FV 129x256, cmip6_180x360, and ne30np4 ne30pg2 grid-files.
You might find multiple grid-files that contain the string 129x256. Grid-file names are often ambiguous. The grid-file global metadata (ncks -M grid.nc) often displays the source of the grid. These metadata, and sometimes the actual data (fxm: link), are usually more complete and/or accurate in files with a YYYYMMDD-format date-stamp. For example, the metadata in file 129x256_SCRIP.20150901.nc clearly state it is an FV grid and not some other type of grid with 129x256 resolution. The metadata in 129x256_SCRIP.130510 tell the user nothing about the grid boundaries, and some of the data are flawed. When grids seem identical except for their date-stamp, use the grid with the later date-stamp. The curious can examine a grid-file (ncks -M -m grid.nc) and easily see it looks completely different from a typical model or observational data file. Grid-files and data-files are not interchangeable.
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