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 most useful grid parameters (besides resolution) are latitude type (lat_typ), longitude type (lon_typ), title (ttl), and, for regional grids, the SNWE bounding box (snwe). The three supported varieties of global rectangular grids are Uniform/equiangular (lat_typ=uni), Cap/FV (lat_typ=cap), and Gaussian (lat_typ=gss). The four supported varieties of longitude types are the first (westernmost) gridcell centered at Greenwich (lon_typ=grn_ctr), western edge at Greenwish (grn_wst), or at the Dateline (lon_typ=180_ctr and lon_typ=180_wst, respectively). Grids are global, uniform, store latitudes from south-to-north, and have their first longitude centered at Greenwich by default. The grid-formula for this is 'lat_typ=uni#lon_typ=grn_ctr#lat_ctrdrc=s2n'. Some examples (remember, this API requires NCO 4.7.6+):
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ncremap -g grd.nc -G latlon=180,360 # 1x1 Uniform grid ncremap -g grd.nc -G latlon=180,360#lon_typ=grn_wst # CMIP6 1x1 Uniform grid, Greenwich-west edge ncremap -g grd.nc -G latlon=129,256#lat_typ=cap # 1.4x1.4 FV grid ncremap -g grd.nc -G latlon=94,192#lat_typ=gss # T62 Gaussian grid ncremap -g grd.nc -G latlon=721,1440#lat_drc=n2s#lat_typ=cap#lon_typ=grn_ctr # ECMWF ERA5 native grid ncremap -g grd.nc -G latlon=1280,2560#lat_typ=gss#lon_typ=grn_ctr#lat_drc=n2s # ECMWF IFS F640 Full Gaussian ncremap -g grd.nc -G latlon=360,720#lat_typ=uni#lon_typ=180_wst # "r05" ELM/MOSART 0.5x0.5 uniform grid |
Regional grids are a powerful tool in regional process analyses, and can be much smaller in size than global datasets. Regional grids are always uniform. Specify the rectangular bounding box, i.e., the outside edges of the region, in SNWE order:
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ncremap -g grd.nc -G ttl="Equi-Angular 1x1 Greenland grid"#latlonlatlon=30,90#snwe=55.0,85.0,-90.0,0.0 # 1x1 Greenland grid |
Intermediate Regridding:
The sections on Prototypical Regridding were intended to be read sequentially and introduced the most frequently required ncremap features. The Intermediate and Advanced regridding sections are an a la carte description of features most useful to particular component models, workflows, and data formats. Peruse these sections in any order.
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