Some input files must be on the model grid - for example Initial Conditions files. Note this is just interpolation, not conservative regridding.
Regular grid interpolation
For interpolating between regular lat-lon grids, interpic can be used. It is provided in CESM code under ./components/cam/tools/interpic_new/.
Unstructured grid interpolation
For interpolating unstructured grids, ncremap ( https://nco.sourceforge.net/nco.html#ncremap ) works for some applications.
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Note that P0 is required to be in the template and input files, so you will have to add it to CESM3 files (e.g., ncks /glade/campaign/acom/MUSICA/grids/templates/P0.nc IC_L93.nc)
Other regridding information:
Regridding meteorological data
Emissions