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• netCDF3 CDF1 format limits fixed variables to sizes smaller than 2^31 B = 2 GiB ~ 2.1 GB, and record variables to that size per record. A single variable may exceed this limit if and only if it is the last defined variable.
• netCDF3 CDF2 format limits fixed variables to sizes smaller than 2^32 B = 4 GiB ~ 4.2 GB, and record variables to that size per record. Any number of variables may reach, though not exceed, this size for fixed variables, or this size per record for record variables.
• The netCDF3 CDF5 and netCDF4 formats have no variable size limitations of real-world import.

If any input or output variable in an E3SM simulation exceeds these limits, choose a PIO format capacious enough, either netCDF3 CDF2, PnetCDF/netCDF3 CDF5, or netCDF4.

NetCDF library:

There are 4 ways to build a library that reads netCDF files.  Our PIO library can interface to all of them at the same time if built properly.  The PIO_TYPENAME is set at runtime to indicate which one is to be used for all I/O operations.  The PIO_TYPENAME is in parenthesis below.

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