It would be useful to enable an "append" mode to allow field constructs to be written to an existing netCDF file, without destroying its original contents. Currently, only new files can be created.

The implemenattion of this would have to consider whether existing netCDF variables in the file can be re-used by the new field and metadata constructs.

This issue was previously raised on an earlier version of cf-python in a different repository (https://bitbucket.org/cfpython/cf-python/issues/27), but not resolved.