Fix/cfradial2 global attrs assignment

[chfer]

This PR fixes a bug in the to_cfradial2 function (located in xradar/io/export/cfradial2.py, line 79).
At that line, the code used:

root = dtree["/"].to_dataset()

The intention is for root to reference the root Dataset so that the following lines can assign the global attributes of the CfRadial2 file. However, DataTree.to_dataset() returns a copy of the dataset, not the live dataset stored in the tree. As a result, the global attributes were being written to this temporary copy and never persisted in the actual DataTree.
The first commit in this branch (fix/cfradial2-global-attrs-assignment, 0eaf495) adds a pytest, tests/io/test_cfradial2.py::test_to_cfradial2_global_attrs, which demonstrates that the global attributes produced by to_cfradial2 do not match the expected values for a CfRadial2 file.
The second commit (3585b62) fixes the issue by replacing:

root = dtree["/"].to_dataset()

with:

root = dtree["/"].ds

Using .ds ensures that the function operates on the actual dataset stored in the DataTree, making the global attribute assignment effective.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.20%. Comparing base (79ba495) to head (bda537c).

Additional details and impacted files

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##             main     #384   +/-   ##
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  Coverage   94.20%   94.20%           
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  Files          29       29           
  Lines        6417     6418    +1     
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+ Hits         6045     6046    +1     
  Misses        372      372           

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[kmuehlbauer]

@chfer Thanks for starting this. This may need a bit of discussion here. This export function was not meant to add these attributes to the original dataset, but only to add or overwrite these attributes to be written to disk.

So if you do something like this with your approach:

dtree.to_cfradial2(fname2)
dtree.to_cfradial1(fname1)

would have created for test: xradar v0.12 CfRadial2 export: xradar v0.12 CfRadial1 export in the CfRadial1 file, even if it was never loaded from CfRadial2 file.

[syedhamidali]

I believe @kmuehlbauer is suggesting something along these lines:

dtree.xradar.to_cf2(fname2)
dtree.xradar.to_cf1(fname1) 

[chfer]

Thank you for bringing this to my attention. Indeed my proposed solution is too simple and causes undesirable side-effects. After conversion to cfradial2 the original DataTree should be preserved. The first thing that comes to my mind is to create a (deep) copy of the DataTree upon entering the function to_cfradial2 , so that the original DataTree will always be preserved:

def to_cfradial2(dtree, filename, engine=None, timestep=None):
    """
    docstring ...
    """
    # let the local dtree reference a copy and not the original DataTree
    dtree = dtree.copy(deep=True)
    # now safely modify dtree
    ...
    # write DataTree
    dtree.to_netcdf(filename, engine=engine)

Maybe a shallow copy of dtree is sufficient, I still have to find this out.

[chfer]

In this last commit, the to_cfradial2 function was updated to create a copy of the input DataTree and use that copy for CfRadial2 export. This ensures that the original DataTree remains unchanged after the function returns.

In principle, a shallow copy would be sufficient: to_cfradial2 creates a new DataTree objects for each sweep, and the global attributes are created or replaced rather than mutated in place. However, to make the code more future-proof, I ultimately chose a deep copy, assuming that the additional memory pressure from radar files with multiple elevations and moments will remain manageable.