Coordinate valid range and intersection issue

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
data

Research direction

Start by reproducing the issue with cube.intersection, iris.save, and loading the resulting NetCDF file using the longitude range described. Trace how the longitude valid_range and points are handled during intersection and reload; done means the saved cube reloads without the warning and preserves the expected -3 through 3 longitude coordinates.

Written by the indexing model from the issue text.

Description

Dragon 🐉 Type: Bug

🐛 Bug Report

I have an issue reloading a cube I've saved:
I start with a cube with longitude coordinate 0-360, and do
new_cube = cube.intersection(longitude=(-3,3))
and I get a new cube with the correct longitudes -3 -2 -1 0 1 2 3

When I save this,
iris.save(new_cube, "file.nc")
and then load back in, the longitude coordinate generates this error/warning:

UserWarning: Failed to create 'longitude' dimension coordinate: The 'longitude' DimCoord points array must be strictly monotonic.Gracefully creating 'longitude' auxiliary coordinate instead.
And has points 9.96920997e+36, 9.96920997e+36, 9.96920997e+36, 0, 1, 2, 3

How To Reproduce

Steps to reproduce the behaviour:

  1. Start with a cube with longitude coordinates in the range 0-360
  2. Make a new cube using cube.intersection to get a cube with longitude crossing the prime meridian, eg from -3 to +3
  3. Save the new cube as netcdf
  4. Load the netcdf file in
  5. The loaded cube produces the above UserWarning and incorrect longitude values for the negative region

Expected behaviour

The cube when loaded back in I expect to have longitude coordinates in the range -180 to +180

Screenshots

Environment

  • OS & Version: Met Office RHEL 7, scitools current default as of 8/6/23
  • Iris Version: 3.3.1

Additional context

Click to expand this section... 
Please add additional verbose information in this section e.g., code, output, tracebacks, screenshots etc

Doing his before saving
`new_cube.coord('longitude').attributes['valid_range']=np.array([-180,180])`
seems to prevent the issue but I'd expect the new_cube to automatically change its valid_range when making the intersection.
Dominant language
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Merged PRs (30d)
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