Area subset fails with extents which cross prime meridian

[JamesVarndell]

Workflows appear to be failing when they contain an area subset operator with extents which cross the 0 degree longitude line (i.e. with a negative western extent and a positive eastern extent).

For example:

wf = ops.Subset(
    ops.Input(
        'tas', ['CMIP6.ScenarioMIP.MIROC.MIROC6.ssp119.r1i1p1f1.day.tas.gn.v20191016']
    ),
    area='-5,50,30,65',
)

Through my WPS adaptor I'm just getting the generic "Sorry, process failed." error message, so I don't have a full traceback that might explain what's happening here - but I can successfully run area subsets for extents which don't cross the prime meridian.

[agstephens]

To check in clisops:

• what is the longitude range in the actual NetCDF file?
• does a longitude extraction work if it does not cross the Greenwich Meridian.

Find (or add) example NetCDF files into the test CMIP6/c3s-cmip6 data that have longitude range:

• 0 to 360
• -180 to 180

Things to test at clisops level:

• Dataset with longitude: 0 to 360:
  • subset request: 10 to 200
  • subset request: -50 to 100
• Dataset with longitude: -180 to 180:
  • subset request: 10 to 200
  • subset request: -50 to 100

[agstephens]

roocs/clisops#35

[agstephens]

Suggested location for new code (assuming that roll works for us):

https://github.com/roocs/clisops/blob/master/clisops/ops/subset.py#L18-L24

Notes:

• only relevant if longitude bounds are set
• need to create a new function to check alignment of requested longitude bounds with the actual longitude in the data
• and if necessary, roll the dataset itself

[ellesmith88]

The longitude range of CMIP6.ScenarioMIP.MIROC.MIROC6.ssp119.r1i1p1f1.day.tas.gn.v20191016 is 0 to 360.

If the longitude bounds selected do not cross the prime meridian then the subsetting is successful.

Further testing:

Dataset with longitude: 0 to 360:

• subset request: 10 to 200: test passes
• subset request: -50 to 100: test raises NotImplementedError: Input longitude bounds ([-50. 100.]) cross the 0 degree meridian but dataset longitudes are all positive.

Dataset with longitude: -180 to 180:
Datasets with longitudes -180 to 180 are not easy to come by! I've done some initial tests with a dataset that has longitude -300 to 60:

• subset request: 10 to 50: Subset is successful
• subset request: -100 to 50: Subset is successful

[ellesmith88]

Looking into using xarray roll here: https://github.com/roocs/clisops/blob/9f38ca78700e18ba676bebdc0c50ce514c1b1a41/tests/test_roll.py#L1-L180

The data has been rolled but still only contains values from 0 to 360.

I tested a subset operation after rolling a dataset by 180 here: https://github.com/roocs/clisops/blob/9f38ca78700e18ba676bebdc0c50ce514c1b1a41/tests/test_roll.py#L140-L157 and it still raises the prime meridian error.

[ellesmith88]

closed with clisops PR #117