Value Error with using recreate_metrics

[jdldeauna]

Hi! I'm using recreate_metrics to calculate distances along cmip6 grids. It can calculate all metrics (dx_t, dy_t, dx_gy, dy_gx, dx_gx, dy_gy, dy_gxgy and dz_t) except for dx_gxgy (dx at the corner point). When using the ff method:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [34], in <cell line: 2>()
      1 # infer dx at the corner point
----> 2 lon0, lon1 = grid.axes["X"]._get_neighbor_data_pairs(
      3     _interp_vertex_to_bounds(ds.lon_verticies.load(), "y")
      4     .isel(bnds=ns_bound_idx)
      5     .squeeze(),
      6     axis_vel_pos["X"],
      7 )

The input positions to _get_neighbor_data_pairs are similar and this error appears:

File /srv/conda/envs/notebook/lib/python3.9/site-packages/xgcm/grid.py:517, in Axis._get_neighbor_data_pairs(self, da, position_to, boundary, fill_value, ignore_connections, boundary_discontinuity, vector_partner, position_check)
    514 valid_positions = ["outer", "inner", "left", "right", "center"]
    516 if position_to == position_from:
--> 517     raise ValueError("Can't get neighbor pairs for the same position.")
    519 if position_to not in valid_positions:
    520     raise ValueError(
    521         "`%s` is not a valid axis position name. Valid "
    522         "names are %r." % (position_to, valid_positions)
    523     )

ValueError: Can't get neighbor pairs for the same position.

I tried it with CMIP6 models where the X axis is either shifted to the right (e.g., IPSL-CM6A-LR) or left (e.g., NorESM2-LM), and the same error appears. Example:

cat_url = "https://storage.googleapis.com/cmip6/pangeo-cmip6-noQC.json"
col = intake.open_esm_datastore(cat_url)
cat = col.search(table_id='Omon',experiment_id='historical',
             variable_id=['wmo','umo','vmo'], grid_label='gn', member_id='r1i1p1f1',source_id='IPSL-CM6A-LR')
ipsl = cat.to_dataset_dict(
        zarr_kwargs={'consolidated':True, 'decode_times': True, 'use_cftime': True},
        preprocess=combined_preprocessing,
        aggregate=False)
wmo = ipsl['CMIP.IPSL.IPSL-CM6A-LR.historical.r1i1p1f1.Omon.wmo.gn.gs://cmip6/CMIP6/CMIP/IPSL/IPSL-CM6A-LR/historical/r1i1p1f1/Omon/wmo/gn/v20180803/.nan.20180803.good.none.none']
umo = ipsl['CMIP.IPSL.IPSL-CM6A-LR.historical.r1i1p1f1.Omon.umo.gn.gs://cmip6/CMIP6/CMIP/IPSL/IPSL-CM6A-LR/historical/r1i1p1f1/Omon/umo/gn/v20180803/.nan.20180803.good.none.none']
vmo = ipsl['CMIP.IPSL.IPSL-CM6A-LR.historical.r1i1p1f1.Omon.vmo.gn.gs://cmip6/CMIP6/CMIP/IPSL/IPSL-CM6A-LR/historical/r1i1p1f1/Omon/vmo/gn/v20180803/.nan.20180803.good.none.none']
umo = umo.rename({'x':'x_c','lon':'lon_u', 'lat':'lat_u'})
vmo = vmo.rename({'y':'y_c','lon':'lon_v', 'lat':'lat_v'})
wmo = wmo.rename({'lev': 'lev_w'})
ds_subset = xr.merge([umo, vmo, wmo], compat='override')
grid = Grid(ds_subset, coords={ 'X':{'center':'x', 'right':'x_c'}, 'Y':{'center':'y', 'right': 'y_c'}, 'Z':{'center':'lev'},},)
ds, metrics_dict = recreate_metrics(ds_subset, grid)


ValueError: Can't get neighbor pairs for the same position.

[jdldeauna]

This error is fixed when ds_subset has lat_verticies and lon_verticies assigned from tracer points (e.g., in the example above assign lat_verticies and lon_verticies from wmo to ds_subset, don't use the coords from either umo or vmo).

[jbusecke]

Ideally one could infer the metrics for each and then have all the grid metrics afterwards, but I have never tried that. I think for now inferring metrics on the tracer (and then interpolating..)is sufficient.