Value Error when using bilinear interpolation method

Hi! I was using the bilinear method and kept encountering an error message:

import gcsfs
import xarray as xr
import xesmf as xe

fs = gcsfs.GCSFileSystem(token='anon', access='read_only')
mapper = fs.get_mapper('gs://cmip6/CMIP6/CMIP/MPI-M/MPI-ESM1-2-HR/historical/r1i1p1f1/Omon/thetao/gn/v20190710/')
ds = xr.open_zarr(mapper, consolidated=True)
target_grid = xe.util.grid_global(1,1)
regridder = xe.Regridder(ds, target_grid, 'bilinear')
var_regrid = regridder(ds)

Error:

ValueError: ESMC_FieldRegridStore failed with rc = 506. Please check the log files (named "*ESMF_LogFile").

Fortunately @jbusecke guided me to a solution:

regridder = xe.Regridder(ds, target_grid, 'bilinear', ignore_degenerate=True)

I was wondering if the error message could be edited to reflect that adding the kwarg ignore_degenerate=True resolves it? Its difficult to work out how to resolve the issue with the existing ValueError message. Thanks!

[sol1105]

Hi @jdldeauna ,

this problem is caused by the data having been published on a grid which still contains the grid halo (rows or columns of duplicated grid cells), which is required during the model run. The grid halo cells are often not properly defined and are collapsing to lines or points and are only sharing the cell centers with their original, which then leads to this ValueError in ESMF (see #109).

This problem exists for quite a few CMIP ocean models. What you can usually do (besides using ignore_degenerated=True), is to remove the grid halo before creating remapping weights with xesmf or other tools. Beware that not removing the grid halo can lead to incorrect results when using the conservative remapping method. Also beware that data not defined on the grid cell centers, but rather on the vertices / edges, require special treatment.

Here is an example with CDOs (Climate Data Operators): https://wiki.mpimet.mpg.de/doku.php?id=analysis:postprocessing_mpiesm:regridding (here, GR15 corresponds to MPI-ESM1-2-LR MPIOM; TP04 to MPI-ESM1-2-HR MPIOM)

And here an example for xarray, for MPI-ESM1-2-LR/HR (see also #109 or this notebook):

# Define paths to the data
path_mpiesmlr="./tos_Omon_MPI-ESM1-2-LR_historical_r1i1p1f1_gn_201001-201412.nc"
path_mpiesmhr="./tos_Omon_MPI-ESM1-2-HR_historical_r1i1p1f1_gn_201001-201412.nc"

# Read the data with halo
ds_mpiesmlr_halo=xr.open_dataset(path_mpiesmlr).isel(time=0)
ds_mpiesmhr_halo=xr.open_dataset(path_mpiesmhr).isel(time=0)

# Read the data without halo (LR - omit first and last column, HR - omit first and last column, first 2 rows)
ds_mpiesmlr=xr.open_dataset(path_mpiesmlr).isel(time=0).isel(i=slice(1, 255))
ds_mpiesmhr=xr.open_dataset(path_mpiesmhr).isel(time=0).isel(i=slice(1, 801), j=slice(2,404))

[jbusecke]

I just wanted to chime in and say that this could be very helpful for the broader community. Since I found the workaround at some point in the past, I have interacted with many folks who experienced this issue.

Also many thanks for raising this issue @jdldeauna!