new default h5netcdf engine raises UnicodeEncodeError

What happened?

When reading+writing a file with waterlevel observations downloaded from CMEMS, I get a "UnicodeEncodeError: 'utf-8' codec can't encode characters in position 4-5: surrogates not allowed" when using the new default engine (h5netcdf)

What did you expect to happen?

This was not the case before, when engine netcdf4 was the default. I can also easily work around this by setting the engine. However, I see the benefits of h5netcdf and I guess this file should be no issue for that engine. Hopefully this can be resolved, either in xarray or h5netcdf.

Minimal Complete Verifiable Example

import xarray as xr
file_cmems = r"dfmtools_cmems_ssh_retrieve_data_temporary_file_6.nc"
ds = xr.open_dataset(file_cmems, engine='h5netcdf')
ds.to_netcdf("temp_file.nc")

File to test with: dfmtools_cmems_ssh_retrieve_data_temporary_file_6.zip

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

UnicodeEncodeError                        Traceback (most recent call last)
File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\spyder_kernels\customize\utils.py:209, in exec_encapsulate_locals(code_ast, globals, locals, exec_fun, filename)
    207     if filename is None:
    208         filename = "<stdin>"
--> 209     exec_fun(compile(code_ast, filename, "exec"), globals, None)
    210 finally:
    211     if use_locals_hack:
    212         # Cleanup code

File c:\data\checkouts\dfm_tools\tests\untitled2.py:10
      8 file_cmems = r"c:\Users\veenstra\Downloads\dfmtools_cmems_ssh_retrieve_data_temporary_file_6.nc"
      9 ds = xr.open_dataset(file_cmems, engine='h5netcdf')
---> 10 ds.to_netcdf("temp_file.nc")

File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\xarray\core\dataset.py:2102, in Dataset.to_netcdf(self, path, mode, format, group, engine, encoding, unlimited_dims, compute, invalid_netcdf, auto_complex)
   2099     encoding = {}
   2100 from xarray.backends.api import to_netcdf
-> 2102 return to_netcdf(  # type: ignore[return-value]  # mypy cannot resolve the overloads:(
   2103     self,
   2104     path,
   2105     mode=mode,
   2106     format=format,
   2107     group=group,
   2108     engine=engine,
   2109     encoding=encoding,
   2110     unlimited_dims=unlimited_dims,
   2111     compute=compute,
   2112     multifile=False,
   2113     invalid_netcdf=invalid_netcdf,
   2114     auto_complex=auto_complex,
   2115 )

File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\xarray\backends\api.py:2107, in to_netcdf(dataset, path_or_file, mode, format, group, engine, encoding, unlimited_dims, compute, multifile, invalid_netcdf, auto_complex)
   2102 # TODO: figure out how to refactor this logic (here and in save_mfdataset)
   2103 # to avoid this mess of conditionals
   2104 try:
   2105     # TODO: allow this work (setting up the file for writing array data)
   2106     # to be parallelized with dask
-> 2107     dump_to_store(
   2108         dataset, store, writer, encoding=encoding, unlimited_dims=unlimited_dims
   2109     )
   2110     if autoclose:
   2111         store.close()

File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\xarray\backends\api.py:2157, in dump_to_store(dataset, store, writer, encoder, encoding, unlimited_dims)
   2154 if encoder:
   2155     variables, attrs = encoder(variables, attrs)
-> 2157 store.store(variables, attrs, check_encoding, writer, unlimited_dims=unlimited_dims)

File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\xarray\backends\common.py:527, in AbstractWritableDataStore.store(self, variables, attributes, check_encoding_set, writer, unlimited_dims)
    523     writer = ArrayWriter()
    525 variables, attributes = self.encode(variables, attributes)
--> 527 self.set_attributes(attributes)
    528 self.set_dimensions(variables, unlimited_dims=unlimited_dims)
    529 self.set_variables(
    530     variables, check_encoding_set, writer, unlimited_dims=unlimited_dims
    531 )

File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\xarray\backends\common.py:544, in AbstractWritableDataStore.set_attributes(self, attributes)
    534 """
    535 This provides a centralized method to set the dataset attributes on the
    536 data store.
   (...)
    541     Dictionary of key/value (attribute name / attribute) pairs
    542 """
    543 for k, v in attributes.items():
--> 544     self.set_attribute(k, v)

File ~\AppData\Local\miniforge3\envs\dfm_tools_env\Lib\site-packages\xarray\backends\netCDF4_.py:555, in NetCDF4DataStore.set_attribute(self, key, value)
    553     self.ds.setncattr_string(key, value)
    554 else:
--> 555     self.ds.setncattr(key, value)

File src\\netCDF4\\_netCDF4.pyx:3087, in netCDF4._netCDF4.Dataset.setncattr()

File src\\netCDF4\\_netCDF4.pyx:1858, in netCDF4._netCDF4._set_att()

File src\\netCDF4\\_netCDF4.pyx:6733, in netCDF4._netCDF4._strencode()

UnicodeEncodeError: 'utf-8' codec can't encode characters in position 4-5: surrogates not allowed

Environment

INSTALLED VERSIONS
------------------
commit: None
python: 3.12.8 | packaged by conda-forge | (main, Dec  5 2024, 14:06:27) [MSC v.1942 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 11
machine: AMD64
processor: Intel64 Family 6 Model 154 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: ('Dutch_Netherlands', '1252')
libhdf5: 1.14.4
libnetcdf: 4.9.2

xarray: 2025.9.1
pandas: 2.2.3
numpy: 2.2.6
scipy: 1.15.1
netCDF4: 1.7.2
pydap: 3.5.3
h5netcdf: 1.5.0
h5py: 3.12.1
zarr: 2.18.4
cftime: 1.6.4.post1
nc_time_axis: None
iris: None
bottleneck: 1.4.2
dask: 2025.1.0
distributed: None
matplotlib: 3.10.0
cartopy: None
seaborn: None
numbagg: None
fsspec: 2024.12.0
cupy: None
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 75.8.0
pip: 25.0
conda: None
pytest: 8.3.4
mypy: None
IPython: 8.31.0
sphinx: 8.1.3

Start by running the supplied MVCE with the attached CMEMS file and inspect the write path around Dataset.to_netcdf, AbstractWritableDataStore.set_attributes, and the h5netcdf/netCDF4 backend shown in the traceback. Determine whether the failure is in xarray or h5netcdf, then add a regression test using the provided file or metadata and confirm that writing no longer raises UnicodeEncodeError.