Mesh files are not CF compliant

[xylar]

The mesh files MPAS-Tools writes fail a CF checker: MpasMeshConverter.x and MpasCellCuller.x set Conventions = "MPAS" with no CF entry, which cfchecks reports as an error, and none of the mesh creation tools write units on the mesh variables. Polaris is working around both in E3SM-Project/polaris#784 (for E3SM-Project/polaris#774) by rewriting the attribute and filling in the metadata from its own table before it writes a mesh, but the metadata belongs with the tools that create the variables.

What happens

Running cfchecks (from the cfchecker package, version 4.1.0) on a mesh from make_planar_hex_mesh() passed through cull() and convert():

ERROR: (2.6.1): This netCDF file does not appear to contain CF Convention data.

because the file carries Conventions = "MPAS". Setting it to CF-1.8 MPAS brings the file to zero errors. jigsaw_to_netcdf() output passed through MpasMeshConverter.x has the same attribute.

With that fixed, the checker still warns about every variable: make_planar_hex_mesh() writes neither units nor long_name, and the C++ converter and culler write long_name but no units (for example xCell, areaCell, dcEdge, angleEdge, weightsOnEdge, meshDensity).

What was expected

• Conventions = "CF-1.8 MPAS" from MpasMeshConverter.x and MpasCellCuller.x (the attribute lists every convention a file follows; CF only needs one entry to be CF-x.y). Any CF entry already on the input should be kept.
• units and long_name on every mesh variable from make_planar_hex_mesh(), jigsaw_to_netcdf(), the converter and the culler, in the plain udunits form CF uses (m, m2, radians, 1 for dimensionless; index and connectivity variables have no units).

Polaris's table of the attributes is polaris/mesh/attrs.yaml from E3SM-Project/polaris#784, and could be the starting point for the same in mpas_tools.

How to reproduce

With the Polaris (or any mpas_tools + cfchecker) environment on PATH:

from mpas_tools.planar_hex import make_planar_hex_mesh
from mpas_tools.mesh.conversion import convert, cull
from mpas_tools.io import write_netcdf

ds = make_planar_hex_mesh(nx=10, ny=10, dc=1e3, nonperiodic_x=False, nonperiodic_y=True)
write_netcdf(ds, 'base_mesh.nc')
write_netcdf(convert(cull(ds)), 'culled_mesh.nc')

then cfchecks base_mesh.nc (warnings only) and cfchecks culled_mesh.nc (the error above).

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Posted by Claude Code on @xylar's behalf. The testing, analysis and wording above are AI-authored; please check them accordingly.