Add support for met_em files

[lpilz]

Change Summary

We already have a discussion about adding support for geo_em files and I wanted to extend this to metgrid met_em files in general. I have identified a number of specific met_em problems and wanted to get some input on how to approach them.

My approach was to call .metpy.quantify() on all variables in a met_em dataset I have and work through the problems sequentially. I finally managed to identify some issues with the variables which I wanted to present und discuss here.

Variable	units attr	Proposed fix
VAR_SSO	meters2 MSL (it's a variance)	overwrite units and description in config
HGT_M	meter MSL	overwrite units and description in config
SM100255	fraction	add upstream?
OL4	whoknows	overwrite in config?
SCB_DOM	category	add upstream?
SOILTEMP	Kelvin	should be working since pint-xarray/issues#26/cf-xarray/pull#197?
COSALPHA_V	none	add upstream?
SEAICE	0/1 Flag	add to _BOOLEAN_UNITS_ATTRS
LANDSEA	0/1 Flag	add to _BOOLEAN_UNITS_ATTRS

Maybe @jthielen can say more to the case (in)-sensitivity but as I understood the issue it should already be upstream??

There are some units missing (like for PRES and ST) but I wanted to discuss how to proceed before simply adding those.

Related issue number

Towards #36

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable

[jthielen]

I think the approach should be to bring these attributes into CF compliance. So, based on using cf_units (a python wrapper of UDUNITS-2, which defines CF compliance for units), all the units except for "Kelvin" in the table above are invalid and should be fixed (or removed if it's actually dimensionless or without real units).

from cf_units import Unit
unit_list = ["meters2 MSL", "meter MSL", "fraction", "whoknows", "category", "Kelvin", "none", "0/1 Flag"]
for unit_str in unit_list:
    try:
        Unit(unit_str)
        print(f"VALID: {unit_str}")
    except:
        print(f"INVALID: {unit_str}")

INVALID: meters2 MSL
INVALID: meter MSL
INVALID: fraction
INVALID: whoknows
INVALID: category
VALID: Kelvin
INVALID: none
ut_scale(): NULL factor argument
ut_are_convertible(): NULL unit argument
ut_divide(): NULL argument
ut_are_convertible(): NULL unit argument
INVALID: 0/1 Flag

Maybe @jthielen can say more to the case (in)-sensitivity but as I understood the issue it should already be upstream??

Yep, CF-appropriate case handling is indeed an upstream issue in MetPy (Unidata/MetPy#1362), and hopefully those same improvements will be brought over to cf-xarray when they eventually happen!

There are some units missing (like for PRES and ST) but I wanted to discuss how to proceed before simply adding those.

I say go ahead and just add them if they're missing!

[kmpaul]

Just popping in here to say I think this is looking really cool!

[jthielen]

Looks good to me, just a suggestion to not lose CF compliance just to make Pint happy.

[lpilz]

Thanks for the review :)

[lpilz]

Just popping in here to say I think this is looking really cool!

Thanks 😊

[lpilz]

Let's merge this PR quickly before it really gets out-of scope ;) Sorry for 9e37255 - fa8fbee, but this just bugged me...

[andersy005]

Thank you, @lpilz! This looks great!

[kmpaul]

This looks great, @lpilz! Thanks for putting this together!

If ready, I say go ahead and merge.