Model_mods which use their own structure to keep track of state variables

[hkershaw-brown]

progvar type:
* mpas_ocn (does not compile)
* mpas_atm
* NCOMMAS (does not compile)
* NAAPS (does not compile)

• wrf has type wrf_static_data_for_dart

• wrf_hydro is making assumptions about the indexing.

• clm has metadata arrays that are the same size as the state:
  607 allocate(lonixy(model_size), latjxy(model_size), levels(model_size), landarea(model_size))
  this you don't want to access with dart_index (i.e. assume the state structure), but instead f(x,y,z,variable).
  Same for the small models have state-length metadata storage. Any model that adds a domain like this dom_id = add_domain(model_size). 9-var has unpack9var.

hot take: I don't think model_mods should have access to dart_index at all.

Some models have their own get_varid_from_kind
POP - using offset
CICE
bgrid_solo

I think you can flush out if a non-toy model_mod is assuming the state order by reversing the order of the variables in the state_structure_mod. i = num_vars, 1, -1, run filter/model_mod_check/whatever and seeing if your results change.

Who cares?

Possible reasons to care:

• General implementation of squished state (Ed's siparcs 2022) project
• Remove missing_r8 from the state so assim_tools, quad_utils can assume all state is valid.
  - same across the ensemble, e.g. POP
  - different across the ensemble, e.g. CLM
  I don't think ensemble manager can stop checking for missing_r8 because a missing_r8 result for a forward operator is valid.
• For wrf, adding a new quantity means updating the model_mod.

[hkershaw-brown]

Note there are perturb routines that assume state variables are contiguous var = state(startindex:endindex)

[hkershaw-brown]

mpas_ocn may be needed for E3SM #769

[hkershaw-brown]

note from chat with Marlee while discussing pull #783
qtys are optional to add_domain (pop not passing qtys) so you cannot assume (take for granted) that you can query state structure for qty, so you have to take that into account for state that is not state (missing values, dry land, etc).
mask vs. get_state_meta_data as a modifier

[hkershaw-brown]

#783 replaces the POP get_varid_from_kind, but POP still using offset. edit: same for CICE