EAMxx: add capability to output time-invariant quantities (possibly after 1 time step)

[mahf708]

It could be useful for quantities we know don't change, say PHIS or some constants.

Maybe a generalized feature here is to enable outputing a single snapshot (not recurring) of some variables, say after X steps.

[bartgol]

Can you make an example? We can already output PHIS (it's part of the "grid data", IIRC). In general, non-time-dependent stuff is outputed at initialization (and it is usually JUST the grid data). It would be helpful to have an example of some other time invariant quantity.

Having instant output ONLY at a specific time is more complicated, unless you are willing to have the WHOLE output stream only output at that single time.

[mahf708]

I want to request a file like this:

# preamble, specs, etc.
fields:
 - PHIS       # (ncol,)
 - RHO_H2O    # () constant scalar
 - GRAVIT     # () constant scalar
 - area       # (ncol,)
 - lat        # (ncol,)
 # ...

And sometimes we are maybe interested in a value at a specific time step, say T_mid_horiz_avg_vert_avg just for curiosity, but we don't need it ever again (no need for a recurring output). Rationale for the latter: you can envision us designing very specific eval tests where we want to see how an emulator predicts a quantity after a very specific timeframe and we don't care about this quantity at any other time. We may be generating these files 1000s of times, so saving on storage will become important

[bartgol]

For constants, I think we should maybe add a separate section in the yaml:

fields:
  - T_mid
  - p_mid
  ...
constants:
  - rho_h20
  - g:=gravit

and then output them as global attributes (rather than variables).

PHIS, lat, and area, may already be saved (though I'm not sure if we add PHIS as "grid geometry data", maybe not)

[mahf708]

Saving them as attrs will be less idea because I don't think they are as accessible in computation, one can do ds.T_mid for a variable (ds.variable['T_mid']) but maybe the same doesn't work for an attrs as one would always need to do ds.attrs['something']. Either way, I would go for the simpler solution, not necessarily the most intelligent/feature-rich

[bartgol]

Yeah, that probably won't work. That said, it seems like a very minor inconvenient... ;)

[bartgol]

#7960 allows to add constants as variables.