Clean up CanopyHydrology

[billsacks]

Before implementing water isotopes in CanopyHydrology (#360 ), there is some general cleanup that needs to be done.

First, we should probably separate this into at least three routines:

• Snow cover fraction
• Snow initialization
• Canopy Hydrology (though it does more than that)

We'll separate flux calculations from state updates so that we can make tracer update calls at the appropriate points.

For now, we'll keep state updates in this module, done at the point where they are currently done (though in separate subroutines). Later, we might merge some state updates (which would change answers).

[billsacks]

Refactoring snow cover fraction calculations in CanopyHydrology

We'll keep in place the original fsca formulation (n&y 07) - probably giving it a better name than just origfflag [edit (2019-04-24) I think this meant 'oldfflag' rather than 'origfflag']. Note that that is the form used in Noah-MP.

Note that, the way things are currently structured, it does the new frac_sno calculation, then possibly overwrites it with the old.

And note that there are order differences for the new vs. old: New needs to compute frac_sno, then update snow_depth based on that; old needs to first update snow_depth, then compute frac_sno based on that. So we should have a routine that updates both frac_sno and snow_depth, and let individual parameterizations do that in whatever order they want.

Then this will be lumped with the FSCA block above it:

https://github.com/ESCOMP/ctsm/blob/5295f61ed21bd6b9ff0d7770d064b1328f030058/src/biogeophys/CanopyHydrologyMod.F90#L548-L554

whereas this will be lumped with the "original fsca formulation" below it:

https://github.com/ESCOMP/ctsm/blob/5295f61ed21bd6b9ff0d7770d064b1328f030058/src/biogeophys/CanopyHydrologyMod.F90#L555-L558

i.e., it is currently assumed that subgridflag==1 is combined with oldfflag==0, and vice versa (see also #502 ).

So we'll end up with code that looks like this:

select case (snow_cover_frac_method)
case (method_snow_cover_frac_clm5)
   call snow_cover_frac_clm5(..., frac_sno, snow_depth)
case (method_snow_cover_frac_niu)
   call snow_cover_frac_niu(..., frac_sno, snow_depth)
end select

subroutine snow_cover_frac_clm5(..., frac_sno, snow_depth)
  ! First, compute frac_sno

  ! Hmmmm, need to determine what to do about the lun%urbpoi check in the current code.
  ! (I feel like we had to deal with something similar for soil hydrology - i.e., urban
  ! always working the old way. Check how we handled that.)
  if (frac_sno(c) > 0._r8)then
     snow_depth(c)=snow_depth(c) + newsnow(c)/(bifall(c) * frac_sno(c))
  else
     snow_depth(c)=0._r8
  end if
end subroutine snow_cover_frac_clm5

subroutine snow_cover_frac_niu(..., frac_sno, snow_depth)
  snow_depth(c)=snow_depth(c)+newsnow(c)/bifall(c)

  ! Then compute frac_sno
end subroutine snow_cover_frac_niu

[billsacks]

The part that actually deals with canopy hydrology is done (will come to master in ctsm1.0.dev041). The other pieces (fractions and snow initialization) still need to be done.