Calculating SMB for Alpine-3D netCDF output about snowpack

[nwever]

It needs to include surfaceFlux.mass[MS_EVAPORATION] as well. There is not (yet) a snowdrift sublimation term. There is a snow drift term, though (Sdata.mass[SurfaceFluxes::MS_WIND]/cos_sl). Maybe it's also good to use Sdata.mass[SurfaceFluxes::MS_HNW] (for snow) and Sdata.mass[SurfaceFluxes::MS_RAIN] (for rain), instead of PSUM.

[EricKeenan]

Thanks for helping me get on track. A few things to still figure out:

1. Still need to determine which variables to divide by cos(slope)
2. Difference between Sdata.mass and surfaceFlux.mass?
3. Synchronize rates and mass contributions in order to have a consistent SMB variable.
4. Is the timestep used in accumulated variables (e.g. MS_EVAPORATION) from SNOWPACK or Alpine-3D?
5. Confirm flux and rate directions (components signs) are correct.

/*
Calculates Surface Mass Balance (SMB) as a function of snowfall, rain, wind erosion, surface sublimation, 
evaporation, and meltwater runoff. Note that SNOWPACK does not yet account for drifting snow sublimation.

Components: 
	Snow rate: MS_HNW (kg/m2/hr)
	Rain rate: MS_RAIN (kg/m2/hr)
	Wind erosion rate: MS_WIND (kg/m2/hr)
	Sublimation: MS_SUBLIMATION (kg/m2, presumably over the timestep)
	Evaporation: MS_EVAPORATION (kg/m2, presumably over the timestep)
	Meltwater runoff: MS_SNOWPACK_RUNOFF (kg/m2, presumably over the timestep)
*/

case SnGrids::SMB:
   value = Sdata.mass[SurfaceFluxes::MS_HNW]
   + Sdata.mass[SurfaceFluxes::MS_RAIN]
   + Sdata.mass[SurfaceFluxes::MS_WIND]
   + (surfaceFlux.mass[SurfaceFluxes::MS_SUBLIMATION] / snowPixel.cos_sl)
   + (surfaceFlux.mass[SurfaceFluxes::MS_EVAPORATION] / snowPixel.cos_sl)
   - (surfaceFlux.mass[SurfaceFluxes::MS_SNOWPACK_RUNOFF] / snowPixel.cos_sl); break;