Adds a porosity parameter for internal frazil salinity

[njeffery]

Modifies how the internal frazil salinity is calculated in interior ocean layers. We currently use the coupling salinity of 4 psu (config_frazil_sea_ice_reference_salinity) or 0 if under ice shelves even in interior ocean layers. This leads to an unphysically large salt flux at depth and too much freshening in upper layers. The new approach is to use the local layer salinity times a frazli porosity namelist parameter (config_frazil_ice_porosity) to estimate the salt content of frazil flocs in the ocean interior. Any frazil accumulated at the surface is then adjusted so that it's salt content is consistent with 4 psu or 0 if under iceshelves.

[njeffery]

A simpler version of the code (equivalent to porosity = 1) was tested in 100-year fully coupled pre-Industrial simulations. See https://acme-climate.atlassian.net/wiki/spaces/HESF/pages/4765679617/20241010.Frazil2.underIce.piControl.anvil

I'm rerunning mpas-analysis to use the cryosphere option.

I've also got a run going that drops the 100 m depth restriction.

[njeffery]

Flagging @proteanplanet , @darincomeau , @xylar , @cbegeman and @vanroekel. I can't find Irena's git name.

[xylar]

@irenavankova, this would be @njeffery's branch to try if you're interested.

[irenavankova]

Thanks, I will give it a try.

[njeffery]

@xylar and @cbegeman : if there is heat in the top layer, then the frazil does cool the layer. This is handled by the layer (i.e. water mass) and temperature tendencies which keep track of the net potential sea ice formation. There is no explicit brine mass in this formulation when salt is lost to additionally cool a layer, though that would be cool. Anyone interested should go forth and model. However, keep in mind that we have a sea ice model for handling mushy layer thermodynamics and brine rejection. An already planned, but longer term approach is to implement consistent multiphase coupling.

[xylar]

I'm mostly wanting to make sure this is handled as expected under ice shelves as well, since the sea ice model doesn't apply there.

[njeffery]

Got it. I looked back at the namelist for the frazil fully coupled runs and config_frazil_under_land_ice = .true. I thought this was supposed to be default false for WC? What happens to the accumulated frazil ice formed under the iceshelves in these runs?

[xylar]

I'm seeing that config_frazil_under_land_ice is false by default (i.e. when ice-shelf fluxes are completely off): https://github.com/E3SM-Project/E3SM/blob/e798f6bd9ca528151e9b000e017ffd35893bb7dd/components/mpas-ocean/bld/namelist_files/namelist_defaults_mpaso.xml#L404

[xylar]

That change is pretty recent so maybe you have a branch where that's not the case?

[njeffery]

What's the flag for when ice-shelf fluxes?

[njeffery]

are off...

[xylar]

What happens to the accumulated frazil ice formed under the iceshelves in these runs?

In runs where config_frazil_under_land_ice = true, the frazil formation through the (top 100 m of the) water column is the same as in the open ocean. But then the frazil mass just gets added to accumulatedLandIceFrazilMassNew and disappears from the ocean. There is no mechanism for that to cause surface cooling or to re-melt directly (though surface melting or freezing of land ice is always happening).

[xylar]

What's the flag for when ice-shelf fluxes are off?

On the current master, config_frazil_under_land_ice = false.

The default is that ice-shelf melt fluxes are off (e.g. in WC runs) and under those circumstances, config_frazil_under_land_ice takes the default value I linked to above of false.

[xylar]

But then the frazil mass just gets added to accumulatedLandIceFrazilMassNew and disappears from the ocean.

So my concern is that it seems like your branch introduces a mechanism for porous frazil to dump its salt at the surface under ice shelves, which is great! But we don't have a mechanism (as it seems there must be via sea-ice coupling in the open ocean) for the top layer to gain or lose the corresponding heat.

[njeffery]

Seems you need a sea ice model under ice shelves!