[MITgcm-support] changes in seaice default behaviour?

[Holland, Paul R.]

Hi Martin

I share your confusion!  These thick ice cells stay at AREA~1, but they are surrounded by open water in summer.  I am guessing that when SEAICEscaleSurfStress=.true., as the ice nearby melts to zero, the wind stress on that ice asymptotes to zero also, and the thick ice nearby is allowed to stay.  When SEAICEscaleSurfStress=.false., the nearby ice gets a proportionately higher and higher stress per unit mass as its AREA drops, and so this is able to partially flush the thick ice out, by shearing it.

My thinking at the moment is that there is an underlying problem in that the rheology should not be allowing this thick ice.  So the stuff above is just compensating for this somehow.  If the rheology prevented the thick ice, then we would be fine to switch back to SEAICEscaleSurfStress=.true., as it should be.  So focussing on SEAICEscaleSurfStress is not necessarily the right thing to do?  I need to ask around some sea ice people to hear the folklore on thick ice near coasts in other models.

Another thing that worries me is that with SEAICEscaleSurfStress=.false., the stress on the ice is inconsistent with the stress passed to the ocean, which is scaled by AREA, right?

Cheers

Paul

> Hi Paul,
>
> I don?t understand why the SEAICEscaleSurfStress parameter makes such a
> big difference. It should only make a difference with ice concenrations < 1
> (really <<1). For AREA < 1 it basically reduces the interfacial stress between
> ice and ocean and ice and atmosphere. Why should that lead to more pile up
> of ice along the coast? Do you have AREA<1 there (i.e. the effect of offshore
> katabatic winds being reduced by that?)
>
> SEAICEpressreplfac = 0. is different, because removing the replacement may
> not be undesirable physically. Replacement pressure was introduced to avoid
> unforced ice motion by a pressure gradient grad(P) in the absense of forcing.
> Fair enough, that avoid unphysical behavoir, but what does that also do? You
> replace P (in grad(P)) by P*Delta/(Delta+DeltaMin) (or
> P*Delta/max(Delta,DeltaMin), but that makes no difference). When ice is
> pushed against a coastline or into a ?bay? (represented by a single grid cell,
> maybe), there will be no motion and Delta -> 0, but DeltaMin is constant at
> O(1e-9). So P->0 and there will be no resistance against compression, so that
> ice can pile up indefinitely. That?s also unphysical. So you have choice
> between two unphysical behaviors, which both stem from a
> regularisation/parameterization. You can probably decrease DeltaMin further
> to avoid either problem, but then it will get harder and harder so solve, ? it?s
> a dilemma.
>
> Martin



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