[MITgcm-devel] Hack to increase mixing near bottom/surface

[Daniel Goldberg]

Hi J-M and Martin:

JM, this is very interesting and looks great. m44 and m43 show very smooth
profiles, not sure if there is a rationale for one over the other. With
regard to the differences in melt rate magnitude, it is difficult to say
but as Martin mentions the melt rate will depend strongly on u*. So while
there may be differences in the temperature and salinity used in the melt
rate calculation (which will differ between shelficeBoundaryLayer and the
various mixing parameterisations) a difference in the way u* is calculated
might make a large difference. Possibly u* is larger in magnitude for wet
point averaging because in the original u* formulation wet cells are being
averaged with dry cells. I saw this as well in the formulation that i
showed you. Due to all the different settings, though, it is difficult to
say which differences are due to the differing u* formulation and presence
of the boundary layer.

Some statements/questions:

1) Is mixSurf a new parameter? that is, based on your notes
http://mitgcm.org/~jmc/pCellMix/pCellMix.notes,
i thought that pCellMix_select=40 had the same effect as mixSurf=4

2) Have you set interDiffKr_pCell=T in any of these tests? I thought this
gave a more accurate vertical discretisation near the surface. Or should it
not impact this at all?

3) In this test, hFacSurfC <= 1, which was the original intention for
shelfice. But for coupling we are interested in the case where hFacC can be
slightly larger than 1, though we try to limit this to ~1.3.

Dan


On Sun, Dec 20, 2015 at 10:28 PM, Jean-Michel Campin <jmc at ocean.mit.edu>
wrote:

> Hi Martin,
>
> Thanks for your explanations.
>
> I updated my "pCellMix" hacked code (http://mitgcm.org/~jmc/pCellMix/)
> which now allows for larger increase of visc & diffK (~ 1/hFac^n,
> n=1,2,3,4).
>
> I repeated some of these tests with Dan's 2-d set-up but modified so that
> it contains some smaller hFac (down to 0.05), and the improvement seen
> before
> (with pCellMix_select=30 or 40) remains.
> I guess to test this in a set-up with rotation, one could use one of the
> isomip experiment ?
>
> Cheers,
> Jean-Michel
>
> On Fri, Dec 18, 2015 at 01:36:08PM +0100, Martin Losch wrote:
> > Hi Jean-Michel,
> >
> > I am not sure if I can contribute  to this, but I can speculate about
> your question:
> >
> > > I wonder if this large sensitivity of melt-rate when using
> SHELFICEuseGammaFrict=T
> > > is common ? or is it due to no-rotation ? or maybe there is something
> wrong in
> > > my wet-point averaging code (now cheked-in) ?
> >
> > when using SHELFICEuseGammaFrict the melt rates become much more
> sensitive to the details of the flow regime. With constant exchange
> coefficients (SHELFICEuseGammaFrict=F), the heat flux/melt rate is
> determined by the temperature difference to the freezing point, i.e. the
> deeper, the more melting with the same potential temperature, and the
> warmer the temperature the more freezing, and it???s just a matter of where
> the warm water is brought into contact with the ice shelf bottom. With
> SHELFICEuseGammaFrict=T, the highest melt rates are found where the
> under-ice currents are larges (and uStar is largest). I assume that
> changing the vertical mixing with your ???hack??? is enough to
> substantially change the circulation and hence the strength of the boundary
> currents under the ice; or modifying uStar by averaging changes the actual
> strength of the exchange and hence the melting.
> >
> > Am I stating the obvious?
> >
> > M
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-- 

Daniel Goldberg, PhD
Lecturer in Glaciology
School of Geosciences, University of Edinburgh
Geography Building, Drummond Street, Edinburgh EH8 9XP


em: D <dgoldber at mit.edu>an.Goldberg at ed.ac.uk
web: http://ocean.mit.edu/~dgoldberg
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