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

[Daniel Goldberg]

Hi J-M and Martin

I have looked at the s/r's and the tests and this looks very compelling.
Regarding interViscAr_pCell and interDiffKr_pCell, these changes do not
seem related to increasing viscosity and diffusivity in thin cells, but
rather to improve upon discretisation that already exists in cells just
above the bottom or below the surface, is this correct?

I see that with pCellMix_select, you override the cell visc and diff with
imposed values (increased by a factor of how much smaller hfacC is than
some threshold value). 2 questions on this -- (1) should there not be an
upper limit to prevent infinite diffusivity and viscosity, and (2) would
these replace any values calculated by a physical package like KPP or GGL90?

I also am unsure where vertical viscosities and diffusivities are
physically located. Are they located at vertical (upward-facing) cell
faces? In the center of these faces or at the boundaries?

Further questions below --

On Fri, Jan 16, 2015 at 7:05 PM, Jean-Michel Campin <jmc at ocean.mit.edu>
wrote:

> Hi Dan and Martin,
>
> I made some simple test with a hack to increase mixing near above bottom
> and/or
> below surface when the bottom/surface grid cell is thin (small hFac).
>
> I put some files here: http://mitgcm.org/~jmc/pCellMix/
> with some documentation in file "pCellMix.notes"
>  and some plots from my simple test.
>
> the hack: it should move to the main code at some point, but
> 1) need more tests (and feedback)
> 2) it involves changing more S/R (> 7 but only 2 right now) since it
>  needs to be pushed where vertical diffusion and viscosity are applied:
>  ( mom_u/v_implicit_r.F mom_u/v_rviscflux.F
>    impldiff.F gad_diff_r.F gad_implicit_r.F )
>  and passing more arguments to these S/R (-> also changing the calling S/R)
>
> Some remarks:
> 1) we should probably always use selectBotDragQuadr=1 or 2 instead of the
> original
>   discretisation (selectBotDragQuadr=0), see
> http://mitgcm.org/~jmc/pCellMix/fig_wt4.ps
> 2) accounting for partial cell in the interior (interViscAr_pCell=T,
>   interDiffKr_pCell=T) is safe when using implicit visc/diff ;
>   but also accounting for hFac in bottom friction (bottomVisc_pCell=T) when
>   using no-slip-bottom=T can be unstable; and we don't have code for
> implicit
>   bottom friction + it would not work with present solve_for_pressure code.
>

I am confused by the 3rd set of tests -- you seem to be comparing the
effect of drag formulation, not the partial cell treatment of viscosity.

Also -- I was unaware that vertical viscosity and diffusivity could be made
implicit, could you tell me which parameters/CPP options enable this?


>
> Regarding how this could be use with pkg/shelfice (to avoid using
> SHELFICEboundaryLayer):
>

This change seems ready to test with the Shelfice package, in that the
surface implementation is coded for ksurfC>1..


> 1) in isomip test experiment, no_slip_shelfice=F and this is good,
> otherwise
>    the velocity below ice-shelf is strongly reduced when hFac is small
> (and it's
>    physical) and make the melt-rate very dependent on hFac.
> 2) using a type of quadratic drag: the equivalent of selectBotDragQuadr >
> 0 is not
>    coded in shelfice_u/v_drag.F but should not be hard (+ I added the new
> argument)
>

There is a parameter call SHELFICEDragQuadratic and it does seem to make
drag quadratic (but it looks as though it can only be implicit, and
probably not implemented the same way as selectBotDragQuadr > 0)


> 3) might need to test different averaging (e.g. wet-point) of the velocity
>    to the grid-cell center (in shelfice_thermodynamics.F)
>    I don't know if there is a diagnostic for this (but could be useful)
>

Should e.g. 0.5 * (uFld(i,j) + uFld(i+1,j)) replace uFld(i,j)? I believe
these currently are the velocities in ksurfC(i,j). Should any thought be
given to circumstances when not all are available? What if, for instance,
maskW(i+1,j,ksurfC(i,j)) = 0 due to variable ice shelf topography?


Thanks for beginning this work,
Dan



> 4) would be interesting to try SHELFICEboundaryLayer=F and
> pCellMix_select=10 ;
>
> Cheers,
> Jean-Michel
>
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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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