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

[Daniel Goldberg]

Hi J-M and Martin --

Jean-Michel, I am following up on the hack to increase mixing, and am now
trying to reproduce your results shown below so that I can apply the hack
to SHELFICE and investigate wet-point averaging in the ustar calculation.

I use the latest version of your modified source files and PCELL_MIX.h. I
tried to reverse engineer the description and figures you show but I am
unable to do so -- I show the results corresponding to type 1 test here
http://www.geos.ed.ac.uk/~dgoldber/pcell_mix_bottom_test/figs/
w_p02 corresponds to your p02. Vertical velocity can be seen elsewhere than
the edge of the step but is still very large in the column at the the edge
of each step.

I am not sure if this is because I have not made the same setup as you, I
did not see your input file so I had to guess at some things -- my input
folder is here
http://www.geos.ed.ac.uk/~dgoldber/pcell_mix_bottom_test/input/
and my guess at obcs velocity (i did not see your magnitude?) is here
http://www.geos.ed.ac.uk/~dgoldber/pcell_mix_bottom_test/figs/obcs_u.jpg

Or alternatively do you still have your input folder? This would be easiest
to see what I did wrong..

Thanks very much
Dan


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