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

[Jean-Michel Campin]

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