[MITgcm-support] bottomDragQuadratic magnitude

[Jean-Michel Campin]

Hi,

Just a minor comment regarding this:
> ... but keep in mind that the bottom drag will be applied as a body force to the bottom-most grid cell,
> i.e. there is a scaling with the grid cell width implied, so that for thin cells the drag will be smaller.
I prefer to view the quadratic bottom drag as a (too ?) simple expression for bottom frictional
stress (i.e., a momentum flux at the bottom) rather than a body force.
It's very/too simple since the magnitude of the bottom stress is just a quadratic function of
the model bottom-cell velocity (does not account for how thick this cell is).
And as this bottom stress is applied to the model velocity, it is naturally converted
to a tendency term for the bottom-cell velocity by dividing the stress by the grid-cell thickness.
So the bottom stress might get smaller when the bottom grid-cell is thin but this is
a side effect (because the model bottom velocity is also getting smaller) and not a direct effect
of the formulation.

Cheers,
Jean-Michel

On Thu, Jun 06, 2024 at 07:04:49PM +0200, Martin Losch wrote:
> Hi Dong,
> 
> I don???t have a good answer for this, but keep in mind that the bottom drag will be applied as a body force to the bottom-most grid cell, i.e. there is a scaling with the grid cell width implied, so that for thin cells the drag will be smaller. To overcome this, a logarithmic law of the wall is implemented for the drag, see https://mitgcm.readthedocs.io/en/latest/algorithm/algorithm.html#vertical-dissipation, eq 2.121 and 2.122, maybe that can be useful for you. With the formulae you could also estimate the bottom drag as a function of roughness length, which may be the more universal parameter.
> 
> Martin
> 
> > On 28. May 2024, at 12:34, Dong Jian (ENV - Postgraduate Researcher) <D.Jian at uea.ac.uk> wrote:
> > 
> > Hi, Dear MITgcm-ers
> >  
> > I am wondering if anyone has experience with the choice of bottomDragQuadratic coefficient? Typical values I can refer to are bottomDragQuadratic = 1.E-3, or bottomDragQuadratic=2.5E-3.
> >  
> > But I am testing a regional model over very rough topography, I do find larger bottomDragQuadratic (even much larger than 2.5E-3 ) gives a closer transport and more similar circulation pattern compared with reanalysis, is there a limit value on the choice of bottomDragQuadratic?
> >  
> > Thanks in advance!
> > Dong
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