[MITgcm-support] KPP scheme and background viscosities

[Abbas Dorostkar]

Hi Jody,

Sorry, my email must be a bit confusing!

I set the explicit horizontal diffusivity to a very small value of 10^-7 m^2
s^{-1} as the 3-DST advection scheme is unconditionally stable and introduces
numerical diffusivity where needed to eliminate grid-scale noise.

I mentioned that the model is not sensitive to the background vertical
viscosity in the range of 10^-7 to 10^-4 m^2 s^{-1} while the background
vertical diffusivity is 10^-7 m^2 s^{-1}. However, increasing the background
vertical viscosity to 10^-3 m^2 s^{-1} gives me a better agreement. From
literature, researchers usually use the constant value of 10^-3 m^2 s^{-1}
for vertical viscosity when the KPP is not used. I am not sure if there is
any relation here.

On the other hand, increasing the background vertical diffusivity from 10^-7
to 10^-4 m^2 s^{-1} while the background vertical viscosity is 10^-3 m^2
s^{-1} gives me a poorer not better agreement.

Please let me know what you think

Thanks

Abbas





On Mon, Apr 19, 2010 at 4:38 PM, Jody Klymak <jklymak at uvic.ca> wrote:

> Hi Abbas,
>
> I'm surprised that increasing the background diffusivity would give you a
> better agreement.  It means either a) that this is simply fortuitous, or b)
> that KPP is not ramping up the turbulence enough.
>
> On Apr 19, 2010, at  8:34 AM, Abbas Dorostkar wrote:
>
> I have simulated basin-scale internal waves in a lake using the hydrostatic
> version of MITgcm on a 400x400 horizontal grid spacing. The smallest
> vertical grid spacing is 0.5 m. A staggered baroclinic time-stepping is
> used for the tracer equation. The tracer advection scheme is a 3-DST so I
> set the horizontal eddy diffusivity to 1E-7. The horizontal eddy viscosity
> is constant with the value of 1. The vertical eddy viscosities and
> diffusivities are computed by the KPP scheme.
>
> Why is your horizontal diffusivity so high?  For a high-resolution run like
> this you should be able to have much smaller horizontal mixing, i.e. 10^-3
> or 10^-4 m^2 s^{-1}. I'm assuming you have solitary waves and the like, and
> a large horizontal viscosity acting on those is going to have a very high
> diapycnal viscosity.
>
> The model is not sensitive to the background vertical viscosity in the
> range of 1E-7 to 1E-4. However, the background viscosity of 1E-3 reduces the
> root-mean-square error between the model and field data by 20% over the
> simulation which uses a value of 1E-5.  Also, the model also does not show
> sensitivity to the background vertical diffusivity ranging 1E-7 to 1E-5.  However,
> using higher values such as 1E-4 gives very poor error statistics.  Does
> anybody have in any inputs?
>
> So higher diffusivity or lower viscosity give poor results?  That's
> confusing.
>
> The model gets unstable if I use background vertical viscosity larger than
> 1E-3 unless I use smaller time step. I was wondering if MITgcm has a viscous
> limitation controlled by the vertical eddy viscosity (ViscAz) such that
> (deltaT)*(ViscAz)/(deltaZ)**2 < 1.
>
> Its pretty hard to understand why you would need such a high background
> turbulent viscosity.  I don't think there is a viscous limitation in the
> model anywhere.
>
> I admit to being somewhat ignorant about exactly what KPP does; Have you
> tried running this with KPP turned off?
>
> You may want to check out a paper we recently published in ocean modelling
> doi:10.1016/j.ocemod.2010.02.005<http://dx.doi.org/10.1016/j.ocemod.2010.02.005>
>  where we use the Thorpe scale to set the vertical viscosity and
> diffusivity in a convective overturn.  This assumes you resolve turbulent
> overturns, which at 0.5 m, I suspect you might.  Not sure if it is useful to
> your situation in particular.
>
> Cheers,  Jody
>
>
>
>
> Thanks in advance for your ideas
>
> Abbas
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>
>
> --
> Jody Klymak
> http://web.uvic.ca/~jklymak/ <http://web.uvic.ca/%7Ejklymak/>
>
>
>
>
>
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>


-- 
Abbas Dorostkar, M.Sc (Eng.)
PhD Student
Room 444, Ellis Hall
Dept. of Civil Engineering
Queen's University
Kingston, ON, CANADA K7L 3N6
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