[MITgcm-support] KPP scheme and background viscosities

[Jody Klymak]

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 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/




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