[MITgcm-support] noise in high resolution run

[Martin Losch]

Hi Nicolas,

I am sorry that I was not precise. I set
viscAh=diffKhT=diffKhS=0. (no harmonic friction)
and
viscA4=diffK4T=diffK4S=1.e12. (bi-harmonic friction, the lowest value  
I could afford with  time step of 600s was is actually 7.e11, but I  
assume that without open boundaries one could go much lower). The  
corresponding parameter to SI is then proportional to (A4.dt)/(dx^4) 
=0.005 for dx=1/6 degree = 6371e3*pi/180/6m, so everything is fine  
here. If you have an isotropic grid, that is you dx=dy locally (dy is  
scaled with cos(lat) on a spherical grid), you have to define  
ISOTROPIC_COS_SCALING (in CPP_OPTIONS.h!!!!) so that (A4.dt)/(dx^4)  
is (nearly) constant over your domain. And this was exactly my  
problem, I used both ISOTROPIC_COS_SCALING _and_ COSINEMETH_III,  
which does not work together. Soon I will fix this COSINEMETH_III issue.

If you want to use harmonic friction, you still should use  
ISOTROPIC_COS_SCALING, COSINEMETH_III does not apply here.

In your plots I see boundary layer problems although your viscAh  
leads to a Munk layer of approximately 2deg and your grid is fine  
enough; weird. What about your diffusion parameters diffKhT/S?. Try  
bi-harmonic friction which is more scale selective.

good luck,

Martin

On Apr 5, 2006, at 4:02 PM, Nicolas Wienders wrote:

>
>> Hi there,
>>
>> my final comment about the noise in my 1/6th degree run:
>> It's gone! With a very high friction parameter of 1e12 for both  
>> viscosity and diffusivity (which is almost the maximum allowed)  
>> and cosPower=4., I have now a 1year run which looks good to me (at  
>> first sight), see http://mitgcm.org/~mlosch/noise6.png
>>
>> My time step is 600s. When I reduce the friction, which I think I  
>> should be able to do, then I run into stability problems after  
>> some integration time (e.g. 5e11 allows me to run for 98 days  
>> before blowing u
>
>
> Dear martin
>
> With the values you mention the stability parameter
>
> Sl=4. (Ah.dt)/(dx^2) is now equal to about 7.10^6
> and this parameter is usually to be left < 0.6 !
> I am surprised you can get any solution.
>
> On our side, very simple configuration, 1/4 or 1/8 degree,
> 25 vertical levels, NATL gyre, gmredi, and cheapaml, i still
> can't get read of the same kind of noise whatever i do with
> the viscosities and diffusivities.
>
> The last try was Ah = viscAh = 3000.
>                              Az = viscAz=10^{-4}
>
> See picture attached.
> The noise is visible for U and V, and W for layers > 1.
>
>
>
> <Picture 2.jpg>
>
>
>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support