[MITgcm-support] noise in high resolution run

[Baylor Fox-Kemper]

Hi Nicolas,
   Things to try:
   1) You are using no_slip_sides=.false., it seems from your  
velocity fields.  Might be interesting to try .true. This will change  
the solutions dramatically, but if it gets rid of the noise then we  
know where to look.
   2) Try usejamartwetpoints=.true.  This may help your w field near  
walls...
   3) You might try different advection schemes for tracers, too.  
There have been some issues of 'digging' near boundaries with some of  
the schemes.  I personally like tempadvscheme=33 and  
saltadvscheme=33.  This may make diagnosis a bit more difficult but  
will reduce certain kinds of 'noise'.
   Hope that helps,
     -Baylor



On Apr 5, 2006, at 10:23 AM, Martin Losch wrote:

> 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>
>>
>>
>>
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