[MITgcm-support] noise in high resolution run

[Martin Losch]

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 up), which I interpret as CFL problems when velocities become  
too large. I did not check where this happens, but I had hoped to be  
able to run with a longer time step. Oh well.

So the take home message: define ISOTROPIC_COS_SCALING in  
CPP_OPTIONS.h (as was pointed out in the thread in Dec2003), do not  
use COSMETH_III with ISOTROPIC_COS_SCALING, which does not appear to  
work.

Martin

PS. there remains a little bit of noise in the w-velocities near  
topography, which I choose to neglect.

On Apr 3, 2006, at 6:07 PM, Martin Losch wrote:

> Hi,
>
> after trying out many things, I may have found a solution to the  
> noise problem. In
> CPP_OPTIONS.h I had to set
> #define ISOTROPIC_COS_SCALING
> #undef COSMETH_III
> because
> 1. setting it in GAD_OPTIONS.h does not apply to the momentum  
> equations (viscosity terms),
> 2. the COSMETH_III does not seem to work with isotropic scaling,  
> maybe it is not meant to work with it, but anyhow, if you turn on  
> COSMETH_III, the scaling is no longer isotropic, which causes the  
> stripes that I observed in my solutions.
> In GAD_OPTIONS.h I commented out the line that says #undef  
> ISOTROPIC_COS_SCALING, so that the isotropic scaling also applies  
> to the tracer fields (diffusivity), but I left the COSMETH_III,  
> because that seems to be correct in this context (although  
> unsetting it here would probably now change things too much).
> Now I can turn off harmonic friction and use a relatively low  
> viscosity/diffusivity with cosPower=4. (in data).
> Some of these aspects are already discussed in a thread in 2003,  
> here's the end of the thread:
> http://dev.mitgcm.org/pipermail/mitgcm-support/2003-December/ 
> 001896.html
>
> So Nicolas, maybe this solves your problem as well. I still have to  
> tune my friction parameters because things are still blowing up,  
> but I do not see this standing noise pattern (so far).
>
> Martin
>
> On Mar 30, 2006, at 3:30 PM, Martin Losch wrote:
>
>> Hi,
>>
>> I seem to have a problem with a 1/6 by 1/6*cos(phi) run with open  
>> boundaries. The domain is the Drake Passage. A plot of bathymetry  
>> and velocities can be found in
>> http://mitgcm.org/~mlosch/noise.png
>>
>> Whatever friction parameters I have tried (viscAh=1e0 to 2e1 and  
>> viscA4=1e8 to 4e10, and similar for diffusivities, I also tried  
>> the Leith/Smagorinski variants), I seem to get noise in the north  
>> western part of the domain. What worries me is, that
>> 1. The noise seems to propagate (compare day 149 to day 214 in the  
>> bottom panels of the figure)
>> 2. The noise seems to be mainly in the x-direction
>>
>> I use mom_fluxform. For mom_vecinv the problem is there, too.
>> I use USE_ISOTROPIC_SCALING (for viscosities because my y-grid  
>> varies with y) and do not use COSINEMETH_III (although that  
>> probably doesn't make much of a difference). When I turn off  
>> USE_ISOTROPIC_SCALING, the noise is still there, but the x- 
>> alignment is slightly less obvious (although very much present).  
>> If the noise were deltaX in both directions, I would be concerned  
>> about my friction parameters. Here I suspect a problem in the  
>> viscosity implementation, but I cannot see how and were.
>> There is some noise that is produced by the open boundaries, but  
>> that usually goes away. I think that the generation of the noise  
>> is connected to the topography around the tip of South America
>>
>> I wonder if anyone has seen something like this before. What do  
>> you think?
>>
>> Martin
>>
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