[MITgcm-devel] Re: [MITgcm-support] noise in high resolution run

[Baylor Fox-Kemper]

Hi Martin,
   Now that you know how all of this works, could you check in  
comments to gad_advection.h and cpp_options.h?
   We shouldn't lose the thread twice.
    -Baylor

On Apr 4, 2006, at 6:32 AM, Martin Losch 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 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
>>>
>>> _______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>
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