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

Tue Apr 4 10:15:09 EDT 2006

Hi Baylor,

have done so for GAD_OPTIONS.h.
For CPP_OPTIONS.h I'd rather wait a little, because to my mind the  
flags are broken. I would like to hear other opinions on this first.  
Also ISOTROPIC_COS_SCALING and COSINEMETH_III do not appear in  
CPP_OPTIONS.h so far.

Martin
On Apr 4, 2006, at 3:52 PM, Baylor Fox-Kemper wrote:

> 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
>>>>
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