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

Wed Apr 5 11:10:52 EDT 2006

It turns out that in mom_fluxform I have to modify mom_u_del2u.F and  
mom_v_del2v.F to get proper isotropic scaling of the biharmonic  
friction term with COSINEMETH_III. I am quite confident that I am  
doing this right,
BUT
in mom_vecinv the file mom_vi_hdissip.F needs cosFacs in the case of  
ISOTROPIC_COS_SCALING in order to get my configuration to work. Now,  
I don't know if that's against the philosophy of mom_vecinv, but I am  
willing to check these changes in (provided they don't modify the  
tests) and then go on a 10 day vacation and let you handle the mess (o:
Could someone please have a look at mom_vi_hdissip.F and tell me  
whether I am right? (attached).

Martin
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On Apr 4, 2006, at 4:15 PM, Martin Losch wrote:

> 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
>>>>>
>>>>> _______________________________________________
>>>>> MITgcm-support mailing list
>>>>> MITgcm-support at mitgcm.org
>>>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>>>
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