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

Tue Apr 4 09:27:13 EDT 2006

Hi,

after solving my problem, I do believe that there is problem in the  
interaction of the CPP-flags ISOTROPIC_COS_SCALING and COSMETH_III.  
The latter does not work properly with the former, because in  
mom_u_del2u and mom_v_del2v, only the x-derivatives are scaled with  
sqcosFacU. I believe there is a
#ifdef ISOTROPIC_COS_SCALING
#ifdef COSMETH_III
      &   *sqcosFacV/U(J,bi,bj)
#endif
#endif
missing in the y-derivative part, so that the y-derivatives are only  
scaled with sqrt(cosFac), while the x-derivatives are scaled with  
cosFac, which is smaller than one and thus explains the 2dx-noise:  
there is too little viscosity in the x-direction, but one cannot to  
increase the overall viscositiy, because then it becomes too large in  
the y-direction (which is larger by a factor of sqrt(cosFac)) and  
leads to an explosion.

Martin
On Mar 31, 2006, at 6:49 PM, Patrick Heimbach wrote:

>
> Hi all,
>
> I am moving this to the devel list.
>
> Given that we've been seeing also somewhat strange behaviour
> in vertical velocity fields near topography and not so stable
> behaviour of the adjoint as a consequence, I am wondering
> (as is Martin) whether this is indicative of a bug.
> Haven't looked carefully recently (in particular after the adjoint
> bug fix for variable aliasing), will dot it soon, but don't think,
> the 4deg adjoint showed this behaviour in much earlier versions.
>
> The adjoint might actually tell us (if we believe it) where to
> look for potential sources of the problem. I'll try to have
> a look next week.
> But maybe it's all ok...
>
> OK, after making such provocative statement, I'll now
> be offline for a few days (and have beers with Martin instead).
>
> -p.
>
>
>
> Quoting Baylor Fox-Kemper <baylor at MIT.EDU>:
>
>> Hi Martin,
>>
>>> useJamartWetPoints is a good suggestion. I will try that next.   
>>> Since the noise seems to be associated with the coasts it sounds   
>>> like a good guess. What about the other flags,
>>
>>> useJamartMomAdv
>>
>> I was told that this one doesn't really make sense in terms of   
>> conservation near boundaries, I believe.  I'm no expert in it,  
>> though.
>>
>>> SadournyCoriolis
>>
>> JMC just told me that this is actually NOT the scheme that  
>> Sadourny  suggests to use, it is really one of the ones that he  
>> suggests NOT to  use in the same paper!  So, I think that for the  
>> moment this is  probably not a good bet either.
>>
>>> I use
>>> no_slip_sides=.false.,
>>> the sidedrag code is a drag to debug. Each time I have a look at   
>>> it, it looks wrong the first time around and then after 3hs of  
>>> mind  warp it turns out to be right. When I have a look at the  
>>> plots I  find it hard to believe that there is not a bug in the  
>>> whatever- part of the code (doesn't even have to be the  
>>> viscosity). I guess I  have to start turning off terms and see  
>>> what happens.
>> sidedrag can't be the problem then...
>>
>> What I fear is what Michael suggests:
>>> we had some problems like this with the NCOM (hydrostatic) at  
>>> one  time and tried all sorts of things.
>>> it turned out to be an indexing problem in the code.
>> The question is, where?
>>   -Baylor
>>
>>> Dimitris,
>>> I don't use KPP so far, although I would like to use in the  
>>> future  (maybe, I have enough problems as it is (o:). My  
>>> experience with  KPP is that it tends to amplify noise, but does  
>>> not generate it  (?). In that case the horizontal filters are  
>>> useful. In this case,  I don't think that it will help too much,  
>>> I am afraid, because the  noise is already there without KPP.
>>>
>>> Thanks for your suggestions.
>>> Martin
>>> On Mar 30, 2006, at 8:06 PM, Baylor Fox-Kemper wrote:
>>>
>>>> Hi Martin,
>>>>   I've seen similar noise in W in some runs, but not so  
>>>> obviously  in U and V.  Have you tried jamartwetpoints?
>>>>   Also, are you using no-slip?  We found a few bugs in the   
>>>> sidedrag code a while back.  It might be worth revisiting...I   
>>>> suppose it is possible that there is a problem elsewhere in the   
>>>> viscosity/viscous terms code, but I don't know why it would be   
>>>> localized in space.
>>>>   -Baylor
>>>>
>>>>
>>>> On Mar 30, 2006, at 8:30 AM, 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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>
>
>
> --------------------------------------------------------
> Patrick Heimbach   Massachusetts Institute of Technology
> FON: +1/617/253-5259                  EAPS, Room 54-1518
> FAX: +1/617/253-4464             77 Massachusetts Avenue
> mailto:heimbach at mit.edu               Cambridge MA 02139
> http://www.mit.edu/~heimbach/                        USA
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