[MITgcm-support] noise in high resolution run

[Martin Losch]

Hi Baylor, Dimitris,

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
SadournyCoriolis
etc.?
Any experience with that?

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.

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
>>
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