[MITgcm-support] noise in high resolution run

[Martin Losch]

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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