[MITgcm-support] Re : Water at the bottom of NA below freezing point

[Christopher L. Wolfe]

On Mar 8, 2008, at 10:48 AM, Jeff Scott wrote:
>
> Anyone else have some insight into this problem and/or possible  
> solutions?
>

I've run into similar issues in my models, but, like you, I'm still  
searching for a solution. I too would like to minimize the overall  
diffusion in the system, but I find that at the resolutions at which  
I'm running (5 km) a flux-limiter is absolutely essential. When I use  
2nd and 4th order centered differences, the temperature field rapidly  
runs out of bounds (towards both positive and negative infinity). I've  
tried using GM (with small kappa) to adiabatically remove the small  
scale noise generated by the centered difference schemes, but without  
much success.

My feeling is that your problems with temperatures overshoots will  
just get worse as you go to higher resolutions and I would be very  
cautious about considering impossibly high/low temperatures as an  
acceptable tradeoff for low diffusivity.

Seeing as (at least part of) the future of ocean modeling is a drive  
to higher and higher resolutions, I'd really like to start a  
discussion about how to simultaneously minimize numerical diffusion  
and overshoots. (I know, I want everything.) As Scott mentioned, even  
the "best" nonlinear schemes (SOM and OS7MP) are still quite  
diffusive ... in my experience OS7MP dissipates as much buoyancy  
variance at 5 km resolution as an explicit diffusivity of about 1e-5  
and SOM is even more diffusive. This puts a pretty stiff limit on my  
ability to study the dependence of the circulation on diffusivity.

I'm interested to hear what other high-resolution modelers are doing  
to minimize diffusion while still keeping their buoyancy fields  
physically bounded.

Cheers,
Christopher

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Dr. Christopher L. Wolfe              	   858-534-4560
Physical Oceanography Research Division    OAR 357
Scripps Institution of Oceanography, UCSD  clwolfe at ucsd.edu
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