[MITgcm-support] Viscosity parameters

[Martin Losch]

Neil,

I think that your viscAhGrid is too small. I’d try something like 0.1
Also you might want to try biharmonic viscA4Grid.

The flow dependent viscosity parameterization (especially Leith, see Dimitris suggestion) are usually the most stable choices.

Martin

On Feb 28, 2014, at 1:44 AM, Neil Patel <nigellius at gmail.com> wrote:

> Hi,
> 
> I'm having trouble understanding the different viscosity parameters. Read the mitgcm manual many time over but still am confused. From what I can tell:
> 
> 1) Vertical eddy viscosity doesn't affect the numerical stability, so choose the value closest to the observations for ViscAr
> 2) Horizontal eddy viscosity does affect the numerical stability and the number chosen should be similar to various energy scaling / grid parameters.
> 
> currently, I'm running a 1/6° resolution simulation. I set 
> 
> viscAr=5.6614E-04,
> 
> and 
> 
> viscAhGrid=0.01,
> viscAhReMax =0.1,     
> viscAhGridMax=0.1,  
> 
> I'm not sure what the difference between the last two. With the current setup I have, will the viscosity always be within the CFL condition? And so, If my model crashes  — "SOLUTION IS HEADING OUT OF BOUNDS" is what I've been getting, with the parameters above could the viscosity parameters be the issue or is it soley due to too large a time step. What would be the negative of setting the viscAh parameters to a very low number? Would it be an unphysical ocean or just a poor parametrization of small flows or neither?
> 
> I've read the details of the Leith / Smagorinsky parameters. I presume they're a replacement to setting viscAhGrid/viscAhGridMax parameters, but not sure what the advantages or disadvantages of either are.
> 
> Thanks for any explanation you can give.
> 
> Thanks,
> 
> Neil
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