[MITgcm-support] Questions about LES / turbulent viscosities

[Martin Losch]

David,
the LES capabilities of the MITgcm are not fully developed.
As you noticed, flow dependent viscosities (Smagorinski, Leith) are  
only implemented for the horizontal viscosities in the momentum  
equations (for both horizontal and vertical velocities). Vertical  
viscosities are constant (if you don't use KPP or other mixed layer  
models).
There is no equivalent for tracers. I generally use flux limiting  
advection schemes to take care of the "sub-grid turbulence". This is  
probably not the best sgs-model, but often sufficient.

For non-hydrostatic modelling, I usually do not use KPP, not sure how  
these two features "cooperate". Also, KPP is supposed to model  
shallow and deep convection. In LES I suspect, that you want to  
resolve these processes explicitly.

Implicit viscosity/diffusion needs to be turned on with KPP (and  
other mixed layer schemes), because large vertical viscosities/ 
diffusivities, as computed by KPP, can make the explicit scheme  
unstable. Otherwise it is usually not required (but you can always  
turn it on).

Implementing 3D smagorinski viscosities is on someone's list, but I  
don't know how far down.

Here are some more earlier postings of this list that deal with  
Smagorinski, etc.
http://forge.csail.mit.edu/pipermail/mitgcm-support/2006-June/ 
004086.html

Martin

On 16 Sep 2008, at 19:44, David Hebert wrote:

> I have been looking at the manual regarding LES viscosities  
> (section 2.21) and am hoping someone can answer the following
>
> I see viscC2Smag appears in the pkg/mom_vecinv routines. Does this  
> imply that vector invariant form must be used with viscC2Smag?
> It appears that this will only affect horizontal viscosities. If I  
> am running in nonhydristatic mode, do I need to use KPP (section  
> 6.4.2 of the manual) for enhanced vertical viscosity based on  
> shear, etc.
> Is viscC2Smag only used for momentum equations? Do I need to use  
> KPP for turbulent mixing of tracers?
> Along those lines, in the KPP section I see there are flags for  
> implicitViscosity and implicitDiffusion. I am having trouble  
> finding what these flags do outside of KPP. Are they required for  
> viscC2Smag? Where can I find more info about the implicit flags?
> Thanks for your help.
>
> David
>
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