[MITgcm-support] Questions about LES / turbulent viscosities

[Martin Losch]

The simplests Richardson number based mixed layer model is  
implemented in the pp81 (Pacanowski+Philander, 1981). The MY82  
package codes the 2nd order MY model, not the 2.5 order model, so  
it's also quite simple. In both case the Richardson number is  
computed from buoyancy frequency and vertical shear of horizontal  
velocities, so again no vertical velocities involved in the  
computation of the viscosity parameter, sorry. But it may be enough  
for your purposes. In the PP81 model you can change the viscosity by  
adjusting PPnu0.

If your flow is primarily horizontal, then you should be fine.

Martin

On 17 Sep 2008, at 19:48, David Hebert wrote:

> Thanks Martin for your input regarding the sub-grid scale  
> parameters. I am looking through the code and see Mellor Yamada 82  
> package. It looks like this computes vertical viscosities and  
> diffusivities based on Richardson number. I am trying to model flow  
> over submerged alps, and perhaps this might suffice for a variable  
> viscosity/diffusivity package. Perhaps this can be used in  
> conjunction with Smagorinski for horizontal parameters? I sill plan  
> on running in non-hydrostatic mode, but would like some turbulent  
> mixing effects downstream of the alps.
>
> Thanks,
>
> David
>
> Martin Losch wrote:
>> 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
>>>
>>> _______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>
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