[MITgcm-devel] mom_calc_visc.F

[chris hill]

Darn - does that mean cnh has to sort it out! I'll take a look.

As I said to David (when I did respond to his original question) - there 
is no 100% _correct_ answer here. The optimal scaling of effective 
dissipation/diffusion with grid spacing does not need to follow the 
spacing exactly .e.g on a lat-lon grid typically the meridional 
component of biharmonic mixing is scaled with a l^~2.5 not l^4 times 
coefficient factor. Nevertheless we should have a consistent transfer 
function and be able to say what it is.

cnh

Jean-Michel Campin wrote:
> Hi David, 
> 
> In the CVS log-message, I found, from cnh (revision 1.25):
>> Change from 1/8 to 1/32 scaling from Baylor.
>> Need to check if there are any verif expts affected.
> May be this could help ?
> 
> Jean-Michel
> 
> On Mon, Mar 09, 2009 at 10:18:34PM -0400, David Ferreira wrote:
>> Since nobody answers my question, I'm doing my own
>> follow-up: Even more bugs in mom_calc_visc.F ?
>>
>> My first below concerned the computation of the viscosities
>> at the div points. There is an equivalent for at vorticity points:
>>
>> CCCCCCCCCCCCC Vorticity Point CalculationsCCCCCCCCCCCCCCCCCC
>> C These are (powers of) length scales
>>         IF (useAreaViscLength) THEN
>>          L2=rAz(i,j,bi,bj)
>>          L4rdt=0.125 _d 0*recip_dt*rAz(i,j,bi,bj)**2
>>         ELSE
>>          L2=2. _d 0/((recip_DXV(I,J,bi,bj)**2+recip_DYU(I,J,bi,bj)**2))
>>          L4rdt=recip_dt/
>>     &     ( 6. _d 0*(recip_DXV(I,J,bi,bj)**4+recip_DYU(I,J,bi,bj)**4)
>>     &      +8. _d 0*((recip_DXV(I,J,bi,bj)*recip_DYU(I,J,bi,bj))**2))
>>         ENDIF
>>
>> As for the divergence, the 2 methods for a carsetian square grid
>> converge for L2 but not L4rdt. Even weirder, the coefficients
>> are now 1/8 and 1/20 (compared to 1/32 and 1/20 at the div points).
>>
>> In the best case scenario, that's only 2 correct coeff out of 4, no ?
>> Comments ?
>> david
>>
>>
>>
>> David Ferreira wrote:
>>> Hi,
>>> I've got a question about the variable bi-harmonic viscosities. In the
>>> MITgcm, there is the possibility to specify a non-dimensional horizontal
>>> bi-harmonic viscosity through viscA4Grid. The model then computes
>>> the actual viscosity taking into account the local size of the  
>>> grid-point.
>>> This local grid-scale can be computed 2 different ways (through the
>>> flag useAreaViscLength):
>>>
>>> C These are (powers of) length scales
>>>         IF (useAreaViscLength) THEN
>>>          L2=rA(i,j,bi,bj)
>>>          L4rdt=0.03125 _d 0*recip_dt*L2**2
>>>         ELSE
>>>          L2=2. _d 0/((recip_DXF(I,J,bi,bj)**2+recip_DYF(I,J,bi,bj)**2))
>>>          L4rdt=recip_dt/( 6. _d 0*(recip_DXF(I,J,bi,bj)**4
>>>     &                             +recip_DYF(I,J,bi,bj)**4)
>>>     &                   +8. _d 0*((recip_DXF(I,J,bi,bj)
>>>     &                             *recip_DYF(I,J,bi,bj))**2) )
>>>         ENDIF
>>>
>>> The default (useAreaViscLength=.FALSE.) compute the grid
>>> factor directly from the length scales, otherwise it is done from
>>> the area of the grid-point (for the cube-sphere for example).
>>>
>>> I was expecting that the 2 formula above would give the same results
>>> for  cartesian square grid-box (dx=dy). This is the case for L2 but  
>>> not for
>>> L4rdt.
>>> If useAreaViscLength is TRUE, then L4rdt = dx^4/(32*deltaT),
>>> otherwise it is L4rdt = dx^4/(20*deltaT).
>>>
>>> Does anyone know which coefficient is the correct one ?
>>> Thanks,
>>> david
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> MITgcm-devel mailing list
>>> MITgcm-devel at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-devel
>>
>> _______________________________________________
>> MITgcm-devel mailing list
>> MITgcm-devel at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-devel
> _______________________________________________
> MITgcm-devel mailing list
> MITgcm-devel at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-devel
>