[MITgcm-support] KPP parameter

[David Ferreira]

Martin Losch wrote:
> Hi Nicolas,
>
> the parameter BVDcon in table 6.5. does not exist in the code, 
> probably a typo, it should be BVScon and the description is not 
> accurate: It's only a lower limit of the N2 in the computation of 
> mixing due to convection below the mixed layer. I have not been able 
> to find the corresponding description in the original KPP paper (Large 
> etal 1994, Rev Geophys. 32(4)), but it's part of the computations of 
> vertical mixing coefficients from shear instability and (I guess) 
> static instability (S/R Ri_iwmix), for shear instabilities it's 
> described in eq(25-28), for buoyancy it's analogous. Here's the code 
> (diffus(i,ki,2) is the local rho0*N2):
>> c  evaluate f of Brunt-Vaisala squared for convection, store in fcon
>>
>>             Rig   = max ( diffus(i,ki,2) , BVSQcon )
>>             ratio = min ( (BVSQcon - Rig) / BVSQcon, c1 )
>>             fcon  = c1 - ratio * ratio
>>             fcon  = fcon * fcon * fcon
> and then diffusivity=fcon*factor. I don't understand, why it has to be 
> done for N2/(dV/dz)^2 and for N2, but maybe someone else can explain 
> that (again, it's not documented in the original paper).
Martin,
This is what I was referring to last week when we were discussing GGL90. 
This piece is the "hidden"
convective adjustment of KPP. It is required to remove statically 
unstable profiles in the ocean
interior. This is why it makes sense to check in KPP that ivdc_kappa is 
not on at the same time
(but not in GGL90).
david





>
> In general you should not have to change these parameters at all. KPP 
> usually works fine with the default parameters,
>
> Martin
>
>
> On Nov 18, 2009, at 10:22 PM, Nicolas Grisouard wrote:
>
>> Hi everyone,
>>
>> I am moving from lab-scale simulations (10x1 m^2) to ocean-scale ones 
>> (400x4 km^2), still using non-hydrostatic simulations and I am 
>> starting my long journey in the world of sub-grid scale 
>> parametrizations. I have a first question about the KPP scheme. 
>> According to the manual (Table 6.5), a parameter to set is the 
>> Brunt-Vaisala frequency squared. The default value is negative, does 
>> that correspond to a convective situation or is it just a matter of 
>> convention? I have a strongly inhomogeneous Brunt-Vaisala profile 
>> (presence of a thermocline), any idea about what to set?
>>
>> This is all... for the moment. ;)
>>
>> Thanks,
>> Nicolas
>>
>> -- 
>> Nicolas Grisouard
>> PhD Student - ERES research group
>> Laboratoire des Ecoulements Geophysiques et Industriels
>> BP53
>> GRENOBLE CEDEX 9 FRANCE
>> tel : +33 (0) 476 825 037 - fax : +33 (0) 476 827 022
>> http://nicolas.grisouard.free.fr
>>
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>
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