[MITgcm-support] KPP parameter

[Martin Losch]

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).

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
>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support