[MITgcm-support] Wind-driven buoyancy flux (WDBF) in KPP

[Jody Klymak]

Hi Liam,

On Jun 12, 2014, at  10:05 AM, Liam Brannigan <Brannigan at atm.ox.ac.uk> wrote:

> The model should trigger extra mixing when static instability is found, though it depends on a couple of factors - the ri_iwmix scheme looks for low gradient Richardson numbers at the base of the mixed layer.  If the Ekman layer is ~20 m and the mixed layer is ~ 100 m then the gradient Richardson number may be positive where the scheme assesses it at the base of the mixing layer, even if there is static instability above.  

Thats not how I am reading the code.  It looks for low local Ri everywhere in the water column.  

If KPP only enhanced mixing in the mixed layer and at the base, I don't think it would be as popular as it is.  You need enhanced mixing elsewhere in the water column sometimes too.  In your hypothetical, the MITgcm's KPP scheme would gives enhanced diffusivity and viscosity in the destabilized upper 20 m.  (so long as EXCLUDE_KPP_SHEAR_MIX is not TRUE).

I'm afraid I do not know the exact form for this extra mixing, but you could figure it out from the code, or appropriate google search.  I also do not know how it interacts with the bulk mixed layer formulation in KPP.  It looks to be additive.  

> I'm also unsure about how KPP feels about static instability high in the mixed layer when setting the mixing layer depth, as it is based on a bulk Richardson number criteria which may not care whether stratification is weakly positive or negative as long as it's less than the critical Ri<0.3.  

Negative stratification means Ri<0, which is certainly less than 0.3!

Cheers,  Jody

--
Jody Klymak    
http://web.uvic.ca/~jklymak/