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

Wed Jun 11 12:18:36 EDT 2014

Hi Liam,

It sounds to me like Lief is saying you need to resolve the submesoscale if you want to do this "right".  

However, your original question was about triggering convection in the mixed layer due to lateral buoyancy fluxes.  If you are just trying to get the coarse mix layer properties right, I think KPP does extra mixing when the Richardson number is negative, Ri<0.  I took a quick look, and in kpp_routines.F there is code around line 945 for "mixing due to internal waves, and shear and static instability".  Zeroth order, downfront winds produce static instabilities in the mixed layer, so KPP should do some enhanced mixing there.  Not the same as convective adjustment, but presuming the viscosities and diffusivities triggered are high enough, and your timesteps not too large, it will have the same result.    

Second order, I believe the idea behind symmetric instabilities, etc is that the mixing is not static instability and that the potential energy is transferred deeper in the water column.  For that, I expect you'd need either resolve the problem or code some coarse parameterization of it.  

Sorry if I've misunderstood what you want. I'm not sure what your model setup is, or what you are trying to diagnose, so maybe you were happy with Leif's suggestion.  

Cheers,   Jody

On Jun 11, 2014, at  3:10 AM, Liam Brannigan <Brannigan at atm.ox.ac.uk> wrote:

> Hi Dimitris
> 
> I put the same question to Leif Thomas and he has come back to me with the following interesting response:
> 
> "The WDBF (which we are now referring to as the Ekman buoyancy flux, e.g. Thomas and Taylor (2010)) is important for the dynamics of submesoscale flows, but I have now come to realize that it does not have to be incorporated into the KPP mixing scheme to capture its main effect on the physics. A positive EBF results in a reduction of PV in the boundary layer, conditioning the flow for submesoscale instabilities. All your model needs is one, to capture the PV dynamics of the boundary layer and two, to resolve the submesoscale instabilities that result.
> 
> A numerical model that resolves the fronts and the Ekman layer, and that uses the original version of KPP should capture the modification of the boundary layer PV by the EBF. The parameterization that I described in Thomas (2005) increases the vertical diffusivity in the boundary layer, but this is of secondary importance to the PV dynamics of the flow.
> 
> I think the more important issue when it comes to simulating wind-forced frontal submesoscale physics is horizontal resolution. If you want to capture symmetric instability for example you need to have a grid spacing smaller than the width of its overturning cells, which scale as H/s_b, where H is the thickness of the boundary layer and s_b=f/(dug/dz) (f is the Coriolis parameter and  dug/dz is the thermal wind shear)."
> 
> Liam 
> 
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> Today's Topics:
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>   1. Wind-driven buoyancy flux (WDBF) in KPP (Liam Brannigan)
>   2. Re: Wind-driven buoyancy flux (WDBF) in KPP (Dimitris Menemenlis)
> 
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Thu, 5 Jun 2014 16:14:13 +0000
> From: Liam Brannigan <Brannigan at atm.ox.ac.uk>
> To: "mitgcm-support at mitgcm.org" <mitgcm-support at mitgcm.org>
> Subject: [MITgcm-support] Wind-driven buoyancy flux (WDBF) in KPP
> Message-ID:
>        <26A3E4BBD1F85546A97B8155F45E02A461902D7C at EXCHNG10.physics.ox.ac.uk>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> Dear MITgcmers
> 
> What's the best way to modify the KPP code to account for wind-driven buoyancy fluxes driven by down-front winds?  This issue arises as down-front winds drive dense water over light water, which triggers convection in the real ocean, but doesn't in the KPP scheme as the non-local convective adjustment is only switched-on for destabilising surface forcing.
> 
> There are a number of ways to do this - the most obvious is described in the appendix of Thomas 2005 (http://journals.ametsoc.org/doi/abs/10.1175/JPO2830.1).  Has anyone implemented this and would consider sharing their code?  I would be happy to help to get it added as a standard option in the model code.
> 
> Liam
> 
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> Message: 2
> Date: Thu, 5 Jun 2014 16:48:02 -0700
> From: Dimitris Menemenlis <dmenemenlis at gmail.com>
> To: MITgcm Support <mitgcm-support at mitgcm.org>
> Cc: Maria Del Mar Flexas Sbert <msbert at jpl.nasa.gov>
> Subject: Re: [MITgcm-support] Wind-driven buoyancy flux (WDBF) in KPP
> Message-ID: <246579D3-355B-4437-ACB0-E872434EBA27 at gmail.com>
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> 
> Liam, I copy your message to Mar Flexas Sbert and Andy Thompson, as they have
> also expressed interest in studying wind-driven mixing near fronts using MITgcm.
> 
> Honestly, I had not realized that KPP is not set-up to deal with dense water in the
> surface level.  What about the local Richardson number term in Ri_iwmix?
> Is this not sufficient?
> 
> Dimitris
> 
> On Jun 5, 2014, at 9:14 AM, Liam Brannigan <Brannigan at atm.ox.ac.uk> wrote:
> 
>> Dear MITgcmers
>> 
>> What's the best way to modify the KPP code to account for wind-driven buoyancy fluxes driven by down-front winds?  This issue arises as down-front winds drive dense water over light water, which triggers convection in the real ocean, but doesn't in the KPP scheme as the non-local convective adjustment is only switched-on for destabilising surface forcing.
>> 
>> There are a number of ways to do this - the most obvious is described in the appendix of Thomas 2005 (http://journals.ametsoc.org/doi/abs/10.1175/JPO2830.1).  Has anyone implemented this and would consider sharing their code?  I would be happy to help to get it added as a standard option in the model code.
>> 
>> Liam
> 
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--
Jody Klymak    
http://web.uvic.ca/~jklymak/