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

Wed Jun 11 06:10:26 EDT 2014

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:

   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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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>
Content-Type: text/plain; charset="iso-8859-1"

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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