[MITgcm-support] Wind-driven buoyancy flux (WDBF) in KPP
Liam Brannigan
Brannigan at atm.ox.ac.uk
Thu Jun 12 13:05:00 EDT 2014
Hi Jody
I'm running at 0.5 km resolution, so I think the processes Leif covered are resolved in my case.
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.
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.
Liam
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Today's Topics:
1. Re: Wind-driven buoyancy flux (WDBF) in KPP (Jody Klymak)
2. Re: Layers package : nothing but zeros for output? (Dan Jones)
----------------------------------------------------------------------
Message: 1
Date: Wed, 11 Jun 2014 09:18:36 -0700
From: Jody Klymak <jklymak at uvic.ca>
To: mitgcm-support at mitgcm.org
Subject: Re: [MITgcm-support] Wind-driven buoyancy flux (WDBF) in KPP
Message-ID: <512D7706-7029-4026-AE55-F7A949984BBF at uvic.ca>
Content-Type: text/plain; charset=us-ascii
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
>
> ________________________________________
> From: mitgcm-support-request at mitgcm.org [mitgcm-support-request at mitgcm.org]
> Sent: 06 June 2014 17:00
> To: mitgcm-support at mitgcm.org
> Subject: MITgcm-support Digest, Vol 132, Issue 3
>
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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)
>
>
> ----------------------------------------------------------------------
>
> 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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--
Jody Klymak
http://web.uvic.ca/~jklymak/
------------------------------
Message: 2
Date: Thu, 12 Jun 2014 07:22:19 +0100
From: Dan Jones <dcjones.work at gmail.com>
To: mitgcm-support at mitgcm.org
Subject: Re: [MITgcm-support] Layers package : nothing but zeros for
output?
Message-ID:
<CAPj3iHS-pHcSyT15U4gQAYZoC1LMd87b0_X5E3x3a6REVCv+eg at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thanks, Ryan! It's working well now.
On Wed, Jun 11, 2014 at 5:00 PM, <mitgcm-support-request at mitgcm.org> wrote:
> Send MITgcm-support mailing list submissions to
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>
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> or, via email, send a message with subject or body 'help' to
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>
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> Today's Topics:
>
> 1. Re: Layers package : nothing but zeros for output?
> (Ryan Abernathey)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 11 Jun 2014 10:12:50 -0400
> From: Ryan Abernathey <ryan.abernathey at gmail.com>
> To: "mitgcm-support at mitgcm.org" <mitgcm-support at mitgcm.org>
> Subject: Re: [MITgcm-support] Layers package : nothing but zeros for
> output?
> Message-ID:
> <CAJAZx5BACfbz053JUvVLu33BTjRWhzJHcEUmFwq=
> 31bJKSnQ_w at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Sorry, I skipped the part of your email where you said the version you were
> running. You will definitely have to use the new syntax. Hopefully my
> example is sufficient to get it working.
>
> -Ryan
>
>
> On Wed, Jun 11, 2014 at 10:10 AM, Ryan Abernathey <
> ryan.abernathey at gmail.com
> > wrote:
>
> > Dan,
> > What version of the code are you using? The syntax for data.layers has
> > changed considerably in the past year or so. If you are using a newer
> > version of MITgcm, you will have to change some things. It is perfectly
> > reasonable to be confused by this, since there is zero documentation. We
> > are working on a new version of layers and accompanying documentation,
> > hopefully coming out in the next few months.
> > -Ryan
> >
> > Here is an example of a "modern" data.layers that does averaging in theta
> > layers
> > -----
> > &LAYERS_PARM01
> > layers_taveFreq=0.,
> > layers_diagFreq=0.,
> > layers_name(1) = 'TH',
> > layers_bolus(1) = .TRUE.,
> > layers_bounds(1:17,1)=
> > -1.182, -1.109, -1.012, -0.882, -0.709,
> > -0.478, -0.169, 0.241, 0.789, 1.52,
> > 2.494, 3.793, 5.526, 7.836, 10.917,
> > 15.025, 20.504,
> > &data.layers
> > ---
> >
> > This syntax allows you to have multiple simultaneous layer computations
> > running (e.g. theta and potential density). The number of possible layers
> > coordinates is set in LAYERS_SIZE.h by layers_maxNum
> > ---
> > PARAMETER( Nlayers = 16 )
> > PARAMETER( FineGridFact = 10 )
> > PARAMETER( FineGridMax = Nr * FineGridFact )
> > PARAMETER( layers_maxNum = 1 )
> > ----
> >
> > Also, we now prefer to do the output via the diagnostics package. For
> > example
> > ---
> > &DIAGNOSTICS_LIST
> > filename(1) = 'DiagLAYERS',
> > frequency(1) = 31104000.,
> > fields(1:3,1)= 'LaVH1TH ','LaHs1TH ','LaPs1TH ', '
> > &
> > &DIAG_STATIS_PARMS
> > &
> > ---
> >
> >
> >
> > On Wed, Jun 11, 2014 at 9:04 AM, Dan Jones <dcjones.work at gmail.com>
> wrote:
> >
> >> Hi all,
> >>
> >> I am interested in implementing the layers package in an idealized
> model.
> >> I can get it to compile, run, and produce output files such as Hs, Hw,
> UH,
> >> and VH. However, these output files contain nothing but zeros! I'm
> very
> >> likely making some trivial error somewhere - please take a look at my
> setup
> >> below:
> >>
> >> -------------------- /code/LAYERS_SIZE.h
> >>
> -------------------------------------------------------------------------------
> >>
> >> C $Header: /u/gcmpack/MITgcm/pkg/layers/LAYERS_SIZE.h,v 1.3 2013/01/08
> >> 21:52:34 jmc Exp $
> >> C $Name: checkpoint64t $
> >> C * Compiled-in size options for the LAYERS package *
> >> C
> >> C - Just as you have to define Nr in SIZE.h, you must define the number
> >> C of vertical layers for isopycnal averaging so that the proper array
> >> C sizes can be declared in the LAYERS.h header file.
> >> C
> >> C - Variables -
> >> C NLayers :: the number of isopycnal layers (must match
> >> data.layers)
> >> C FineGridFact :: how many fine-grid cells per dF cell
> >> C FineGridMax :: the number of points in the finer vertical grid
> >> C used for interpolation
> >> C layers_maxNum :: max number of tracer fields used for layer
> >> averaging
> >> INTEGER Nlayers, FineGridFact, FineGridMax, layers_maxNum
> >> PARAMETER( Nlayers = 10 )
> >> PARAMETER( FineGridFact = 10 )
> >> PARAMETER( FineGridMax = Nr * FineGridFact )
> >> PARAMETER( layers_maxNum = 1 )
> >>
> >> -------------------- /input/data.layers
> >>
> -------------------------------------------------------------------------------
> >>
> >> &LAYERS_PARM01
> >> # averaging
> >> layers_taveFreq=45000.,
> >> # no instantaneous output
> >> layers_diagFreq=0.,
> >> layers_G=-0.2,0.6,1.5,2.3,3.2,4.0,4.8,5.7,6.5,7.4,8.2,
> >> &
> >> # Note: Some systems use & as the
> >> # namelist terminator. Other systems
> >> # use a / character.
> >>
> >> As you can see in the header file, I'm running MITgcm c64t. Based on
> the
> >> thread linked below, I assume that I'm using the default layer
> coordinate
> >> (i.e. theta), so as long as I'm picking reasonable values of theta I
> should
> >> have well-defined layers. Is that correct?
> >>
> >> http://mitgcm.org/pipermail/mitgcm-support/2011-December/007569.html
> >>
> >> Thanks in advance for taking a look!
> >> Dan
> >>
> >> ---------------------------------------------------------
> >>
> >> Dr Dan Jones
> >> British Antarctic Survey, NERC
> >> Cambridge, UK
> >>
> >> Phone: +44 (0)1223 221505
> >> Fax: +44 (0)1223 362616
> >> Skype: dcjones.work
> >>
> >> ---------------------------------------------------------
> >>
> >> _______________________________________________
> >> MITgcm-support mailing list
> >> MITgcm-support at mitgcm.org
> >> http://mitgcm.org/mailman/listinfo/mitgcm-support
> >>
> >>
> >
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