[MITgcm-support] only eta as open boundary ?
Jean-Michel Campin
jmc at ocean.mit.edu
Mon Feb 1 18:10:28 EST 2016
Hi kaveh,
few comments below.
On Mon, Feb 01, 2016 at 09:18:36PM +0100, kaveh Purkiani wrote:
> Dear Michel,
>
> Thank you for your reply.
>
>
> As you mentioned it is possible to calculate flow if one uses Orlanski.
> As far as I understood from the code, applying Orlanski with nonlinFreeSurf
> is not yet possible. So, is it then possible to force the model with only
> eta (to calculate velocity field) ?
I think that the answer is no; the way OBC are implemented in MITgcm is
such that (again) the pressure gradient accross the OB does not enter the equations.
Also it's worth to mention that Orlanski works well with the (slow) baroclinic
component of the flow, but does not a good job in dealing the (fast) barotropic
component.
> Applying similar way as it is mentioned in the link you introduced, to
> convert eta to bartropic velocity, forcing with barotropic velocities,
> model works and all BCs are applied to the domain. While, using only
> OB(W,E,N,S)eta at BCs is not applied to domain. It shows only identical
> values at edges as they are introduced to the BCs.
Yes, it's also what I was expected: if the only prescribed OBCS is eta, since it
does not generate any pressure gradient, not much is happening.
Cheers,
Jean-Michel
>
> I modified data name-list with Martin's hint:
>
> implicitFreeSurface=.TRUE.,
> exactConserv=.TRUE.,
> nonlinFreeSurf=4,
> select_rStar=2,
> hFacInf=0.2,
> hFacSup=2.0,
> hFacMin=0.2,
>
> and data.obcs:
> useOBCSprescribe = .TRUE.,
> OB(N,S,W,E)etaFile = ...
>
>
> In attached you see eta values at 3 different points, blue line (mid
> domain), red (western boundary) and white (eastern boundary).
>
> regards,
>
> Kaveh.
>
>
>
> On Mon, Feb 1, 2016 at 6:19 PM, Jean-Michel Campin <jmc at ocean.mit.edu>
> wrote:
>
> > Hi Kaveh,
> >
> > I think there has been some discussions you can find in the support
> > archive:
> > For instance, when I type "eta at OBCS" on the MTgcm seach tool,
> > the 1rst one that come up:
> > http://mitgcm.org/pipermail/mitgcm-support/2005-October/003491.html
> > contains some relevant information for your case.
> > And just to repeat it, the way OBC are implemented in MITgcm is that
> > the flow is prescribed (or computed, e.g. if using Orlanski) at the OB
> > and the pressure gradient accross the OB does not enter the equations.
> >
> > Cheers,
> > Jean-Michel
> >
> > On Mon, Feb 01, 2016 at 10:56:38AM +0100, kaveh Purkiani wrote:
> > > Dear Martin,
> > >
> > > I tried with all values larger than 0, 1 to 4, none of them could help
> > this
> > > to solve.
> > > I will have a look at documentation again and report you back soon.
> > >
> > > Kaveh.
> > >
> > > On Mon, Feb 1, 2016 at 10:45 AM, Martin Losch <Martin.Losch at awi.de>
> > wrote:
> > >
> > > > I suggest to read the documentation to learn about the parameters
> > > > necessary for the nonlinear free surface:
> > > > <
> > http://mitgcm.org/public/r2_manual/latest/online_documents/node42.html>
> > > > where you will see, that ???1??? is not the appropriate choice for
> > > > nonlinFreeSurf
> > > >
> > > > Also, did you find a verification example with obcs and nonlinear free
> > > > surface? I suggest verifcation/exp4/input.nlfs
> > > >
> > > > Martin
> > > >
> > > > > On 01 Feb 2016, at 10:35, kaveh Purkiani <
> > kavehpurkiani at googlemail.com>
> > > > wrote:
> > > > >
> > > > > Dear Martin,
> > > > >
> > > > > I use nonlinearFreesurf in my configuration similar to as sinusoidal
> > > > elevation (simple M2) at BC. Also the nonlinFreeSurf = 1 at data
> > namelist.
> > > > > Should not this work? and show non-zero elevation inside the model as
> > > > well as boundaries?
> > > > >
> > > > > The method for converting the elevation into bartropic velocities
> > seems
> > > > to work, but I wanted to apply a realistic condition to the model (
> > > > components of M2, M4, S2 and O1 ) therefore, using eta BC seems to be
> > more
> > > > accurate.
> > > > >
> > > > > regards
> > > > >
> > > > > Kaveh.
> > > > >
> > > > >
> > > > > On Mon, Feb 1, 2016 at 10:23 AM, Martin Losch <Martin.Losch at awi.de>
> > > > wrote:
> > > > > Kaveh,
> > > > >
> > > > > you probably use a linear free surface in your model configuration?
> > With
> > > > a ***linear*** free surface, the open boundary conditions for Eta have
> > no
> > > > effect (basically, there are none to be specified). But your method of
> > > > converting the elevation into bartropic velocities seems to work,
> > doesn???t
> > > > it?
> > > > >
> > > > > Otherwise you need to use a non-linear free surface, which will make
> > the
> > > > experiment more complicated.
> > > > >
> > > > > Martin
> > > > >
> > > > > > On 30 Jan 2016, at 20:50, kaveh Purkiani <
> > kavehpurkiani at googlemail.com>
> > > > wrote:
> > > > > >
> > > > > > Hi MITgcm users,
> > > > > >
> > > > > > I have been trying to use eta as the only boundary condition in
> > > > > > a configuration.
> > > > > >
> > > > > > below you see the relative part of configuration regarding the bcs
> > at
> > > > data and data.obcs
> > > > > >
> > > > > > data
> > > > > >
> > > > > > periodicExternalForcing=.TRUE.,
> > > > > > externForcingPeriod=900.,
> > > > > > externForcingCycle =43200.,
> > > > > > &
> > > > > >
> > > > > >
> > > > > > data.obcs
> > > > > >
> > > > > > OBCSfixTopo=.FALSE.,
> > > > > > OB_Jnorth= 80*-1,
> > > > > > OB_Jsouth= 80*1,
> > > > > > OB_Ieast = 42*-1,
> > > > > > OB_Iwest = 42*1,
> > > > > > useOBCSprescribe = .TRUE.,
> > > > > >
> > > > > > OBNetaFile = 'OBNeta.bin', (Nx,time)
> > > > > > OBSetaFile = 'OBSeta.bin', (Nx,time)
> > > > > > OBWetaFile = 'OBWeta.bin', (Ny,time)
> > > > > > OBEetaFile = 'OBEeta.bin', (Ny,time)
> > > > > >
> > > > > > The boundary conditions are obviously read in (only at boundaries
> > > > output shows identical to what is prescribed as boundary condition),
> > > > > > but they are not applied inside the domain. Except the boundaries,
> > > > rest of the domain does not show any elevation.
> > > > > >
> > > > > > When I convert the sea surface elevation (eta) into barotropic
> > > > velocity and prescribe the OB(N,S,E,W)(u,v) and no elevation at
> > boundaries,
> > > > it works
> > > > > > properly, with meaningful eta, zonal and meridional values.
> > > > > >
> > > > > > I appreciate any suggestions.
> > > > > >
> > > > > > regards,
> > > > > >
> > > > > > Kaveh.
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > >
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