[MITgcm-support] OBCS (open boundary forcing) problem

Tue Jan 28 12:00:19 EST 2014

Hi Jonny,

>  OB_Jnorth=   450*-0.1,
>  OB_Jsouth=   450*0.1,
>  OB_Ieast=   800*-0.1,
>  OB_Iwest=   800*0.1,
This is not right. All these 4 vectors are integer (indices), they cannot
be specified as real. 

Cheers,
Jean-Michel

On Tue, Jan 28, 2014 at 04:30:47PM +0000, Jonny Williams wrote:
> Hi Martin
> 
> Currently, the top of my data.obcs file reads...
> 
> &OBCS_PARM01
>  OB_Jnorth=   450*-0.1,
>  OB_Jsouth=   450*0.1,
>  OB_Ieast=   800*-0.1,
>  OB_Iwest=   800*0.1,
>  OBCS_u1_adv_T=1,
>  OBCS_u1_adv_S=1,
> 
> 
> Have I got this the wrong way round as you suggest that I should have...
> 
> OB_Ieast = 450*-1,
> OB_Iwest = 450*1,
> 
> ?
> 
> I have checked that my OBCS boundary conditions are indeed constant in time
> as you suggest.
> 
> Jonny
> 
> 
> 
> On 28 January 2014 16:15, Martin Losch <Martin.Losch at awi.de> wrote:
> 
> > Hi Jonny,
> >
> > I am assuming that your boundaries are specified like this:
> >  OB_Ieast = 450*-1,
> >  OB_Iwest = 450*1,
> > or something like that to have the open boundary on the first and the last
> > (Nx'th) grid point. The boundary conditions are implemented in such a way
> > that you are supposed to have exactly the prescribed value on the boundary.
> > You can easily check that when you prescribe bcs that are constant in time.
> > Your figure suggests to me that theses values on the boundaries are not
> > exactly the same (although I cannot be sure). Rather because the velocity
> > is opposite to the remaining domain and the color scale of matlab
> > automatically accomodates the high negative (westward) velocities, the
> > values look similar. Is that possible? I would check if you haven't made a
> > sign error when generating the boundary values.
> >
> > For more, I'd have to have a closer look at your configuration (data.*
> > files and the "code" directory).
> >
> > Martin
> >
> >
> > On Jan 28, 2014, at 5:02 PM, Jonny Williams <Jonny.Williams at bristol.ac.uk>
> > wrote:
> >
> > > Thanks very much for that Martin
> > >
> > > Your explanation of the periodic boundary conditions makes sense so
> > that's much appreciated.
> > >
> > > What I still do not understand however is why when I move from 1 degree
> > to 0.1 degree resolution why the periodic boundary conditions seems to
> > impinge on both sides. The attached figure should make this clear for the u
> > velocity in the horizontal direction. The only things which are different
> > in the two runs are the resolution dependent factors in data and data.exf
> > and the resolution of the initial condition T and S and the OBCS forcing
> > files. This seemingly strange behaviour does not happen when I run at 1
> > degree resolution.
> > >
> > > Thanks again.
> > >
> > >
> > >
> > > On 28 January 2014 11:22, Martin Losch <Martin.Losch at awi.de> wrote:
> > > Hi Jonny,
> > >
> > > there is probably no problem at all: The default boundary condition is
> > double periodic and the (FORTRAN) field sizes are the same for all of  your
> > fields, so even for u and v you have 45x80 grid points. In the netCDF
> > output, however, the extra column (for u) and row (for v) for the vector
> > components on the u and v-points in added for convenience, so that you
> > don't have to wrap around your self when you want to compute, say averages
> > on C-points or gradients, etc. These columns/rows are just copies of i/j=1.
> > In the  case of OBCS that does not make too much sense, because the default
> > period boundary conditions are overruled. You can safely ignore Nx+1/Ny+1
> > and focus on Nx/Ny.
> > >
> > > The penultimate colum/row (I had to look that up) are probably constant
> > because here you prescibe constant u/v ? If not, you should be worried.
> > >
> > > Martin
> > >
> > > On Jan 28, 2014, at 11:46 AM, Jonny Williams <
> > Jonny.Williams at bristol.ac.uk> wrote:
> > >
> > > > Hi there
> > > >
> > > > I am currently running two separate regional versions of the MITgcm,
> > one at 1 degree resolution (45 longitudes x 80 latitudes) and one at 0.1
> > degree resolution (450 longitudes x 800 latitudes) with both EXF and OBCS
> > packages turned on.
> > > >
> > > > They are essentially identical in that they use the same forcing data,
> > albeit regridded to differing resolutions.
> > > >
> > > > My problem is essentially that although the temperature and salinity
> > forcing seem to be behaving themselves, the forcing of the u (zonal) and v
> > (meridional) velocities are not. In the output NetCDF files I get, T and S
> > of dimension 45 x 80, u of dimension 46 x 80 and v of dimension 45 x 81.
> > This makes sense since velocities and tracers are usually on staggered
> > grids in GCMs. What is strange however is that the 1st, 2nd and last
> > columns/rows of data for u/v are identical and constant, that is, the open
> > boundary conditions seem to be being applied at BOTH sides of the geometry.
> > The penultimate row/column of u/v are also constant throughout the
> > simulation. This constancy is good as it shows that two different sets of
> > OBCS boundary conditions are being applied at least!
> > > >
> > > > Does anyone have any ideas about what could be going on? I can provide
> > more details if need be!
> > > >
> > > > Many thanks
> > > >
> > > > Jonny
> > > >
> > > > --
> > > > Dr Jonny Williams
> > > > School of Geographical Sciences
> > > > University of Bristol
> > > > University Road
> > > > BS8 1SS
> > > >
> > > > +44 (0)117 3318352
> > > > jonny.williams at bristol.ac.uk
> > > > bit.ly/jonnywilliams
> > > > _______________________________________________
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> > > > MITgcm-support at mitgcm.org
> > > > http://mitgcm.org/mailman/listinfo/mitgcm-support
> > >
> > >
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> > >
> > >
> > >
> > > --
> > > Dr Jonny Williams
> > > School of Geographical Sciences
> > > University of Bristol
> > > University Road
> > > BS8 1SS
> > >
> > > +44 (0)117 3318352
> > > jonny.williams at bristol.ac.uk
> > > bit.ly/jonnywilliams
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> >
> >
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> 
> 
> 
> -- 
> Dr Jonny Williams
> School of Geographical Sciences
> University of Bristol
> University Road
> BS8 1SS
> 
> +44 (0)117 3318352
> jonny.williams at bristol.ac.uk
> bit.ly/jonnywilliams

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