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

[Martin Losch]

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
> > _______________________________________________
> > MITgcm-support mailing list
> > MITgcm-support at mitgcm.org
> > http://mitgcm.org/mailman/listinfo/mitgcm-support
> 
> 
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support
> 
> 
> 
> -- 
> 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
> <MITgcm_u.png>_______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support