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

[Martin Losch]

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
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