= NEMO Workplan 2011: Summary of work on OBC-BDY merge =

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The plan for this year was to merge the OBC and BDY modules in NEMO. Due to shortage of time this has not been completed. However, a completely rewritten version of the BDY module has been produced which will form the basis for the merged module in the future. 


== Summary of new BDY module ==

 * The new module is more flexible. The open boundary conditions are defined for tracers (T and S), the barotropic solution (SSH, depth-mean currents) and the baroclinic velocities separately, and it is easy to implement new algorithms for each set of fields. 

 * Boundary conditions for barotropic and baroclinic velocities are always applied separately. There is still an option to input full-velocity boundary conditions but these are split into barotropic and baroclinic parts when they are read in. 

 * The boundary arrays are arranged in structures and dynamically allocated. (Previously hardcoded sizes). 

 * The new module has a clearer separation between the initialisation of the open boundary definition (obcini.F90), handling of the external boundary data (all in obcdta.F90), and application of the boundary conditions (obctra.F90, obcdyn2d.F90, obcdyn3d.F90).

 * There is one obc_dta routine to handle the reading in of boundary data. Old version had two very similar routines.

 * The new module uses the fldread module (called from obc_dta) to manage the input of external boundary data from files. This is more flexible - one can define the file for each field separately and whether the field is climatological or real-time data. One can choose to interpolate the input boundary data to the "before", "now" or "after" time level depending on whether one is using implicit or explicit schemes. In the future it may be possible to use the online interpolation facility in fld_read to take boundary data directly from climatological or model datasets.  

 * The open boundary can be defined in an obc.coordinates.nc file analagous to the usual NEMO coordinates.nc file or defined in a namelist as straight-line segments. The obc.coordinates file will be used in conjunction with the SCRIP interpolation tool to generate boundary conditions from 3D data sets '''(TO BE DONE)'''. 

 * One can define more than one open boundary with different boundary conditions on each boundary.

 * One can define initial conditions as boundary conditions separately for each of the tracers, barotropic variables and baroclinic velocities. 

 * The new module requires the data in the data files to be in order of increasing ''nbr'' (distance from edge). Also if more than one data file is specified for a particular boundary (eg. for baroclinic and barotropic boundary conditions), the data must be in the same order in each file. I have a written a fortran utility to re-order data files produced for the old version of the code.