Open Boundary Conditions
The boundaries pass from q-point to q-point. There are u,v masks for setting velocity points as normal to the boundary.
There is a "Flather" option for each of the four edges of the domain, which applies boundary conditions to the flow normal to the corresponding side. It applies these conditions to both the baroclinic and barotropic velocities, Orlanski and Flather, respectively. It sets the outer h/eta values, but does nothing for the flow parallel to the boundary just outside.
There are also OBC_U and OBC_V which require providing more information about the boundary conditions via some other means, possibly coded in via the user directory.
Whenever a boundary value is needed, it uses one set during initialization. Updating of these exterior values is desired and being looked into.
There has been discussion about the location of the land mask with respect to the open boundary. Experimentally, the CCS1 domain blows up when "Flather" points are inside the land mask. Besides, there is code to set the outside halo points to sea when there's a "Flather" open boundary.
The boundary conditions only work in symmetric mode.
We'd like some means of specifying pieces of open boundary, each being aligned with the i- or j-axis.
We don't promise to support convex interior corners such as would be required by converting the peninsula between segment 1 and segment 2 in the figure to an open boundary.
A ROMS grid has all the interior computational points, plus a round of boundary points of all four types (u,v,T,q):
Here, the red lines are through points considered to be part of the boundary condition. Note that it is symmetric - the other way from MOM6.
Continent A is like North America in the CCS1 domain. One could extract just the interior points and set the outside land mask to match that of the interior. Anything that isn't land is open - as long as the eastern side is not opened.
Continent B is like Asia in an Arctic domain, where part of the western side is open and part is closed, including places where the land mask is only in the exterior. One could bring the whole grid into the MOM6 interior points, put the "edge" one grid in, setting the openness of each edge point by the land mask. If so, there needs to be a way to shut down the regular computations where the black arrows are drawn - the land mask should take care of only half those shown.
Does it make sense to support both modes? Try mode A first, then mode B? Figuring out mode B is a prerequisite to figuring out the above more general case.