| A radiation open boundary condition based on vertical normal modes is introduced here. This boundary condition has been applied to both a nested and a coupled Equatorial Kelvin wave propagation test using the Naval Research Laboratory Coastal Ocean Model (NCOM), which is a sigma-z-level hybrid coordinate model. This boundary condition has also been applied to the more computationally difficult problem of coupling the Naval Research Laboratory Layered Ocean Model (NLOM), which is an isopycnal coordinate model, to the NCOM for the test case.; The NCOM is a primitive equation ocean model which employees the hydrostatic, Boussinesq, and incompressible approximations and uses a free surface. The NLOM is a primitive equation ocean model, which is hydrodynamic (isopycnal) and utilizes a free surface. Because both of these models use the full primitive equations, vertical normal mode theory shows that the form of the modal equations is identical for the two models in the linear limit. This allows a physical linkage between the two models that can be used in a boundary condition application that is advanced in many ways over standard radiation boundary condition approaches.; This approach is shown to be superior to standard radiation open boundary conditions in NCOM coupling tests in which the number of vertical levels in the forcing domain are fewer than the number of vertical levels in the area of interest. The superiority comes both in the reduction of reflected outgoing waves, and in the ability to recognize and adjust the differences in the physics, of the two models, that affect wave speed. An advantage to the latter superiority is that the noise created by the numerics of the boundary condition and the error created by the differences in the physics of the two models are distinct from each other. Another advantage is that the forcing data does not need to be vertically interpolated at any time to be used in this calculation, thus reducing one source of numerical noise or error.; The lack of vertical interpolation and the use of a physically motivated linkage are advantages over numerical techniques when the forcing model uses different physical equations than the model used to investigate an area of interest. |
