On the implementation of open boundary conditions for a general circulation model: The threedimensional case

This article discusses the results of an experimental survey on the performance of a select group of open boundary conditions (OBCs) for threedimensional, primitive equation models. The focus is on passive conditions, where the model response at the open boundaries is dictated by the interior dynamics. The performance of the OBCs is tested in a series of numerical experiments conducted in a rotating channel which includes variable bottom topography and density stratification. The experiments are sele. cted to enhance nonlinear phenomena that comprises advection and the propagation of dispersive waves. The first two experiments study coastal upwelling with constant and time-varying wind forcing. The third experiment analyses the barotropic and baroclinic response of a coastal ocean due to the passage of a traveling storm. The open boundary condition with the best overall performance is the flow relaxation scheme on sea surface elevation and transport, a radiation condition for internal velocities, and a combined scheme of flow advection and relaxation for the temperature field. A modified gravity wave radiation scheme provides reasonable responses when combined with radiation conditions for internal velocities and an advection equation for temperature if the forcing is not changing direction rapidly at the open boundary. Schemes using the method of characteristics or traditional wave advection schemes fail when combined with radiation conditions for the baroclinic mode.