This is a new question regarding the NBQ mode and boundary conditions. I am building my way up to imposing a time-dependent stratified flow at the western boundary in the non-hydrostatic mode. As in my earlier post I am using a simple rotating, non-hydrostatic geostrophic adjustment problem (define NBQ and UV_COR) to learn about the model. The flow is nominally d/dy=0 (NS_PERIODIC with Mm=3). In this case the calculation works as expected when the x-direction boundary conditions are either EW_PERIODIC or EW-walls. However, if I define OBC_EAST and OBC_WEST (attempting to get a simple radiation condition) the calculation fails almost immediately. The velocities immediately adjacent to the two x-boundaries become very large. I suspect that this could be related to the acoustic mode, but am uncertain whether this is the case, and how to handle it. There are no problems with OBCs in the hydrostatic mode. Any help is appreciated.