Revisiting open boundary conditions from the point of view of characteristic variables

This paper emphasizes the peculiar role of characteristic variables in the design of open boundary conditions (OBCs). It is shown that local OBCs leading to positive results in previous comparative studies do fulfil two requirements: they make use of incoming characteristic variables (i.e. privilege the hyperbolic aspect of the equations), and satisfy a consistency relationship between the model solution and some external data. The classical OBCs used in atmosphere and ocean modeling are revisited from this point of view. It is shown that several usual boundary conditions should be avoided, while conditions satisfying the two preceding criteria are pointed out. Finally, the application of these criteria to the design of OBCs for primitive equations is discussed.