A nonlinear time-dependent radiation condition for simulations of internal gravity waves in geophysical fluid flows

This paper examines the development of a time-dependent nonreflecting boundary condition (or radiation condition) for use in simulations of the propagation of internal gravity waves in a two-dimensional geophysical fluid flow configuration. First, a linear radiation condition, originally derived by Campbell and Maslowe, is implemented in some linear test cases. It involves the computation of a Laplace convolution integral which is nonlocal in time and thus requires values of the dependent variable at all previous time levels. An approximation for the integral is implemented here to reduce the expense of the computation and the results obtained are shown to be more accurate than those obtained using steady boundary conditions. For larger amplitude waves, nonlinear equations are required and the application of the linear radiation condition gives rise to instabilities. A new nonlinear time-dependent nonreflecting boundary condition is introduced which takes into account wave mean flow interactions in the vicinity of the outflow boundary by including a component corresponding to the vertical divergence of the horizontal momentum flux. This prevents the development of numerical instabilities and gives more accurate results in a nonlinear test problem than the results obtained using the linear radiation condition.