A Time-Dependent Lateral Boundary Scheme for Limited-Area Primitive Equation Models

Before high-resolution numerical models can be of use operationally, they must be restricted to a limited domain, thus necessitating lateral boundary conditions which allow the changes outside the limited domain to influence the results while not contaminating the forecast with spurious boundary-reflected energy. Such a set of time-dependent lateral boundary conditions are presented in this paper. This boundary condition set is investigated using the linear analytic and finite-difference advection equations, the non-linear finite-difference shallow-water equations, and the hydrostatic primitive equations. The results illustrate how the boundary condition transforms long- and medium-length interior advective and gravity waves into short waves which can then be removed by a low pass filter, thereby giving the appearance that the exiting wave simply passed through the boundary. The results also indicate that large-scale advective and gravity waves enter the forecast domain with little degradation. Thus, from the tests performed, the described boundary condition scheme yields a practical solution for prescribing time-dependent lateral boundaries for a limited-area model.