A simple immersed boundary forcing for flows over steep and complex orography

Simulation and parametrization of flow over the Earth's orographic features is an important element of any climate or weather model. With increasing resolution, especially in the limited-area context, the orographic features are not only becoming better resolved but, importantly for numerical stability, the slopes are becoming ever steeper and the currently employed terrain-following coordinates can cause convergence issues as the curvilinear terms become relatively large. This paper discusses this specific issue and suggests a possible solution based upon an immersed boundary forcing method for representing steep and complex terrain. The method is simple and straightforward to incorporate into any atmospheric model.