The Nonlinear Response of the Atmosphere to Large-Scale Mechanical and Thermal Forcing.

Abstract The subject of large-scale mountain waves is reviewed briefly. Existing mountain wave theory based on a linear system is shown to give an inadequate description of the balance of angular momentum. The response of the atmosphere to mechanical forcing in a nonlinear framework is then discussed, using a two-level quasi-geostrophic long-wave spectral model based on spherical coordinates, including diabatic heating, surface friction and mountains. The nonlinear theory shows that there exists a critical mountain height Hc, which is a function of the frictional coefficient as well as the phase difference between the mountain and the surface pressure field. If, and only if, the mountain height is less than this critical value, can the deflection effect of the mountain be neglected and the response regarded as approximately linear. This critical mountain height is only about 1 km. Thus most of the atmospheric response to large-scale mountains must be nonlinear. In the nonlinear case, as the mountain heigh...