The Penetration of Mountain Waves into the Middle Atmosphere

Abstract

A linear nonhydrostatic model of gravity waves forced by a bell-shaped ridge is used to investigate the penetration of mountain waves into the stratosphere and mesosphere during winter and fall. Gravity waves with horizontal scales less than 30 km are found to be trapped near the tropopause and the stratopause in regions of strong winds. The effect of trapping these modes produces a disturbance whose structure broadens with height. In the mesosphere the disturbance appears 20?40 km downstream from the forcing depending on the strength of the intervening winds. Wavebreaking associated with the mountain wave is predicted in the lower stratosphere as a result of wave superposition; no individual harmonic reaches breaking amplitude. In the mesosphere, wave breakdown is more prevalent, and the disturbance spectrum is relatively more monochromatic as a result of the filtering of the shorter scale modes by the lower atmosphere.