Lee Waves and Mountain Waves ∗

Buoyancy perturbations develop when stably stratified air ascends a mountain barrier. These perturbations often trigger disturbances that propagate away from the mountain as gravity (or buoyancy) waves. Gravity waves triggered by the flow over a mountain are referred to as mountain waves or lee waves. Mountain waves sometimes reveal their presence through dramatic cloud formations, such as smooth lenticular clouds (see Figs. 4 and 5) and ragged rotor clouds. Large-amplitude mountain waves can generate regions of clear-air-turbulence that pose a hazard to aviation. Large-amplitude mountain waves may also produce very strong winds that blow down the lee slope of ridge-like topographic barriers (see downslope winds). What happens to mountain waves after they are generated? If the wave amplitude becomes large in comparison to the vertical wavelength, the streamlines in a vertically propagating mountain wave steepen and overturn in a manner roughly analogous to a breaking wave in the ocean. Such “convective” overturning often occurs as the waves enter the lower stratosphere where they encounter increased static stability and decreasing horizontal wind speeds. The convective overturning of vertically propagating waves is also promoted by the systematic decrease in atmospheric density with height. Those waves that do not breakdown due to convective overturning before reaching the mesosphere are ultimately dissipated by the vertical transfer of infra red radiation between the warm and cool regions within the wave and the surrounding atmosphere (“radiative damping”). Horizontal momentum is transported by mountain waves from the regions of wave dissipation to the surface where a net pressure force is exerted on the topography. A decelerative force is exerted on the large-scale atmospheric circulation in those regions where the wave undergoes dissipation. The basic structure of a mountain wave is determined by the size and shape of the mountain and by the vertical profiles of temperature, wind speed and moisture in the impinging flow. The overall character of the wave can often be predicted on the basis of linear theory, in which the mountain is assumed to be small in comparison with