What are Mountain Waves?
(Part of and article from “Fliegermagazine" by Manfred and Stephan Kreipl)
A mountain wave is a disturbance of the atmosphere, such that with relatively stable layers of air, wave-like layering of the air exists accompanied by strong turbulence and vertical air currents. These waves occur in conjunction with mountain ranges lying at right angles to the wind. This phenomenon is comparable to water flowing over an obstruction causing velocity changes and lee waves.
This double activity can be also seen in mountain waves. It can be described by a two layer model.
- As soon as the lower layer passes over the ridge line of a mountain range it follows the descending ground and sweeps out pockets of older air from the valley. Then this down-going air shoots up forming a rotor cloud.
- In the upper layer there is an internal lee wave that builds up downwind of the ridge top above the rotor zone.
The interaction of these two effects determines the amplitude of the wave and its altitude in relation to the mountains. Beyond that, mountain waves are also influenced by the rate of change of wind with altitude and by the influence of the stratosphere as a third layer.
What favors the formation of wave?
The most intensive waves are fostered by strong winds at right angles to the mountain range. As a rule of thumb, a 25 knot wind across a ridge will probably generate wave. Wave can develop even on rather low ridges and isolated mountains. Waves have been observed to develop from small hills to altitudes of 1,000 to 24,000 feet. The higher the wind velocity, the stronger the effect on the lee side of the obstruction. Normally the wind speed increases rapidly with increasing height of the mountain range and up to 1,000 feet over the ridge top. Above that, the wind is strong and smooth up to the tropopause and sometimes higher.
The character of the wave varies with different wind profiles. An unusually high wind gradient with altitude (for instance a 60 knot increase from ridge top to 5,000 ft higher) can cause the wave to disappear and leave stagnant air in the valley.
Strong wave development occurs often in connection with a favorable jet stream orientation with respect to the range. Then you have to expect strong winds in the upper troposphere in the neighborhood of 75 to 150 knots and strong associated turbulence.
translated by David Noyes, Ohio
