Mountain and Lee Waves

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What are Mountain Waves?

Mountain Waves are common forms of lee waves, which are forms of internal gravity waves. To understand mountain waves, we need to clarify some definitions.

Intertia-Gravity Waves are waves in a fluid perturbed (started) and influenced by gravity and inertial forces. If the case where the Coriolis effect can be neglected, then these waves are simply gravity waves. (H&H, p.141) These waves tend to be very fast compared to Rossby Waves (waves created by the influence of the Coriolis effect, which are global and slow-moving). (CR, p.278) Under the influence of these gravity waves, particles in the atmosphere or other fluid will oscillate vertically, and will transport momentum and energy in the vertical direction. (Tsuda)

Lee Waves are waves produced by waves blowing over the edge of a mountain on the lee (downslope) side. Mountain Waves are one type of lee wave that is most commonly referred to and will be referred to from here on.

Mountain waves are internal gravity waves created by wind in a stably-stratified atmosphere blowing over a mountain or otherwise irregular landscape. (CR p.412) These waves propagate away from the mountain as internal gravity waves. (These are gravity waves within a fluid below the surface, such as in water or in the atmosphere.)

Stratification in the atmosphere refers to horizontal layers of air stacked on top of each other, each layer at different densities and temperatures, which lighter and colder air on top of warmer and denser air. In this case, the stratification is stably stratified.

The determining factors in the production of a mountain wave are the width of the mountain ridge, wind speed, stratification frequency, wavelength of mountains, the vertical profile of temperature, and moisture in the atmosphere. Mountain waves are more likely to occur when a jet stream is crossing over a mountain area. (Vieira 2005)