Gravity Waves

• Chapter IV: Physical Background Of Lee Waves


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• Trapped And Vertically Propagating Lee Waves
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• Rotor Clouds
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• Breaking Waves
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• The Brunt-Väisälä Frequency
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• The Scorer Parameter
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• The Wave Length
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• The Froude Number
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• Exercises
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Breaking Waves

Breaking waves can be observed when vertical wind shear (also directional shear) exceeds a critical value. Wave slopes get steeper and steeper with increasing vertical shear until the top of the wave overruns the lower part (similar to breaking waves of the sea). Subsequently the laminar flow turns into a turbulent flow, inverting the preexisting stable stratification by mixing cold air with warmer air below. Breaking waves are found in both the lower atmospheric levels and in the jet level with negative wind shear below the tropopause (see chapter 5b).

Figure 6: Breaking waves over Mount Duval in Australia. ©GRAHAMUK, CC BY-SA 3.0

The more stable the atmosphere, the higher the wind shear can be before waves start to break. This also means that a less stable atmosphere favors turbulence in a region with lee waves.