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The Influence of Inversions on Rotors

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  • 1 CoRA Division of Northwest Research Associates, Boulder, Colorado
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Abstract

Rotors are small-scale circulations about a horizontal or quasi-horizontal axis that usually form in conjunction with high-amplitude mountain waves. The moderate to severe turbulence often found within rotors is a hazard to aviation. Observations and numerical model studies have revealed two types of mountain-wave–rotor systems. The first type is associated with trapped waves, whereas the second, less common, type resembles a hydraulic jump. It has long been known that an upstream, near-mountaintop inversion plays an important role in mountain-wave/rotor formation. In this study, the role of the upstream inversion strength and height in an environment with sheared flow and over a barrier with a steep lee slope is investigated. It is found that the second mountain-wave/rotor type is more likely to form when a strong near-mountaintop inversion is present. Baroclinic generation of horizontal vorticity within the inversion along the lee slope leads to overturning in an upstream direction and spreading of inversion isentropes. The sign and magnitude of vertical shear within the upstream inversion has a modifying influence, with positive shear favoring the formation of trapped-wave systems. Stronger inversions lead to higher-reaching, more turbulent mountain-wave/rotor systems, regardless of type.

Corresponding author address: Rolf F. Hertenstein, CoRA Division of Northwest Research Associates, 3380 Mitchell Lane, Boulder, CO 80302. Email: herten@cora.nwra.com

Abstract

Rotors are small-scale circulations about a horizontal or quasi-horizontal axis that usually form in conjunction with high-amplitude mountain waves. The moderate to severe turbulence often found within rotors is a hazard to aviation. Observations and numerical model studies have revealed two types of mountain-wave–rotor systems. The first type is associated with trapped waves, whereas the second, less common, type resembles a hydraulic jump. It has long been known that an upstream, near-mountaintop inversion plays an important role in mountain-wave/rotor formation. In this study, the role of the upstream inversion strength and height in an environment with sheared flow and over a barrier with a steep lee slope is investigated. It is found that the second mountain-wave/rotor type is more likely to form when a strong near-mountaintop inversion is present. Baroclinic generation of horizontal vorticity within the inversion along the lee slope leads to overturning in an upstream direction and spreading of inversion isentropes. The sign and magnitude of vertical shear within the upstream inversion has a modifying influence, with positive shear favoring the formation of trapped-wave systems. Stronger inversions lead to higher-reaching, more turbulent mountain-wave/rotor systems, regardless of type.

Corresponding author address: Rolf F. Hertenstein, CoRA Division of Northwest Research Associates, 3380 Mitchell Lane, Boulder, CO 80302. Email: herten@cora.nwra.com

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