Rotor types associated with steep lee topography: influence of the wind profile

Turbulent rotors in the lower troposphere are usually associated with high-amplitude lee waves. Two types of rotors have been observed. The first type, often visible as harmless-looking cumulus or cumulus fractus lines paralleling the mountain range, comprises a well-defined circulation under the crests of resonant mountain waves. This type of rotor contains moderate or severe turbulence and is often confined below the height of a frequently-observed upstream, near-mountain-top inversion. A second, less common, rotor type extends much higher than the upstream inversion. This type has been observed to contain severe or extreme turbulence, and is thought to be associated with a high-amplitude mountain-wave system resembling a hydraulic jump. Both rotor types present a hazard to aviation, although the second type of rotor is far more dangerous. We present high-resolution two-dimensional simulations of two distinct rotor types, which reveal dramatic differences in internal structure and turbulence intensity. The evolution of the lee-side horizontal vorticity provides insight into the formation mechanism involved. Horizontal vorticity within the initial upstream inversion is modified due to baroclinic generation as the flow spills down the lee slope. The magnitude of the modified horizontal vorticity is regulated by shear in the initial upstream inversion. At the same time, horizontal vorticity of the opposite sign forms in a shallow surface layer. The type of rotor that forms depends on the sign of the dominant horizontal vorticity as near-surface flow separation occurs along the lee slope. The simulations point to the vital role of an upstream near-mountain-top inversion, its deformation in the lee of the mountain, and the initial vertical shear within the inversion in rotor formation.