Atmospheric Rotors and Severe Turbulence in a Long Deep Valley

Abstract

The conceptual model of an atmospheric rotor is reexamined in the context of a valley, using data from the Terrain-Induced Rotor Experiment (T-REX) conducted in 2006 in the southern Sierra Nevada and Owens Valley, California. All T-REX cases with strong mountain-wave activity have been investigated, and four of them (IOPs 1, 4, 6, and 13) are presented in detail. Their analysis reveals a rich variety of rotorlike turbulent flow structures that may form in the valley during periods of strong cross-mountain winds. Typical flow scenarios in the valley include elevated turbulence zones, downslope flow separation at a valley inversion, turbulent interaction of in-valley westerlies and along-valley flows, and highly transient mountain waves and rotors. The scenarios can be related to different stages of the passage of midlatitude frontal systems across the region. The observations from Owens Valley show that the elements of the classic rotor concept are modulated and, at times, almost completely offset by dynamically and thermally driven processes in the valley. Strong lee-side pressure perturbations induced by large-amplitude waves, commonly regarded as the prerequisite for flow separation, are found to be only one of the factors controlling rotor formation and severe turbulence generation in the valley. Buoyancy perturbations in the thermally layered valley atmosphere appear to play a role in many of the observed cases. Based on observational evidence from T-REX, extensions to the classic rotor concept, appropriate for a long deep valley, are proposed.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…Processes controlling whether ambient flow overflows or flushes the valley atmosphere [207][208][209][210][211] have obvious air-quality implications for large urban areas in mountains [142,212]. • Inversion layers above mountain tops control the amplitude and wavelength of propagating and trapped wave modes (Figure 5b).…”

Mountain Waves in High Resolution Forecast Models: Automated Diagnostics of Wave Severity and Impact on Surface Winds

“…Mountain waves, induced by stable tropospheric flow over terrain, are a well-known source of aviation hazards, such as strong up-and down-drafts, downslope windstorms [1][2][3][4], clear air turbulence (CAT) associated with tropospheric wave breaking [5][6][7][8][9][10], low-level turbulence and 'rotors' associated with wave-induced boundary layer separation [11][12][13][14][15][16][17] and hydraulic jump-like flows [12,18], as well as hazards to surface transport [19], such as very strong or gusty winds [20].…”

The amplitude of lee waves on the boundary‐layer inversion

“…On the one hand, the strong updraughts allow long and smooth glider flights to high altitudes. On the other hand, they can lead to severe low‐level turbulence in atmospheric rotors (Doyle and Durran, ; Strauss et al , ) and pose a potential hazard for aviation.…”