Observations and Modeling of Atmospheric Rotors

Atmospheric rotors are frequently associated with low-level severe turbulence and intense up- and downdrafts near the ground on lee slopes in regions of complex terrain. Pioneering investigations of the phenomenon date back to the 1930s, while the major elements of its current understanding have been obtained through dedicated field campaigns in the 1950s (Sierra Wave Project and Mountain Wave Experiment), 1970s (Colorado Lee Wave Program) and 2000s (Sierra Rotors Project and the Terrain-induced Rotor Experiment (T-REX)). It is now well established that atmospheric rotors are associated with non-hydrostatic part of the terrain-generated wave response, linking large-amplitude lee waves aloft with a separated boundary layer near the ground.

In this presentation we summarize a few recent findings on atmospheric rotors, including:
1) The intensity of turbulence within rotors is comparable to that in hydraulic jumps and in mid-tropospheric wave-breaking regions. Rotors react quickly to mesoscale forcing and may move steadily against the background wind on the lee side of a mountain as they dissipate. Supporting evidence includes Doppler cloud radar measurements made in 2006 in the lee of the Medicine Bow Mountains (Wyoming, USA). These observations are interpreted with the aid of mesoscale numerical simulations.
2) The complexities of rotor structure and dynamical evolution that are inherent to a valley environment are illustrated with findings from the T-REX campaign, held in 2006 in Owens Valley in the lee of the Sierra Nevada (California, USA).
3) Rotors may occur, in connection with an undular bore, even when the primary mountain wave propagates vertically and is essentially hydrostatic. This finding is supported by idealized simulations of linearly stratified and vertically uniform flow over a mountain.