Mountain waves forced by long ridges are often accompanied by low-level vortices that have horizontal axes parallel to the ridgeline. These vortices, known as rotors, can be severe aeronautical hazards and have been cited as contributing to numerous aircraft upsets and accidents, including occurrences involving modern commercial and military aircraft. In spite of their obvious importance, mountain-induced rotors still remain poorly understood. In this study, the dynamics of rotors forced by three-dimensional topography are investigated through a series of high-resolution simulations with the non-hydrostatic COAMPS model using idealized and real-data simulations.

High-resolution simulations of two rotor events that formed over the Owens Valley, CA on 19 November 1996 and 29 October 2000 are performed using five nested grids with a horizontal grid increment of 333 m on the finest mesh and 60 vertical levels. The Owens Valley is characterized by exceptionally steep topography and is located in between the Sierra Nevada ridge to the west and the Inyo and White Mountains to the east. These simulation results enable us to deduce some common characteristics of rotors in three dimensions. Rotor formation occurs in conjunction with strong cross-mountain flow in excess of 20 m/s and large amplitude trapped lee waves. The results suggest that rotors are a boundary-layer separation phenomenon. The development of separated flow along the lee slope is forced by adverse pressure gradients associated with the trapped lee waves. A region of extreme turbulence with values of turbulent kinetic energy in excess of 20 m² s^-2 is positioned along the leading edge and immediately above the rotor circulation. The model results indicate that preferential regions of rotor generation exist due to three-dimensional forcing from the Sierra Nevada range. The development of subrotors, regions of enhanced horizontal vorticity that develop on a horizontal scale much smaller than the rotor itself, will be addressed using the real-data and idealized simulations.