The internal structure and dynamics of rotors that form in the lee of topographic ridges is explored using a series of high-resolution eddy resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel shear instability breaks this vortex sheet into small intense vortices or sub-rotors. The strength and evolution of the sub-rotors and the internal

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... ture of the main largescale rotor are substantially different in 2-D and 3-D simulations. In 2-D, the sub-rotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of a relatively laminar vortex inside the large-scale rotor. In 3-D, even for flow over a uniform infinitely long barrier, the sub-rotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between the main rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic When an isolated peak is added to a 3-D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Sub-rotors passing through such regions are intensified by stretching and may develop values of the ridgeparallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such sub-rotor circulations likely pose the greatest hazard to aviation.