Mountain waves forced by long, quasi-two-dimensional ridges are often accompanied by low-level vortices or rotors that have horizontal axes parallel to the ridgeline. The dynamics of rotors associated with mountain waves are investigated through a series of high-resolution two-dimensional simulations with the non-hydrostatic COAMPS model using free-slip and no-slip lower boundary conditions. Boundary-layer separation is a prerequisite for rotor formation. The development of separated flow at a point along the lee slope is facilitated by adverse pressure gradients forced by trapped lee waves. Strong adverse pressure gradients present in free-slip simulations are associated with strong rotors in the presence of surface friction. A nondimensional pressure gradient greater than 2.2 appears to be essential for viscous rotor generation indicating that not all lee waves are capable of supporting rotors. Additionally, high shear in the boundary layer can be sustained without rotor development when no trapped waves are present. Although transient rotors can be generated with a free-slip lower boundary, realistic steady rotors appear to develop only in the presence of surface viscosity. Mechanical shear in the planetary boundary layer is the primary source of a sheet of horizontal vorticity associated with the rotor that is lifted vertically into the lee wave at the separation point. Increasing surface roughness beyond values typical for a smooth surface (roughness length=0.01 cm) decreases the rotor strength. Surface heating along the lee slope increases the vertical extent of the rotor circulation and turbulence with a concomitant decrease in the strength of the reversed flow. The importance of three-dimensional effects such as vortex stretching for the development of sub-rotor structures will be addressed.