In this paper the results of a number of idealized simulations of the airflow over a mountain ridge, using a fully compressible non-hydrostatic mesoscale atmospheric model is presented. First, the simulations were conducted with a steady background flow field as lateral boundary conditions. The results are interpreted with a special emphasis on formation and properties of lee waves and rotors. The effects of the changes in the atmosphere, topography and external wind field on lee wave and rotor properties are examined. It was found out that the flow fields generated by the present model are significantly different and much more complicated than those predicted by the linear theories. Most of the conditions produced single or multiple rotors that caused boundary layer separation on the lee side. Secondly, the effect of unsteadiness of the background flow on the lee wave dynamics was examined. It was found out that a gradual drop of wind speed could cause the lee wave train to move upstream and to cause a considerable rotor-like motion over the mountain ridge. An attempt is made to link this behavior of the model under the unsteady wind with the observed abundance of thunder-storm activities over mountain ridges compared with surrounding lowlands.