Recent Developments in the Theory of Atmospheric Rotors

MARCH 2004 AMERICAN METEOROLOGICAL SOCIETY | he Sierra Nevada Range is one of the most prominent and steepest mountain barriers in the United States and, not surprisingly, is a well-known location for a multitude of topographically forced atmospheric phenomena. As the prevailing westerly winds pass over the Sierra Nevada, gravity waves are frequently generated. Occasionally these mountain waves result in spectacular topographically forced phenomena such as trapped lee waves, downslope windstorms, rotors, and attendant wave and rotor cloud structures, as shown in the examples in Fig. 1. In situations such as those shown in Fig. 1, severe downslope winds near the surface, sometimes in excess of 50 m s-1, decelerate rapidly in the lee and give way to a return flow back toward the mountain crest that is the lower branch of an intense horizontal circulation, as illustrated in the schematic in Fig. 2. These horizontal vortices, known as rotors, are common to the steep eastern slopes of the Sierra Nevada, and particularly over the Owens Valley, where they are notorious for their intensity (e.g., Whelan 2000). Rotors have also THE MAP ROOM