Mountain Waves and Downslope Winds Forced by a Midlatitude Cyclone Encountering an Isolated Ridge

The interaction of a mid-latitude cyclone with an isolated north-south mountain barrier is examined using numerical simulation. A prototypical cyclone develops from an isolated disturbance in a baroclinically unstable shear flow upstream of the ridge, producing a cold front that interacts strongly with the topography. The structure and evolution of the lee waves launched by the topography are analyzed, including their temporal and their north-south variation along the ridge.

Typical mountain wave patterns are generated by a 500-m high mountain, but these waves often exhibit significant differences from the waves produced in 2D or 3D simulations with steady large-scale flow structures corresponding to the instantaneous conditions over the mountain in the evolving flow. When the mountain height is 2 km, substantial wave breaking occurs, both at low-levels in the lee and in the lower stratosphere. Despite the north-south uniformity of the terrain profile, large north-south variations are apparent in wave structure and downslope winds. In particular, for a 24-h period beginning after the cold front passes by the upstream side of the mountain, strong downslope winds occur only in the lee of the northern half of the ridge. Just prior to this period, the movement of the cold front across the northern lee-slopes is complex and accompanied by a burst of strong downslope winds and intense vertical velocities.