Lee cyclogenesis

From Glossary of Meteorology

lee cyclogenesis

(Also called orographic cyclogenesis.) The synoptic-scale development of an atmospheric cyclonic circulation on the downwind side of a mountain range.

The "lee" side is relative to the mean background airflow. Weak development can occur due to a redistribution of uniform vorticity as large-scale flow passes over a mountain barrier (
see lee trough). Stronger cases of lee cyclogenesis occur when the mountain range interacts with a developing baroclinic wave. In this instance the mountain acts to position the cyclone or generate a secondary cyclone in the lee. Often this leaves a weaker parent cyclone that is typically poleward of the development and far from the mountains. Lee cyclogenesis is a multistage process involving a phase of rapid deepening followed by a transition to slower baroclinic deepening. The mountain barrier disrupts the orderly advection of low-level cold air that would occur behind the cyclone over flatter terrain and induces a quasigeostrphic imbalance. As it tends to restore this imbalance (
see geostrophic adjustment), the atmosphere produces the beginnings of a cyclonic system near the surface. The lack of cool air at low levels and the descending upper-level air make the mountain lee environment statically less stable than the surroundings. This enhances the possibility for vertical coupling of potential vorticity maxima associated with the approaching upper-level trough and the incipient lee disturbance at lower levels. Lee cyclogenesis is common on the leeward side of the major mountain ranges of the world including the Alps, the Himalayas, the Rockies (both east and west), and the Andes. Many minor ranges support lee cyclogenic activity.
See also Genoa cyclone, Colorado low, Alberta clipper, pampero.