Downslope windstorm

From Glossary of Meteorology
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downslope windstorm

A very strong, usually gusty, and occasionally violent wind that blows down the lee slope of a mountain range, often reaching its peak strength near the foot of the mountains and weakening rapidly farther away from the mountains.

Gust speeds in such winds may exceed 50 m s-1 and occasionally strong vortices capable of doing F1 to F2 damage (
see Fujita scale) may occur in association with these winds. Such windstorms are most likely to the lee of elongated quasi-two-dimensional mountain ranges and can be distinguished from gap winds, which are confined to within or downstream of notable gaps or breaks in a mountain barrier and are generally weaker and less gusty. Downslope windstorms of great severity require an upstream mountain range having a crest at least roughly 1 km in height above terrain to its lee, and with a steep leeside slope. Meteorological conditions favoring downslope windstorms are strong synoptic-scale flow across the mountain barrier at the level of its crest, with the cross-range component of the flow either decreasing with height or not increasing too rapidly with height above the crest. Also favorable is high static stability at the level of the mountain crest in the flow approaching the mountain range, decreasing with height above. A mean-state critical level in the middle troposphere, where the flow component across the mountain drops to zero and reverses sign, is often very favorable for downslope windstorms. Downslope windstorms can be considered a gravity wave phenomenon in the sense that vertically propagating gravity waves launched by the passage of stable air over high-amplitude terrain become very steep or break, creating an internal region above the mountain that is characterized by turbulence and a lapse rate approaching the dry adiabatic. Such a region restricts the vertical propagation of energy, allowing the flow near the surface of the mountain to accelerate downslope. Downslope windstorms can also be considered hydraulic jump phenomena in which flow becomes supercritical above and to the lee of a mountain barrier. Downslope windstorms are often known by local names in areas where they occur throughout the world (e.g., the bora along the northeastern shore of the Adriatic Sea and the Taku wind along the Gastineau Channel in southeast Alaska).