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   February 2000 

Mountain winds take off

High, broad mountains with steep windward slopes, such as this mountain ridge on Kauai, one of the Hawaiian islands, can strongly affect coastal weather, according to a recent study by scientists at the National Center for Atmospheric Research. Photo from John Bortniak, NOAA.

Using an idealized numerical model, scientists at the National Center for Atmospheric Research have studied the effects that mountains such as the Sierra Nevada range and the Rocky Mountains have on cold fronts along the coast. Previous studies have shown that during the winter months the mountains can affect coastal weather, creating strong winds and heavy rainfall that cause mudslides. Two studies published in the February 1999 and October 1999 issues of the Journal of the Atmospheric Sciences examine what determines the strength of coastal winds, and how mountains affect the evolution of fronts.

Researchers Scott Braun, Richard Rotunno and Joseph B. Klemp ran two sets of simulations in their study. One focused on dry, inviscid (frictionless or ideal) dynamics, the other on the effects of surface friction. Creating simulations that used orography typical of the Western United States and Canada—high, broad mountains with steep, windward slopes—the scientists found that when coastal winds hit steep mountains they decelerate abruptly, creating a pressure imbalance. This pressure imbalance results in southerly winds accelerating to speeds above 50 miles per hour in some cases. The strength of this high-speed wind, called a barrier jet, largely depends on the mountain’s width.

The simulations also show that the eastward movement of fronts could be hindered along the coast by a combination of blocking by the mountains and increased friction over land. The lifting of air by the mountains and the slower frontal movement can contribute to heavy rainfall in the coastal zone. Winds associated with the front can combine with those of the barrier jet. Strong winds can form before a front when southerly frontal winds combine with the southerly barrier jet. However, weak winds can result behind the front as northerly winds behind the front combine with the southerly barrier jet, effectively canceling each other.

—Livia Levis
contributing writer