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.

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.