Strong Downslope Winds at Boulder, Colorado

Abstract

Twenty wind storms, with maximum gusts exceeding 33 m sec?1, are analyzed. At the height of the storms, mean hourly speeds in Boulder were 20 m sec?1 with mean maximum gusts of 36 m sec?1. At the same time wind speeds were about 50% lower along the lee slopes to the west and within 10 km to the east into the plains. The phenomena are non-stationary, and the result is a great diversity in surface wind speed patterns at the stations. Observational data show that the storms are the result of a deep lee trough or pressure jump. Air is accelerated down the slopes toward the local pressure minimum beneath the disturbance and is decelerated again rapidly to the east. Air mass characteristics that lead to the development of the disturbance are a stable layer or inversion at a critical level above mountain top and relatively strong winds at that level; strong winds in the upper troposphere are not necessary.