The Influence of Canyon Winds on Flow Fields near Colorado's Front Range

Abstract

A network of sodars was operated in the late summer and fall of 1993 to monitor the occurrence of nocturnal winds from a canyon in Colorado's Front Range near the Rocky Flats Plant and to determine the influence of those winds on the flow fields over the plant. The canyon flows could be broadly classified into two categories: well developed and irregular. The well-developed flows were generally stronger, deeper, and more continuous than the irregular ones, and the canyon's influence on the wind fields near the plant site was confined primarily to periods with these flows. These periods, in turn, usually followed days during which a deep mixed layer formed over the plains to the east of the mountains. Following days with shallower mixed layers, the canyon winds tended to be weaker and shallower. Numerical simulations with a nested mesoscale numerical model were used to examine the mechanisms responsible for this behavior. The nighttime simulated temperature gradients between the air near the mountain slopes and the free air over the plains were found to be larger after days with deep mixed layers, resulting in stronger down-canyon flows at night. In addition, for the deeper mixed-layer cases, air parcels descending the slope do so in more neutrally stratified conditions so that their buoyant acceleration down the slope is maintained. A notable exception to this behavior occurred on one night when nonstationary waves appeared to disrupt the canyon flows. Marker particles released into the simulated flow fields were used to follow the motion of air parcels from the mountains out over the plains. They revealed a tendency for air parcels to remain elevated when they exit the valley on nights with lighter canyon winds and shallower afternoon mixed layers, thereby reducing the canyon's potential effect on the near-surface winds over the Rocky Flats Plant. Particle trajectories were also used to examine the concept of a well-defined airshed feeding a draining valley; the concept was found to be of limited validity for the topography in this area.