Observations of complex-terrain flows using acoustic sounders: experiments, topography, and winds

Acoustic sounders have been used extensively in a series of nocturnal drainage flow experiments conducted by the U.S. Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program. Doppler acoustic sounders, located in three different valleys during the sequence of experiments, reveal drainage-wind profiles that depend strongly upon the ambient meteorological conditions and the elevation of each observing site relative to surrounding terrain. In elevated sites that drain easily, Doppler-sounder-derived wind profiles show a simply structured flow. In lower lying areas, subject to topographic construction and cold-air pooling, and where Archimedean forces are comparable to those caused by synoptic and mesoscale pressure gradients, the wind profiles show considerable vertical and temporal variation. In particular, in the Geysers area of northern California, the sea breeze and the depth of the Pacific coast marine inversion affect the initiation of drainage winds and their subsequent evolution.