Observed variations of ?? and ?? in the nocturnal drainage flow in a deep valley

Abstract

The variations of Σθ and ΣΦ in the drainage flow in the Brush Creek valley of western Colorado are investigated using data from Doppler acoustic sodars and instrumented towers. The data were obtained on two experimental nights during the 1984 ASCOT field study. There is good agreement between the Σ variations derived from low-level observations of the sodars and those derived from the towers located throughout the valley. The observed hourly average Σθ and ΣΦ in the nocturnal drainage flow are about 20 ° to 25 ° and 5 °, respectively; these values are much larger than those generally observed over flat terrain during nighttime stable conditions. After sunrise (about 0600 MST), as the valley warms and the flow direction changes to up-valley, these parameters increase sharply to their peak values at about 0800 MST and then decrease to their normal daytime values after about two hours.