The Dispersion of Atmospheric Tracers in Nocturnal Drainage Flows

Abstract

This paper summarizes the results of a series of perfluorocarbon tracer experiments that were carried out in the Brush Creek Valley in western Colorado under the auspices of the Atmospheric Studies in Complex Terrain (ASCOT) program. The results indicate that tracers entrained within the valley's nocturnal drainage flows displayed well defined plumes that were not influenced significantly by the larger scale flows above this deep and narrow valley. Thus, the spatial distributions of the tracers were primarily governed by the structure of the drainage flows. None of the tracers released within the valley were detected in significant quantities on the adjoining mesas or within the adjacent valleys prior to sunrise. The process of ventilating the tracers out of the valley was initiated shortly after sunrise by the upslope flows generated along the valley sidewall exposed to the morning sun. The rate of ventilation was influenced by the solar intensity, the ambient meteorology, and the location of the plumes within the valley. The maximum rate occurred about one to two hours after sunrise on the mesa. The volume fluxes of the main valley drainage flows as well as those from Pack Canyon were estimated with the use of the tracer data. The Pack Canyon flow was estimated to contribute a maximum of 13% to the main valley flow with a more probable value of a few percent.