Tributary Fluxes into Brush Creek Valley

Abstract

Measurements in a tributary to Brush Creek Valley during the September and October 1984 ASCOT campaign with laser anemometers, tethersondes, a minisodar, and smoke release were used to calculate the contribution by tributaries to nocturnal drainage flow from the main valley. Four experimental nights with different mesoscale wind regimes were used in the study. It was found that a simple picture of mass flux proportional to drainage area is not sufficient to predict the relative contributions of drainage basins. The exposure of the slopes within the tributaries to the external wind regime was found to be a significant factor in the contribution of the upper regions of the tributary; but drainage from the well-protected lower region was found to be stronger when the external wind direction was opposite to the drainage direction than when the external wind was along the drainage direction. A circulation cell that introduces mass into the tributary system both from the main canyon flow and from the side opposite the main canyon flow is proposed to explain this effect. Data from the morning hours after flow reversal has occurred in the main canyon show continued drainage in the tributary for up to two hours. The data are used to show that this is probably due to continued radiational cooling of the protected sidewalls in the tributary.