Numerical Simulation of Nocturnal Drainage Flows in Idealized Valley–Tributary Systems

Abstract

Numerical simulations of nocturnal drainage flow and transport in idealized valley?tributary systems are compared with the Atmospheric Science in Complex Terrain (ASCOT) meteorological field data and tracer studies from the Brush Creek valley of western Colorado. Much of the general valley?tributary flow behavior deduced from observations is qualitatively reproduced in the numerical results. The spatially complex, unsteady nature of the tributary flow found in the field data is also seen in the simulations. Oscillations in the simulated tributary flow are similar to some field observations. However, observed oscillations in the valley flow at the mouth of the tributary could not be reproduced in the numerical results. Thus, hypotheses of strongly coupled valley?tributary flow oscillations, based on field data, cannot be supported by these simulations. Along-valley mass flux calculations based on model results for the valley?tributary system indicate an increase of 5%?10% over a valley without a tributary. Enhanced valley mass fluxes were found from 8 km above the tributary to almost the valley mouth. However, the valley mass fluxes for topography with and without a tributary were nearly equal at the valley outflow. ASCOT field data suggested a tributary mass flow contribution of 5%?15% for a Brush Creek tributary of similar drainage area to the model tributary employed here. Numerical simulations of transport in the nocturnal valley?tributary flow strongly support ASCOT tracer studies in the Pack Canyon tributary of Brush Creek. These results suggest that the valley?tributary interaction can significantly increase plume dispersion under stable conditions. Overall, the simulation results presented here indicate that simple terrain geometries are able to capture many of the salient features of drainage flow in real valley?tributary systems.