11.7 THE INTERACTION OF THERMALLY-DRIVEN CIRCULATIONS AND THEIR EFFECT ON VERTICAL MIXING PROCESSES IN THE SALT LAKE VALLEY Jerome D. Fast Pacific Northwest National Laboratory, Richland, Washington

Our understanding of how various meteorological processes redistribute pollutants within basin and valley atmospheres at night has been limited by a lack of observations. Pollutants released within the stable boundary layer may remain highly concentrated since the strong atmospheric stability and weak turbulent motions limit the amount of vertical diffusion. Complex flows associated with basin and valley topography, however, can complicate the vertical distribution of pollutants. In this study, a mesoscale model, a Lagrangian particle dispersion model, and the extensive observations made during the Vertical Transport and MiXing (VTMX) field campaign are used to examine the interaction of thermally-driven valley, canyon, and slope flows and their effect on tracer transport and turbulent mixing. The mesoscale model predictions have been compared with data from surface stations, sonic anemometers, radiosondes, radar wind profilers, and a Doppler lidar. The predicted mean winds and turbulence quantities from the mesoscale and dispersion models are compared with data from a series of perfluorocarbon tracer (PFT) release experiments that provide quantitative information concerning transport and turbulent mixing patterns during the evening and morning transition periods. The models are also used to describe how mean vertical motions and turbulence associated with converging flows affect the horizontal and vertical distribution of tracers in the Salt Lake Valley. Some of the VTMX measurements are described next, followed by a discussion of the model results.