Canyon Drainage Induced Mixing over a Large Basin

The Salt Lake City basin has been identified by the Department of Energy’s Vertical Transport and Mixing Program (VTMX) as the target region for an observational and modeling experiment to investigate mixing processes within stable boundary layers. One of the key topographic features surrounding the valley are the deeply incised canyons along the Wasatch Front and the presence of a river valley linking this basin to the Great Salt Lake to the north and the Utah Valley to the south. Canyon drainage winds are often observed to flow into the basin in early evening and they can act as a disturbance mechanism to the evolving nocturnal stable layer.

In this paper, we will present modeling studies, using the Regional Atmospheric Modeling System, that are designed to look at the evolution of the stable boundary layer over the basin in the presence of canyon drainage winds. We have initially chosen to investigate this regime using idealized basin, canyon, and valley topography to isolate the interaction of cold drainage currents from the canyons and the evolving stable layer over the basin. We will also include increased roughness and prescribed temperature difference within the basin to simulate the effects of urban areas on mixing processes.

The preliminary results indicate that the depth and strength of the canyon drainage has an important influence on the stable layer development. Following classical gravity current theory, the drainage current can generate bore waves that propagate ahead of the drainage current and create significant mixing along the top of the evolving radiatively-induced nocturnal stable layer. We further seek to identify the significance of these phenomena as a catalyst for vertical transport of material within the basin, and also to identify how we might observe this process during the upcoming VTMX experiment within the Salt Lake basin.