The VTMX (Vertical Transport and Mixing) field experiment took place in October 2000. It was designed to observe the processes that lead to vertical transport and mixing under stable conditions in complex terrain. An increased understanding of these processes is crucial for improving weather and pollution forecasts in urban basins.

As part of VTMX, observations were made by NCAR at a site located at the southern end of the Great Salt Lake Basin to characterize the flow between the Great Salt Lake Basin and the Utah Lake Basin. Instrumentation at this site included a multi-level tether sonde, a mini-sodar, wind profiler, an aerosol backscatter lidar, surface met stations, and soundings. Channeled flow between basins is known as gap flow. Gap flow was observed at the NCAR site on several days that were characterized by weak synoptic forcing and strong nighttime radiative cooling (i.e., clear skies). The direction of the gap flow varied diurnally. Daytime winds at the NCAR site tended to be northerly due to a combination of up-valley flow and the passage of the lake breeze front in mid afternoon. At night, down-valley flow channeled through the gap produced periods of gusty wind from the south. The timing, duration, strength, and depth of the nighttime gap flow events observed during VTMX are discussed. Vertical mixing atop the nocturnal boundary layer appears to have been enhanced by Kelvin-Helmholtz waves triggered by the gap flow. A mesoscale model is employed to better understand the interaction between processes (radiative transfer, boundary layer physics, mesoscale dynamics, topographic flow) that drive the gap flow and determine its characteristics.