The modulation of canyon flows by larger-scale influences

During several nights of the Vertical Transport and Mixing (VTMX) project, the synoptic meteorology supported the formation of thermally-driven flows in the Salt Lake City basin (Doran et al. 2002). One of the thermally-driven flows occurred on a scale larger than the basin, as evidenced by daytime northerly flow that reversed to southerly flow after sunset. The southerly flow took the form of a low-level jet (LLJ) that dominated the nighttime basin flow (Banta et al. 2004). Smaller-scale flow features also formed, including nighttime canyon outflows. The National Oceanic and Atmospheric Administration/Earth System Research Laboratory's Doppler lidar measured these canyon outflows as they extended into the Salt Lake City basin. These flows were often elevated, escaping detection by surface wind sensors. The speed, height above ground, timing, and extent of the basin penetration of these canyon outflows varied nightly. Through the analysis of Doppler lidar range-height scans pointed toward canyon openings (Emigration, Parleys, and Big Cottonwood canyons) and the Jordan Narrows gap to the south of the basin, we have created time-height plots of the component of flow aligned with each canyon and the gap for five of the VTMX intensive operating periods (IOPs). By comparing these time-height series with Doppler lidar vertical profiles of the horizontal wind, we assessed how the canyon outflows were modulated by the larger-scale flows (e.g., the winds at and above the ridge-tops of nearby mountain ranges and the LLJ). Canyon outflows for each IOP had distinct characteristics, but there were some similarities. For instance, during 4 of the 5 IOPs the onset of the southerly LLJ shut down the canyon outflows at low levels, causing the wind flow aligned with the canyons to reverse from an outflow to an inflow direction, even though the canyons have an alignment that is somewhat perpendicular to the axis of the LLJ. With time, the canyon outflows' penetration into the basin either continued to be suppressed by the LLJ, or became reestablished in a weaker, shallower form. Other similarities and differences among the IOPs will be presented.