Characteristics of valley exit jets in the Rocky Mountains of the United States

Locations where major valleys or canyons open onto plains are important as sites where cities have become established. The nighttime meteorology of these locations is dominated by thermally forced outflows of cold air, or exit jets, when the synoptic flow is less than 5 m/s at ridgetop. These outflows have a significant affect on air quality, dispersion, winds, minimum temperatures, and other important forecast variables. The thermal structure of these outflows is stable, because of strong surface cooling. Consequently, Doppler lidar studies of their flow structure, obtained at two locations in the Rocky Mountains, indicate a strongly layered structure, and a time-dependent evolution of the outflow properties. Lidar scan data also indicate that this layered structure often originated well back in the mountainous terrain, especially where the valley geometry is complex. Published mesoscale numerical modeling results of these outflows indicates that the models were able to capture the gross features of the outflow jets, but the details of the timing and structure in the horizontal and vertical, which would be important for applications, were not well represented.