WCD-05. Ground-based remote sensing of the atmospheric boundary layer in the East River Valley in the Colorado Mountains during SPLASH

The East River Valley in the Colorado Mountains stretches from Crested Butte to the town of Gunnison, which is famous for cold air pools during wintertime that often lead to record low temperatures. Due to its importance for the watershed hydrology of the Colorado River Basin, the valley was chosen as the study area for the Surface-Atmosphere Integrated field Laboratory (SAIL) and Study of Precipitation, the Lower Atmosphere and Surface for Hydrometeorology (SPLASH) campaigns. Several passive remote sensing instruments, including infrared spectrometers (AERI and ASSIST) and microwave radiometers (MWR), were simultaneously deployed along the valley axis during a three-month period starting in October 2021. Using an optimal estimation physical retrieval (TROPoe), profiles of temperature and humidity were retrieved from these instruments with high temporal resolution. Fair-weather conditions during the first few weeks of the deployment were followed by several storms that brought substantial snow to the area. The instrumentation allowed for a study of the evolution of the atmospheric boundary layer in the valley over a period during which the surface conditions changed from snow free to snow covered. When the large-scale synoptic forcing was weak, thermally driven circulations regularly formed in tandem with a pronounced diurnal cycle in the atmospheric boundary layer. A strong surface inversion typically formed during nighttime in the valley, accompanied by weak winds near the surface. The spatial variability in vertical temperature structure along the valley axes and the factors which determine if the surface inversion was mixed out during daytime or not are some of the foci of this study.