Bianca Adler

and 11 more

Thermally-driven upvalley wind in the upper East River Valley in the Colorado Rocky Mountains often unexpectedly stops in mid-morning and reverses back to downvalley wind. We use a comprehensive observational data set for a nearly two-year long period to analyze the wind system and boundary layer evolution in this high-altitude valley and determine the reason for this early wind reversal. Days with short upvalley wind predominantly occur during the warm season when the valley floor is free of snow and the convective boundary layer grows well above the height of the surrounding ridges. Upvalley wind persists throughout the day only on a few days during the warm season. We link differences in valley wind evolution to wind direction at upper levels at and above ridge height and propose forced channeling mechanisms to describe coupling between valley and upper-level wind when the convective boundary layer grows above ridge height. The frequency distribution of upper-level wind direction is such that channeling in the downvalley direction is favored, which explains the predominance of days with short upvalley wind. The deep convective boundary layer is supported by the presence of a deep weakly stably stratified residual layer with high aerosol content, which is regularly present over the mountain range during the warm season. On days when the convective boundary layer does not grow above ridge height, for example when the valley floor is covered by snow, thermally-driven upvalley wind is able to persist throughout the day independent of upper-level wind direction.

Bianca Adler

and 16 more

The structure and evolution of the atmospheric boundary layer (ABL) under clear-sky fair weather conditions over mountainous terrain is dominated by the diurnal cycle of the surface energy balance and thus strongly depends on surface snow cover. We use data from three passive ground-based infrared spectrometers deployed in the East River Valley in Colorado’s Rocky Mountains to investigate the response of the thermal ABL structure to changes in surface energy balance during the seasonal transition from snow-free to snow-covered ground. Temperature profiles were retrieved from the infrared radiances using the optimal estimation physical retrieval TROPoe. A nocturnal surface inversion formed in the valley during clear-sky days, which was subsequently mixed out during daytime with the development of a convective boundary layer during snow-free periods. When the ground was snow covered, a very shallow convective boundary layer formed, above which the inversion persisted through the daytime hours. We compare these observations to NOAA’s operational High-Resolution-Rapid-Refresh (HRRR) model and find large warm biases on clear-sky days resulting from the model’s inability to form strong nocturnal inversions and to maintain the stable stratification in the valley during daytime when there was snow on the ground. A possible explanation for these model shortcomings is the influence of the model’s relatively coarse horizontal grid spacing (3 km) and its impact on the model’s ability to represent well-developed thermally driven flows, specifically nighttime drainage flows.