The Short Life of Upvalley Wind during the Warm Season in Colorado’s East River Valley

Comprehensive atmospheric measurements were conducted in the high-altitude East River Valley in Colorado for a nearly 2-year period from 2021 through 2023 in the framework of the NOAA Study of Precipitation, the Lower Atmosphere, and Surface for Hydrometeorology (SPLASH) and the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program Surface Atmosphere Integrated Field Laboratory (SAIL) campaigns. In this study, we used temperature, humidity, and wind profiles from ground-based remote sensing instruments and radiosondes in the upper part of the valley, as well as near-surface meteorological observations from sites distributed along the valley axis. Temperature and humidity profiles with high temporal resolution were retrieved from infrared spectrometer radiances with the optimal estimation physical retrieval TROPoe.

We found that the diurnal cycle of the flow in the boundary layer was very different from a typical thermally driven wind system on many days, especially during the warm season. On the majority of the days, thermally driven upvalley flow already turned back to downvalley direction in mid to late morning when the convective boundary layer grew above ridge height. At the same time, humidity rapidly decreased and turbulence increased within the boundary layer. Similar changes with respect to humidity and turbulence occurred even on days when upvalley wind persisted. A distinct difference between days with short upvalley wind, i.e. when the flow turned back to downvalley in the morning, and long upvalley wind days, i.e. when upvalley flow persisted throughout the day, was upper-level wind direction at and above ridge height. Short upvalley wind days were characterized by a southerly upper-level wind component, while long upvalley wind days had a westerly to northerly component. We hypothesize that once the convective boundary layer reached above ridge height, upper-level momentum was transported down into the valley and channeled along the valley axis either in upvalley wind direction (long upvalley wind days) or downvalley direction (short upvalley wind days) depending on upper-level wind direction. This process has been described as forced channeling in previous studies. This explanation is supported by the finding that upvalley wind could persist throughout the day despite an upper-level southerly wind component on days when the convective boundary layer did not grow up to ridge height. This was mostly the case when snow cover in the valley was high during the other seasons.