Multi-season analysis of the boundary layer structure and evolution in a high-altitude valley in Colorado

Comprehensive atmospheric measurements were conducted in the 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. The main focus of these research initiatives is to enhance weather and water prediction capabilities by measuring, evaluating, and understanding integrated atmospheric and hydrologic processes relevant to water resources. The East River Valley is embedded in the East River Watershed which is a representative mountainous headwater catchment of the Colorado River Basin and is a primary source of water for much of the southwestern United States. The valley floor is located at more than 2500 m above mean sea level and the surrounding ridges extend above 4000 m.

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 5 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. The data set allows one to investigate the seasonal and diurnal cycle of the boundary layer and to investigate the impact of varying spatial snow coverage including the melt period. We show that the diurnal cycle of the boundary layer conditions on many days is very different from a typical thermally driven wind system, especially when snow coverage is low, and we discuss possible factors contributing to the boundary evolution.