Sublimation of Snow

Snow is a vital part of water resources, and sublimation may remove 10%-90% of snowfall from the system. To improve our understanding of the physics that govern sublimation rates, as well as how those rates might change with the climate, we deployed an array of four towers with over 100 instruments from NCAR's Integrated Surface Flux System from November 2022 to June 2023 in the East River watershed, Colorado, in conjunction with the U.S. Department of Energy's Surface Atmosphere Integrated Field Laboratory (SAIL) and the National Oceanic and Atmospheric Administration (NOAA)'s Study of Precipitation, the Lower Atmosphere and Surface for Hydrometeorology (SPLASH) campaigns. Mass balance observations, snow pits, particle flux sensors, and terrestrial lidar scans of the evolving snowfield demonstrated how blowing snow influences sublimation rates, which we quantified with latent heat fluxes measured by eddy-covariance systems at heights 1-20 m above the snow surface. Detailed temperature profiles at finer resolutions highlighted the role of the stable boundary layer. Four-stream radiometers indicated the important role of changing albedo in the energy balance and its relationship to water vapor losses. Collectively, these observations span scales from seconds to seasons, from boundary layer turbulence to valley circulation to mesoscale meteorology. We describe the field campaign, highlights in the observations, and outreach and education products we are creating to facilitate cross-disciplinary dialogue and convey relevant findings to those seeking to better understand Colorado River snow and streamflow. SIGNIFICANCE STATEMENT: Snow provides over 80% of water for the overallocated Colorado River, and in recent years, less runoff has occurred per unit snowfall. Sublimation, the conversion of ice to water vapor, results in less water for runoff, but due to a historic lack of observations, this process is hard to constrain. Variations in how sublimation is represented in models have led to a large divergence of projected water resource availability for the Colorado River basin over both current and future climates. The field campaign described here provides the first comprehensive examination of how snow accumulates, blows around, evolves, and sublimates over 8 months in the Colorado Rocky Mountains, providing a critical benchmark for process understanding and model development.