Estimating snow sublimation in complex terrain: a season of intensive field measurements and the role of vertical water vapor flux divergence

Understanding the role of snow sublimation in the alpine water balance is critical to predicting future water resource availability. During Winter 2022–2023, the Sublimation of Snow campaign in Colorado’s East River watershed used 12 eddy covariance (EC) instruments (2–20 m height) to measure sublimation and micrometeorology on the valley floor. ECs measured 33–42mm of snow water equivalent sublimated (8–10% of seasonal peak snow accumulation). Mid-winter sublimation was driven by blowing snow and springtime sublimation by positive net radiation. During blowing snow, EC water vapor fluxes increased between 3–10 m, on average by 26% and by up to 200% during individual events (positive vertical turbulent flux divergence). During non-blowing snow conditions, fluxes decreased between 3–20 m, on average by 36% (negative vertical turbulent flux divergence). Estimates of transport terms in a water vapor conservation equation suggest that positive divergence arose from blowing snow sublimation and negative divergence arose from vertical water vapor advection, although horizontal advection remains unquantified, limiting our conclusions. We found that keeping one instrument functional over the entire winter is more important than having instruments at multiple heights. Seasonal uncertainty in measured total sublimation due to instrument height is estimated at ±12% due to blowing snow sublimation and water vapor advection; however, for shorter deployments, this uncertainty may be larger. The optimal instrument height for estimating total sublimation, 10 meters at our site, is likely to vary by location and further work is needed to understand the role of advection.