Estimating Snow Sublimation in Complex Terrain: A Season of Intensive Field Measurements and the Role of Vertical Water Vapor Flux Divergence

Abstract Understanding the role of snow sublimation in the alpine water balance is critical to predicting future water resource availability. During winter 2022–23, 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 33–42 mm of snow water equivalent sublimated (8%–10% of seasonal peak snow accumulation). Midwinter sublimation was driven by blowing snow and springtime sublimation by positive net radiation. During blowing snow, EC water vapor fluxes increased with height between 3 and 10 m, on average by 26% and by up to 200% during individual events (positive vertical turbulent flux divergence). During nonblowing snow conditions, fluxes decreased with height between 3 and 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 m at our site, is likely to vary by location, and further work is needed to understand the role of advection. Significance Statement Mountain snowpacks act as water reservoirs for populations worldwide, and snow sublimation, which is rarely measured, removes water from those reservoirs. A recent measurement campaign offered the unique opportunity to compare sublimation measurements from 12 instruments, which reveal that sublimation estimates vary both across a winter season and with instrument height. Sublimation rates are higher during times with blowing snow compared to times without. During blowing snow, higher instruments (above 5 m) measure 26% more sublimation than lower instruments. Otherwise, higher instruments (above 10 m) measure 36% less sublimation. This work concludes that when measuring alpine sublimation, instrument height can introduce uncertainty, particularly if instruments are deployed for short periods of time. To best estimate total sublimation, instruments should be deployed over an entire winter season.