Evaluating Multiple Canopy-Snow Unloading Parameterizations with Time-lapse Photography Characterized by Citizen Scientists

Abstract:

Snow in the canopy can sublimate back into the atmosphere or unload to the surface. Snow unloading results in a drastic, and sometimes sudden, decrease in the land surface albedo. Snow unloading is a complex physical process that is difficult to parameterize due to limited observations. Time-lapse photos of snow in the canopy were characterized by citizen scientists to create a dataset of snow interception observations at multiple locations across the western United States. This novel interception dataset was used to evaluate three snow unloading parameterizations in the Structure for Unifying Multiple Modeling Alternatives (SUMMA) modular modeling framework.

SUMMA was modified to include Roesch et al. (2001), a temperature and wind dependent snow unloading parameterization. This parameterization is compared to a meltwater drip unloading parameterization (Andreadis et al., 2009) and a exponential unloading parameterization (Hedstrom & Pomeroy, 1998). The parameterizations were calibrated at Niwot Ridge, CO where the Roesch et al. (2001) parameterization performed best with 87% of the observed snow interception events were captured. When the parameterizations were transferred to Grand Mesa, CO and a maritime site in the Olympic Mountains, WA, they unloaded snow from the canopy too rapidly. This suggested that parameterizations would need to be recalibrated for the unique interception physics of those domains. As a result, Andreadis et al. (2009), which retained snow in the canopy longest, captured the duration of snow in the canopy best when transferred to these additional sites. At Niwot Ridge, cumulative sublimation from the canopy was 87.6 mm using Andreadis et al. (2009) parameterization, which is 6% of the total winter precipitation, compared to Roesch et al. (2001), which sublimated 1% of the total precipitation. More work needs to be done to understand the transferability of model parameterizations and the physics of unique snow interception and unloading events. While canopy-snow unloading parameterizations are often overlooked in land surface models, results show that the unloading scheme can have an effect on the duration of snow in the canopy, impacting the land-surface albedo, and whether canopy-snow contributes to streamflow or is sublimated back to the atmosphere.