Forest Snow Albedo
Snow albedo regulates the surface energy budget of snow-covered regions which governs snowmelt processes, this makes snow albedo a critical parameter for climate models.Spaceborneand airborne remote sensing measurements provide high spatial and temporal resolution snow albedo data buthavelimitedaccuracyin forested regions due to the canopy shading effect. Vegetation induces spatial and temporal variability in snow albedo by depositing forest litter on the snow surface, reducing albedo in affected areas.Thus, accurate measurements of snow albedo and forest litter are needed to determine snow albedo characteristics and variability. Here, Ipresent a unique set of field-basedresults detailing snow albedo and the snowpack energy budgetin a temperate and coniferous forest. Data were collected during the 2019 snow season at theSagehen Creek Field Station in the California Sierra Nevada. Three meteorological stations continuously measuredthesnowpack energy budgetand snow depth across a forest density gradient. A forest density gradient spanned the forest, forest edges, and open meadow at the study site. Using a Spectral Evolution RS-3500 Portable Spectroradiometer, Imeasured snow spectral albedo during the accumulation and ablation periods. Spectral data and snow samples were collected over the period 14 February –12 May 2019 and the net radiometer data were collected over the period 22 March –15 May 2019. These measurements were used to characterize spectral and broadband snow albedo during the ablation season across the forest density gradient. Surfaceand subsurfacesnow samples were collected at each site and filtered to determine the mass fraction of forest litterandquantifyblack carbon content.Results highlightthe spatial and temporal differences of net shortwave radiation and snow albedo across a forest density gradient in the California Sierra Nevada. Differences in spectrallyintegrated and broadband snow albedo are driven by forest litter concentration, which is variable due to forest density. Forest litter concentration is a function of forest density and increases at the snowpack surfacethrough the accumulation to theend of theablation period.Snow albedo is 22% less in forest than in open sites because of greater forest litter concentrations.Strong and significant relationships exist between both percent canopy cover and distance to forest tree stand evaluated with spectral snow albedo measurements.In the forest, net shortwave radiation is determined by canopy shading and snow albedo. Inopen areas of forest net shortwave radiation is completely dependent on snow albedobecause canopy shading is not present. Net shortwave radiation is the largest term inthe snowpack energy budget across the forest density gradient. In the forest,net shortwave radiation is a reduced component of the total snow energy budget when compared to open forest.As forest density decreases, albedo and net shortwave radiation are more important for the snowpack energy budget. This implies that, for estimating energy budget in snowy forests, sub-canopy snow albedo does not need to be known as accurately as snow albedo in open areas.
Alex Greenwald and Keith Jennings measuring spectral albedo of snow at Sagehen Creek Field Station
Spectral and broadband albedo measurements from winter 2019