Influence of canopy shading and snow coverage on effective albedo in a snow-dominated evergreen needleleaf forest

The presence of a forest canopy above highly reflective snow results in overall lower surface albedo, even when snow is intercepted by the forest canopy. The effective forest snow albedo (FSA) explains the overall upwelling radiation from the forest relative to the incoming radiation. FSA is strongly influenced by the complex pathways of radiation as it travels through the 3D canopy structure. Current errors in calculations of FSA arise due to uncertainties in how models should treat masking of snow by vegetation. Improvement of distributed models is currently limited by a lack of measurements that demonstrate both spatial and temporal variability over forests. We present above-canopy measurements of winter-time effective forest snow albedo using up- and down-looking radiometers mounted on an octocopter UAV for a total of fifteen flights on eight different days. Ground-view fractions across the flight path were between 0.12 and 0.81. Correlations between FSA and both ground-view fraction and maximum canopy height were statistically significant during 14 out of 15 flights, but correlation strength varied between flights as a function of solar angle and snow cover. Measured effective albedo across the flight path differed by up to 0.33 during snow-on canopy conditions. A subsequent comparison between maximum interception and no interception showed effective albedo differed by up 0.27. A similar variation (0.26) in effective FSA was measured during low (44°) and high (67°) solar zenith angles. This study therefore demonstrates that temporal and spatial variations in effective albedo caused by canopy shading of the snow surface are therefore as important as temporal variations caused by interception of snow by the canopy. Calculation of effective albedo of forested areas requires careful consideration of canopy height, canopy coverage, solar angle and interception coverage. The results of this study should be used to inform snow albedo and canopy structure parametrizations in local and larger scale land surface models.