Measuring spatiotemporal variation in snow optical grain size under a subalpine forest canopy using contact spectroscopy

• Published in: Water resources research 2016-09, Vol.52 (9), p.7513-7522
• Main Authors: Molotch, Noah P., Barnard, David M., Burns, Sean P., Painter, Thomas H.
• Format: Article
• Language: English
• Subjects: Accumulation; Analytical methods; Biogeosciences; Canopies; Canopy; Canopy gaps; Cryosphere; Crystals; Density; Depth; Distribution; Evolution; Excavation; Forest canopy; Forests; geometry; Glaciology; Grain size; Grain size distribution; Grains; Growth; Hydrology; Hydrometeorology; Ice; Instruments and Techniques: Monitoring; Measurement; Metamorphism; Metamorphism (geology); Meteorological conditions; Particle size; Profiles; Resolution; Snow; Snow accumulation; Snow and Ice; Snow depth; snow grain size; snow temperature; Snowmelt; Snowpack; snow‐forest interactions; Spatial discrimination; Spatial distribution; Spatial resolution; Spectroscopic analysis; Spectroscopy; Spectrum analysis; Spring; Spring (season); Subalpine environments; Technical Reports: Methods; Temperature effects; Temperature gradients; Temporal variations; Trees; Trenches; Uncertainty; Vertical profiles; Walls; water; Water/Energy Interactions; Winter
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1002/2016WR018954

The distribution of forest cover exerts strong controls on the spatiotemporal distribution of snow accumulation and snowmelt. The physical processes that govern these controls are poorly understood given a lack of detailed measurements of snow states. In this study, we address one of many measurement gaps by using contact spectroscopy to measure snow optical grain size at high spatial resolution in trenches dug between tree boles in a subalpine forest. Trenches were collocated with continuous measurements of snow depth and vertical profiles of snow temperature and supplemented with manual measurements of snow temperature, geometric grain size, grain type, and density from trench walls. There was a distinct difference in snow optical grain size between winter and spring periods. In winter and early spring, when facetted snow crystal types were dominant, snow optical grain size was 6% larger in canopy gaps versus under canopy positions; a difference that was smaller than the measurement uncertainty. By midspring, the magnitude of snow optical grain size differences increased dramatically and patterns of snow optical grain size became highly directional with 34% larger snow grains in areas south versus north of trees. In winter, snow temperature gradients were up to 5–15°C m−1 greater under the canopy due to shallower snow accumulation. However, in canopy gaps, snow depths were greater in fall and early winter and therefore more significant kinetic growth metamorphism occurred relative to under canopy positions, resulting in larger snow grains in canopy gaps. Our findings illustrate the novelty of our method of measuring snow optical grain size, allowing for future studies to advance the understanding of how forest and meteorological conditions interact to impact snowpack evolution. Key Points: Contact spectroscopy excels at characterizing spatiotemporal variation in snowpack microstructure Proximity to tree canopy influences snowpack temperature gradient and snow grain growth Rates of snow metamorphism are significantly greater on south versus north sides of trees