Vapor transport, grain growth and depth-hoar development in the subarctic snow

Measurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow, the fundamental processes of dry-snow metamorphism. Due to extreme temperature gradients, the subarctic pack undergoes extensive depth-hoar metamorphism. By the end of the winter a five-layer structure with a pronounced weak layer near the base of the snow evolves. Grain-size increases by a factor of 2-3, while the number of grains per unit mass decreases by a factor of 10. Observed growth rates require significant net inter-particle vapor fluxes. Stable-isotope ratios show that there are also significant net layer-to-layer vapor fluxes. Soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow, while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack. End-of-winter density profiles for snow on the ground, compared with snow on tables, indicate the net layer-to-layer vapor flux averages 6 x 10(-7) kg m(-2) s(-1), though detailed measurements show the net flux is episodic and varies with time and height in the pack, with peak net fluxes ten times higher than average. A model, driven by observed temperature profiles, reproduces the layer-to-layer flux pattern and predicts the observed weak layer at the base of the snow. Calculated layer-to-layer vapor fluxes are ten times higher than inter-particle fluxes, which implies that depth-hoar grain growth is limited by factors other than the vapor supply. This finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow, and it explains why grains can metamorphose into different forms so readily.