Depth hoar

Abstract: The growth of ice particles in dry seasonal snow is caused by vapor diffusion among particles due to temperature gradients imposed on the snow cover. The diffusion is calculated by using the potential field solutions for electrostatically charged particles. The stereography of snow is represented by using a log-normal distribution function for a geometrical enhancement factor defined here. Reasonable crystal growth rates and supersaturations are found. The transition between the growth of highly faceted crystals and the growth of highly rounded crystals is determined by the critical supersaturation for the onset of dislocation aided growth. Thermal convection and the continuous movement of vapor around the particles due to the imposed temperature gradient are accommodated in the theory, although both have little effect relative to the interparticle diffusion. The growth of a layer of faceted crystals just below semi-permeable crusts is explained by showing that crusts can cause the local supersaturation to exceed the critical value. Faceted crystals are shown to grow most rapidly in the lower, warmer portions of the snow cover because of temperature effects on growth rate.

Abstract: [1] We carried out numerical simulations of the conductivity of snow using microtomographic images. The full tensor of the effective thermal conductivity (keff) was computed from 30 three-dimensional images of the snow microstructure, spanning all types of seasonal snow. Only conduction through ice and interstitial air were considered. The obtained values are strongly correlated to snow density. The main cause for the slight scatter around the regression curve to snow density is the anisotropy of keff: the vertical component of keff of facetted crystals and depth hoar samples is up to 1.5 times larger than the horizontal component, while rounded grains sampled deeply in the snowpack exhibit the inverse behavior. Results of simulations neglecting the conduction in the interstitial air indicate that this phase plays a vital role in heat conduction through snow. The computed effective thermal conductivity is found to increase with decreasing temperature, mostly following the temperature dependency of the thermal conductivity of ice. The results are compared to experimental data obtained either with the needle-probe technique or using combined measurements of the vertical heat flux and the corresponding temperature gradient. Needle-probe measurements are systematically significantly lower than those from the two other techniques. The observed discrepancies between the three methods are investigated and briefly discussed.

Abstract: 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-layered 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.