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

Abstract

ABSTRACT. M easurements from the sub arctic snowpack are used to explore the relation ship hctween grain growth and vapor flow, th e fundamental processes of dry-snow m eta m or phi sm. Du e to extreme temperature grad ients, the suba rctic pack undergoes extensive deplh-hoa r m etamorphi sm. By the end of the winter a fi ve-l ayered stru ctu re with a pronounced weak laye r near th e base of the snow evolves. Grain-size increases by a factor of 2-3, wh i le t he number of g ra i ns per unil mass d ecreases by a fac tor of 10. Obse rved growth rates require significant net inter-particle vapor fluxes. Stable-isoto pe ratios show that there are a lso significa nt net layer-to -I aye r vapor flu xes. Soil moi sture enters the base of th e p ack and mixes wit h the bottom 10 cm of snow, while iso topieally light water vapor fractionates from the basal layer a nd is d eposiled up to 50 cm high er in the p ack. End-of-winter density profi les for snow on the gro und, eomf,ared with snow on tabl es, indi cate th e ne t layer-to-laye r vapor flu x averages 6 x 10 kg m -2 S-I, th ough detail ed measurements show the net flu x is episod ic a nd varies with time and height in the pack, with peak net fl uxes ten tim es higher th a n average. A model, driven by observed temperatu re profiles, reproduces th e layer-lo-layer flu x pattern a nd predicts the observed weak layer at the base of the snow. Calcu lated layer-to-Iaye r vapor fluxes a re ten times higher than inter-particle flu xes, which implies that depth-hoar g rain g rowt h is limited by factors other than the vapor supply. This finding suggesls that gain and loss of water molecules due to sublimation from g rains ta kes place at a rate m a ny times hi gher than th e rate at which gr ains grow, a nd it expl ains why g rain s can meta morphose into different form s so r eadily.

Influence of Snow and Ice Crystal Formation and Accumulation on Mercury Deposition to the Arctic

“…Deeper layers were sampled from a 2 m long pit excavated to the base of the snowpack (usually ∼30 cm deep) with a polyethylene shovel. Layers were characterized by their thickness, lateral continuity and grain characteristics (27,28). Snow from individual layers was collected into FEP Teflon bottles.…”

“…Depth hoar forms through snow metamorphism when there is redistribution of water vapor in the snowpack driven by strong vertical temperature gradients (27). Depth hoar crystals grow as hexagonal cups at the base of the snowpack.…”

“…For primary crystals, the carapace is derived from atmospheric moisture. For metamorphosed grains in the snowpack, a significant volume of the grain can be derived from recycled vapor from other snow grains (27). Postdepositional metamorphosed snow grains may be missing the nucleus and mote, or if they are present, they will be minute in comparison to the whole crystal.…”

“…The numerator is the mercury balance and is largely controlled by the chemical properties and processes. The denominator gives the water balance, with NH 2O and SH 2O describing the same processes that drive snow grain growth and metamorphism (27,31). Each term in eq 1 is a function of time (t) and temperature (θ), with grain size and character ( ), light intensity and penetration (I), permeability (P), and relative humidity (h) also affecting rates and processes.…”

“…Most snow grains undergo multiple sublimation and condensation events, sometimes cycling on a diurnal basis. Particularly in a high-temperature gradient environment like Barrow, the flux of water molecules can be quite high (27,31). This may be why older snow has a wide range of mercury concentrations.…”