Role of Thermal Convection in Heat and Mass Transport in the Subarctic Snow Cover

摘要

This study investigated the role of air convection in moving heat and water vaporin snow. To detect convection, the three-dimensional temperature field in the Fairbanks snow cover was measured hourly during 3 winters (1984-1987). Measurements of snow density, compaction, and grain size were made monthly to determine water vapor flux and textural changes. The snow metamorphosed into depth hoar, producing a sequence of five layers, including a basal layer with horizontal c-axes. C-axes in the overlying layers were vertical or randomly oriented. As the depth hoar developed, its air permeability increased to several times higher than previously measured for any snow, while the number of snow grains per unit volume decreased by an order of magnitude as a few select grains grew while others sublimated away. Simultaneously, there was a net transfer of mass from the base to the top of the snow due to mass flux gradients. Convection occurred sporadically in the winter of 1984-85 and continuously in the winters of 1985-86 and 1986-87. The evidence was (1) simultaneous warming and cooling at different locations in a horizontal plane in the snow; and (2) horizontal temperature gradients of up to 16C/m. The convection was time-dependent, with perturbations such as high wind or rapid changes in air temperature triggering periods when horizontal temperature gradients were strongest, suggesting these were also periods when the air flow was fastest. During the winter, warm and cold zones developed in the snow and remained relatively fixed in space. The zones were probably the result of a diffuse plume-like convection pattern linked to spatial variations in the temperature of the snow/soil interface.