The role of thermal convection in heat and mass transport in the subarctic snow cover

Abstract

The purpose of this study was to investigate the role of air convection in moving heat and water vapor in snow. To detect convection, the three dimensional temperature field in the Fairbanks snow cover was measured hourly during three winters (1984-1987). Measurements of snow density, compaction, and grain size were made monthly to determine the 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 random. As the depth hoar developed, its air permeability increased to a value 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 top of the snow due to mass flux gradients that averaged 3 $\times$ 10$\sp{-6}$kg m$\sp{-2}$s$\sp{-1}$m$\sp{-1}$, but were occasionally 10 times higher. Convection occurred sporadically in 1984-85 and continuously in 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 16 K m$\sp{-1}$. 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. Air flow was inferred to have been horizontal near the base of the snow and vertical elsewhere. Flow averaged 0.2 mm s$\sp{-1}$, with a maximum of 2 mm s$\sp{-1}$. During average flow conditions, convection moved about a third of the total heat, but did not move significant mass. However, the coincidence of crystals with horizontal c-axes and the horizontal flow lines at the base of the snow suggests that convection may have affected crystal growth direction.