NATURAL-CONVECTION IN THE SUB-ARCTIC SNOW COVER

The purpose of this study was to determine if air convects in a natural snow cover. To detect convection, the temperature field in the subarctic snow cover in Fairbanks, Alaska, was measured hourly during three winters (1984-1987) using an array of thermistors which were suspended on threads and allowed to be buried by snowfall. The results indicate that convection occurred sporadically in 1984-1985 and almost continuously in 1985-1986 and 1986-1987. 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-degrees-C m-1. During the winter, warm and cold zones developed in the snow and remained relatively fixed in space. We interpret these zones to be the result of a diffuse plumelike convection pattern linked to spatial variations in the temperature of the snow-soil interface. Air flow was inferred to have been primarily horizontal near the base of the snow and vertical elsewhere. Calculated flow speeds were of the order of 0.2 mm s-1, with a maximum value of 2 mm s-1. The convective circulation was time-dependent, with perturbations such as high wind or rapid changes in air temperature triggering periods when horizontal temperature gradients were strongest, suggesting that these were also periods when the air flow was fastest. The coincidence of depth hoar crystals with horizontal c axes and the horizontal flow lines at the base of the snow suggests that convection may have affected crystal growth directions.