Sublimation rate and the mass-transfer coefficient for snow sublimation

Sublimation of snow is a fundamental process that affects the crystal structure of snow, and is important for ice core interpretation, remote sensing, snow hydrology and chemical processes in snow. Prior investigations have inferred the sublimation rate from energy, isotopic, or mass-balance calculations using field data. Consequently, these studies were unable to control many of the environmental parameters which determine sublimation rate (e.g. temperature, relative humidity, snow microstructure). We present sublimation rate measurements on snow samples in the laboratory, where we have controlled many of these parameters simultaneously. Results show that the air stream exiting the snow sample is typically saturated under a wide range of sample temperature and air-flow rate, within measurement precision. This result supports theoretical work on single ice grains which found that there is no energy barrier to be overcome during sublimation, and suggests that snow sublimation is limited by vapor diffusion into pore spaces, rather than sublimation at crystal faces. Undersaturation may be possible in large pore spaces (i.e. surface- or depth-hoar layers) with relatively high air-flow rates. We use these data to place bounds on the mass-transfer coefficient for snow as a linear function of Reynolds number, and find that hm = 0.566 Re + 0.075.