Seasonal changes in snow surface roughness characteristics at Summit, Greenland: implications for snow and firn ventilation

Air-snow transfer processes impact both ice-core interpretation and exchange affecting atmospheric chemistry. An understanding of seasonal differences in the character of the surface snow will facilitate evaluation of possible preferential seasonal exchange of reactive chemical species. Both diffusive processes and advective (ventilation) processes can serve to alter the physical, chemical and isotopic character of snow and firn. In this paper, we examine measurements of surface roughness over the course of a year at Summit, Greenland, and the implications for snow and firn ventilation. At Summit, during the winter-over experiment, summer and fall sastrugi amplitudes were approximately 5 cm and had smoothly curved profiles. The average amplitudes experienced mild increases in january, but by the end of February through March the amplitude increased to approximately 20 cm, and the profiles exhibited more abrupt geometries. Calculations are performed to show the potential impact of the changing roughness on interstitial ventilation rates in the snow, assuming that the permeability profile does not change in 1 time. Under high winds, ventilation velocities in the near-surface snow can be up to 3 cm s(-1) in the winter, compared to 1 cm s(-1) in the summer. The frequency of 12 m s(-1) winds in the summer, however, is less than in the winter. Under low-wind conditions, the summer roughness causes ventilation rates that are comparable to diffusion rates. However, in winter even 5 cm s(-1) wind conditions can cause the interstitial airflow due to ventilation to exceed the diffusion rates.