The mass balance of polythermal ice masses is critically dependent on the proportion of surface-generated meltwater that subsequently refreezes in the snowpack and firn. In order to quantify this effect and to characterize its spatial variability, we measured near-surface (<10 m) snow and firn densities at an elevation of ∼1945 m a. s. l. in the percolation zone of the Greenland ice sheet in spring and autumn 2004. Results indicate that local snowpack depth above the previous end-of-summer 2003 melt surface increased by ∼5% (7.6 cm) from spring to autumn while, over the same period, snowpack density increased by >26%, resulting in a 32% increase in net accumulation. This 'seasonal densification' increased at lower elevations, rising to 47% 10 km closer to the ice-sheet margin at 1860 m a. s. l. Density/depth profiles from nine sites within 1 km2 at ∼1945 m a.s.l. reveal complex stratigraphies that change over short spatial scales and seasonally. We conclude that estimates of mass-balance change cannot be calculated solely from observed changes in surface elevation, but that near-surface densification must also be considered. However, predicting spatial and temporal variations in densification may not be straightforward. Further, the development of complex firn-density profiles both masks discernible annual layers in the near-surface firn and ice stratigraphy and is likely to introduce error into radar-derived estimates of surface elevation
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.