| Summary: | 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 similar to 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 similar to 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% 10km closer to the ice-sheet margin at 1860 m a.s.l. Density/depth profiles from nine sites within 1 km(2) at similar to 1945ma.s.l. reveal complex stratigraphies that change over short spatial scales and seasonally. We conclude that estimates of massbalance 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.
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