Densification and refreezing in the percolation zone of the Greenland Ice Sheet: implications for mass balance measurements

• Main Author: Parry, Victoria
• Other Authors: Nienow, Pete, Mair, Doug, Wadham, Jemma, Hubbard, Bryn
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2009
• Subjects: Geography; Glaciology; Greenland; Ice Sheet
• Online Access: http://hdl.handle.net/1842/3076

In order to increase coverage, mass balance changes of the world’s ice sheets are increasingly derived from surface elevation changes measured via satellite. Across the percolation zone of the Greenland Ice Sheet, meltwater, percolation and refreezing cause a re-distribution of mass through densification which may result in elevation change with no associated mass loss. Therefore, densification processes need to be quantified, spatially and temporally, and accounted for in mass balance measurements. This thesis investigates the relationships between patterns of elevation change and temporally and spatially variable accumulation and densification processes. In doing so, it provides an important contribution to the validation of the European Space Agency’s CryoSat-2 mission by placing error bars on the accuracy to which changes in satellite-measured ice-mass surface elevation represent real changes in ice mass. Temporal variability in near-surface (<10 m) snowpack and firn density and structure was measured in snowpits, shallow cores and using a neutron probe in the spring and autumn of 2004 at ~1945 m elevation (T05, 69o 51N, 47o 15W) in the percolation zone of the Greenland Ice Sheet. Results show that average snowpack density increased by 26% from spring to autumn, with a 5% (7.6 cm) increase in elevation, and a corresponding 32% increase in mass. Spatial variability was investigated at 11 sites along two transects at spatial scales of 1 m – 10 km. Whilst there was little variability in small scale (1 - 100 m) density changes, ‘seasonal densification’ increased at lower elevations, rising to 47% 10 km closer to the ice sheet margin at 1860 m a.s.l. The spatial variability in seasonal densification was further investigated in spring 2006 at seven sites located at ~10 km intervals along a 57 km transect spanning a 350 m elevation range. Snowpits and shallow cores reveal no significant variation in spring (prior to melt) snowpack density but following summer melt and refreezing cycles, seasonal densification ...