Characteristics and small-scale variability of GPR signals and their relation to snow accumulation in Greenland's percolation zone

We investigate snowpack properties at a site in west-central Greenland with ground-penetrating radar (GPR), supplemented by stratigraphic records from snow pits and shallow firn cores. GPR data were collected at a validation test site for CryoSat (T05 on the Expeditions Glaciologiques Internationales au Groenland (EGIG) line) over a 100 m x 100 m grid and along I km sections at frequencies of 500 and 800 MHz. Several internal reflection horizons (IRHs) down to a depth of 10m were tracked. IRHs are usually related to ice-layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season. Warm conditions in the following melt season can change these characteristics by percolating meltwater. In cold conditions, smaller melt volumes at the surface can lead to faint IRHs. The absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data. Our GPR-derived depth of the 2003 summer surface of 1.48 m (measured in 2004) is confirmed by snow-pit observations. The distribution of IRH depths on a I km scale reveals a gradient of increasing accumulation to the northeast of about 5 cm w.e. km(-1). We find that point measurements of accumulation in this area are representative only over several hundred metres, with uncertainties of about 15% of the spatial mean.