Otosaka, Inès N., Shepherd, Andrew, Casal, Tânia G. D., Coccia, Alex, Davidson, Malcolm, Di Bella, Alessandro, Fettweis, Xavier, Forsberg, René, Helm, Veit, Hogg, Anna E., Hvidegaard, Sine M., Lemos, Adriano, Macedo, Karlus, Kuipers Munneke, Peter, Parrinello, Tommaso, Simonsen, Sebastian B., Skourup, Henriette, and Sørensen, Louise Sandberg
Greenland Ice Sheet surface melting has increased since the 1990s, affecting the rheology and scattering properties of the near‐surface firn. We combine firn cores and modeled firn densities with 7 years of CryoVEx airborne Ku‐band (13.5 GHz) radar profiles to quantify the impact of melting on microwave radar penetration in West Central Greenland. Although annual layers are present in the Ku‐band radar profiles to depths up to 15 m below the ice sheet surface, fluctuations in summer melting strongly affect the degree of radar penetration. The extreme melting in 2012, for example, caused an abrupt 6.2 ± 2.4 m decrease in Ku‐band radar penetration. Nevertheless, retracking the radar echoes mitigates this effect, producing surface heights that agree to within 13.9 cm of coincident airborne laser measurements. We also examine 2 years of Ka‐band (34.5 GHz) airborne radar data and show that the degree of penetration is half that of coincident Ku‐band. Plain Language Summary: Radar waves emitted by satellites can be used to measure changes in surface elevation of the Greenland Ice Sheet. However, they do not reflect off the ice sheet surface itself but penetrate into the snow to a depth of about 15 m for radar wavelengths of 2.3 cm. When the snow melts, meltwater can percolate into the snow or refreeze at the surface. Layers of refrozen ice sharply reduce the degree of radar penetration and may be mistaken for an elevation increase in radar measurements. Here, we combine firn cores and modeled firn densities with 7 years of airborne radar data collected during field campaigns in West Central Greenland to quantify this effect. We identify internal layers corresponding to annual stratigraphy within the snowpack, and we show that more melt means less radar penetration into the firn. The unprecedented surface melting which occurred across Greenland in 2012 caused a sharp reduction in the degree of radar penetration, from 11.5 to 5.3 m. However, if the effects of penetration are corrected for, radar altimeters can accurately measure the surface elevation of the ice sheet. Key Points: We use airborne radar, firn cores, and firn models to confirm that summer melting is the principal source of radar penetration fluctuationsThe largest fluctuations are recorded after the extreme melt event of 2012, which caused a 6.2 ± 2.4 m reduction in radar penetrationEcho retracking compensates for radar penetration fluctuations, leading to surface heights that agree with laser data to within 13.9 cm [ABSTRACT FROM AUTHOR]