Effects of Greenland ice sheet surface melt on radar backscatter and elevation change from CryoSat-2

In July 2012, an unprecedented 99 % of the surface area of the Greenland ice sheet experienced melting as a result of anomalously high near surface temperatures, with approximately one third of the ice sheet undergoing melt for the first time in a decade. This event created a frozen melt layer near the ice sheet surface, resetting the radar scattering horizon and complicating elevation retrievals from radar altimeters. Surface melt on the Greenland ice sheet and poorly understood fluctuations in snowpack scattering characteristics introduce considerable uncertainty into estimates of ice mass balance from radar altimeters. Using a backscatter model which retrieves the depth distribution of radar backscatter from Low Resolution Mode CryoSat-2 waveforms we are able to, for the first time, map changes in radar penetration depth across the interior of the Greenland ice sheet between 2011 and 2018. As a result of the extreme melt event in the summer of 2012, we estimate a step-like reduction in penetration depth of 2.2 m on average at surface elevations greater than 2000 m and - due to the subsequent accumulation of snowfall on the ice sheet surface - observe a continuous increase in radar penetration depth until the end of 2017, to within approximately 50 cm of that recorded before July 2012. Such improved knowledge of snowpack scattering properties have the potential to inform more accurate estimates of ice mass balance from satellite radar altimetry - we explore this here by using estimated changes in the radar scattering horizon to correct elevation trends across the interior of the Greenland ice sheet.