Insights into Greenland Ice Sheet Surface Roughness from Ku-/Ka-band Radar Altimetry Surface Echo Strengths

Abstract. Surface roughness is an important factor to consider when modelling mass fluxes at the Greenland Ice Sheet (GrIS) surface (i.e., surface mass balance, SMB). This is because it can have important implications for both sensible and latent heat fluxes between the atmosphere and the ice sheet and near-surface ventilation. While surface roughness can be quantified from ground-based, airborne and spaceborne observations, satellite radar datasets provide the unique combination of long-term, repeat observations across the entire GrIS and insensitivity to illumination conditions and cloud cover. In this study, we investigate the reliability and interpretation of a new type of surface roughness estimate derived from the analysis of Ku- and Ka-band airborne and spaceborne radar altimetry surface echo powers by comparing them to contemporaneous laser altimetry measurements. Airborne data are those acquired during the 2017 and 2019 CryoVEx campaigns while the satellite data (ESA CryoSat-2, CNES/ISRO SARAL, and NASA ICESat-2) are those acquired in November 2018. Our results show that because surface roughness across the GrIS is primarily scale-dependent, a revised empirical mapping of quantified radar backscattering to surface roughness gives a better match to the coincident laser altimetry observations than an analytical model that assumes scale-independent roughness. We also show that the radar altimetry-derived surface roughness is best interpreted as the wavelength-baseline linear projection of the scale-dependent surface roughness observed at hundreds of meter scales and is therefore not representative of individual small-scale features. These results provide critical context for interpreting the datasets and evaluating their applicability in modelling GrIS SMB.