Empirical evidence of overestimated Ku-band sea ice radar freeboards in satellite altimetry

Abstract. Pan-Arctic sea ice thickness estimates are routinely produced from Ku-band radar altimetry observations. State-of-the-art waveform retracking algorithms rely on the uncertain assumption that Ku-band radar waves penetrate through the snow and that the dominant return originates from surface scattering at the snow/ice interface. However, growing evidence suggests that Ku-band radar altimetry freeboards may not always accurately track this elevation. We investigate this question by analyzing the evolution of spaceborne radar and laser freeboards over immobile regions of landfast first-year ice (FYI) and multi-year ice (MYI) off Greenland's coast from mid-winter to mid-summer 2022. Our results suggest that the radar freeboards over FYI trace the snow/ice interface during most of the cold season (up to mid-May, at this location), providing empirical support for the validity of the assumption of full snow penetration at Ku-band frequency. Over MYI, the retracked heights correspond to locations well below the air/snow interface most of the time, at least 60 % deep in the snow, but the exact depth could not be reliably assessed. However, these data also provide evidence for a positive bias in Ku-band radar freeboards during short intervals throughout the winter and the melt season, for both ice types. In particular, our results suggest that, during winter, the Ku-band radar freeboards tend to be biased high: i) for ice with a saline snow cover during a warming event (brine volume>1 %), and ii) during and immediately after strong snowfall events. Winter warming events are often accompanied by snowfall, leading to a cumulative bias for the areas with saline snow – i.e. most FYI in the Arctic. In the period of snow melt onset in May/June, biased radar freeboards appear to be related to saline snow only, but biases are on average larger (up to 10 cm) than during the winter period (under 4 cm). Though our results indicate a positive bias in satellite radar freeboards under specific snow conditions, periods of biased freeboard are short-term during the winter – in total accounting for approximately 15 % of the time. Our findings therefore generally support the assumption of full snow penetration for Ku-band sea ice thickness and dual-altimetry snow depth retrievals during winter, such as planned for the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission.