Sensitivity of CryoSat-2 Arctic sea-ice freeboard and thickness on radar-waveform interpretation

Abstract

Abstract. In the context of quantifying Arctic ice-volume decrease at global scale, the CryoSat-2 satellite was launched in 2010 and is equipped with the K u band synthetic aperture radar altimeter SIRAL (Synthetic Aperture Interferometric Radar Altimeter), which we use to derive sea-ice freeboard defined as the height of the ice surface above the sea level. Accurate CryoSat-2 range measurements over open water and the ice surface of the order of centimetres are necessary to achieve the required accuracy of the freeboardto-thickness conversion. Besides uncertainties of the actual sea-surface height and limited knowledge of ice and snow properties, the composition of radar backscatter and therefore the interpretation of radar echoes is crucial. This has consequences in the selection of retracker algorithms which are used to track the main scattering horizon and assign a range estimate to each CryoSat-2 measurement. In this study we apply a retracker algorithm with thresholds of 40, 50 and 80 % of the first maximum of radar echo power, spanning the range of values used in the current literature. By using the selected retrackers and additionally results from airborne validation measurements, we evaluate the uncertainties of sea-ice freeboard and higher-level products that arise from the choice of the retracker threshold only, independent of the uncertainties related to snow and ice properties. Our study shows that the choice of retracker thresholds does have a significant impact on magnitudes of estimates of sea-ice freeboard and thickness, but that the spatial distributions of these parameters are less affected.

About the consistency between Envisat and CryoSat-2 radar freeboard retrieval over Antarctic sea ice

“…In this case radar altimeter range measurements generally relate to ice freeboard. However, the generality of this assumption has been recently questioned by several publications (Willatt et al, 2010(Willatt et al, , 2011Ricker et al, 2014;Kurtz et al, 2014;Price et al, 2015;Kwok, 2014).…”

“…A full description is given in Ricker et al (2014). Leads are cracks in the ice cover and usually have a distinct specular radar echo, while open-ocean and sea-ice surfaces have wider waveforms, resulting from diffuse reflection due to the higher surface roughness (see Fig.…”

“…We use the geolocated level 1b Synthetic Aperture Radar (SAR) and interferometric SAR (SARIn) waveform products over the Southern Ocean (K u band, 13.575 GHz) provided by the ESA (https://earth.esa.int/web/ guest/-/how-to-access-cryosat-data-6842). The surface elevations are processed along individual CS-2 orbits using the Threshold First-Maximum Retracker Algorithm (TFMRA) described by Helm et al (2014) and Ricker et al (2014) in detail. Specifically, the main scattering horizon is tracked at the waveforms' leading edge of the first local maximum by using a power threshold (see Fig.…”

“…Different is the waveform-based surface type classification, where individual radar echoes are segmented in the classes lead, sea ice and unclassified. While CS-2 waveforms are classified using a multi-parameter approach (Ricker et al, 2014), Envisat waveform parameters for classification are limited to the PP defined as (Peacock and Laxon, 2004) …”

“…The capability of sea-ice thickness retrieval using satellite radar and laser altimetry data has been demonstrated for Arctic and Antarctic sea ice (Ricker et al, 2014;Laxon et al, 2013;Kurtz et al, 2014;Zwally et al, 2008;. The altimetry sea-ice thickness retrieval algorithm is based on estimations of freeboard, the height of the ice (ice freeboard) or snow surface (total or snow freeboard) above the local sea level.…”