Sensitivity of CryoSat-2 Arctic sea-ice volume trends on radar-waveform interpretation

Abstract

Abstract. Several studies have shown that there is considerable evidence that the Arctic sea-ice is thinning during the last decades. When combined with the observed rapid reduction of ice-covered area this leads to a decline in sea-ice volume. The only remote sensing technique capable of quantifying this ice volume decrease at global scale is satellite altimetry. In this context the CryoSat-2 satellite was launched in 2010 and is equipped with the Ku-band SAR radar altimeter SIRAL, which we use to derive sea-ice freeboard defined as the height of the ice surface above the local sea level. 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 Ku-band SAR radar altimeter SIRAL, 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 in the order of centimeters are necessary to achieve the required accuracy of the freeboard to thickness conversion. Besides uncertainties of the actual sea-surface height and limited knowledge of ice and snow properties, the penetration of the radar signal into the snow cover 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 measurement. In this paper 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 current literature. For the 40% threshold we assume that the main scattering horizon lies at a certain depth between the surface and snow-ice interface as verified through coincident CryoSat-2 and airborne laser altimetry measurements. This contrasts with the 50% and 80% thresholds where we assume the ice-snow interface as the main scattering horizon similar to other published studies. Using the selected retrackers we evaluate the uncertainties of trends in sea-ice freeboard and higher level products that arise from the choice of the retracker threshold only, independently from the uncertainties related to snow and ice properties. Our study shows that the choice of retracker thresholds does have a non-negligible impact on magnitude estimates of sea-ice freeboard, thickness and volume, but that the main trends in these parameters are less affected. Specifically we find declines of Arctic sea-ice volume of 9.7% (40% threshold), 10.9% (50% threshold) and 6.9% (80% threshold) between March 2011 and March 2013. In contrast to that we find increases in Arctic sea-ice volume of 27.88% (40% threshold), 25.71% (50% threshold) and 32.65% (80% threshold) between November 2011 and November 2013. Furthermore we obtain a significant increase of freeboard from March 2013 to November 2013 in the area for multi-seasonal sea-ice north of Greenland and the Canadian Archipelago. Since this is unlikely it gives rise to the assumption that applying different retracker thresholds depending on seasonal properties of the snow load is necessary in the future.

Sea surface height and dynamic topography of the ice‐covered oceans from CryoSat‐2: 2011–2014

“…Ricker et al . [] used a modified version of the PP that considers the neighborhood of the peak of surface return. Both the PP and SSD are measures derived from the entire waveform, which include off nadir returns, and therefore these measures are sometimes confounded by strong off‐nadir returns (sometimes referred to as “snagging”).…”

“…Returns from sea ice are more variable and complex due to large variability in surface relief, but returns from the flat surface of open water leads or thin ice are somewhat simpler. While different CS‐2 retracking techniques over sea ice [e.g., Wingham et al ., , Laxon et al ., , Ricker et al ., , Kurtz et al ., , Jain et al ., ] have been suggested and devised, and sensitivities of specific techniques analyzed [ Ricker et al ., ], an optimal retracking approach that addresses the scattering issues in radar echoes especially in footprints that contain mixtures of ice and water has been difficult to establish. Since the focus here is on retrieving SSHs, we describe only those waveform characteristics than allow us to unambiguously identify the nearly pure sea surface returns.…”

“…For discrimination of ice and leads in CS-2 data, Laxon et al [2013] utilized a pulse ''peakiness'' (PP) parameter (a relative measure of peak signal strength) together with a stack standard deviation (SSD) parameter, which provides a measure of the variation in along-track surface backscatter [Wingham et al, 2006]. Ricker et al [2014] used a modified version of the PP that considers the neighborhood of the peak of surface return. Both the PP and SSD are measures derived from the entire waveform, which include off nadir returns, and therefore these measures are sometimes confounded by strong off-nadir returns (sometimes referred to as ''snagging'').…”

Evaluation of CryoSat-2 derived sea-ice freeboard over fast ice in McMurdo Sound, Antarctica

“…The ability to accurately measure freeboard and to include information on snow morphology is vital, as any errors in these input factors are greatly magnified in the eventual sea-ice thickness estimation. The European Space Agency’s (ESA) Synthetic aperture radar Interferometric Radar ALtimeter system (SIRAL) on-board CryoSat-2 (CS-2) is the most advanced satellite radar altimeter instrument for sea-ice freeboard retrieval in operation to date (Drinkwater and others, 2004; Wingham and others, 2006), and at the time of writing is improving understanding of the Arctic sea-ice thickness distribution (Laxon and others, 2013; Kurtz and others, 2014; Ricker and others, 2014). Given the more heterogeneous and thinner state of Antarctic sea ice, primarily due to its exposed oceanic setting and its highly variable snow distribution and morphology (Massom and others, 2001; Ozsoy-Cicek and others, 2013), the uncertainty in resultant thickness estimates from CS-2 in the Southern Ocean is likely to be higher.…”

“…Waveform fitting is the basis for the ESA’s Level 2 product ( ESAL2 ) and the Waveform Fitting ( WfF ) procedure as described by Kurtz and others (2014). The Threshold-First-Maximum-Retracker-Algorithm employed at 40% ( TFMRA40 ) is an empirical approach presented by Helm and others (2014) and applied over sea ice by Ricker and others (2014).…”