An improved CryoSat-2 sea ice freeboard retrieval algorithm through the use of waveform fitting

Abstract

Abstract.We develop a physical model capable of simulating the mean echo power of CryoSat-2 SAR-and SARInmode waveforms over sea-ice-covered regions. The model simulations are used to show the importance of variations in the radar backscatter coefficient with incidence angle and surface roughness for the retrieval of surface elevation of both sea ice floes and leads. The physical model is used to fit CryoSat-2 waveforms to enable retrieval of surface elevation through the use of lookup tables and a bounded trust region Newton least-squares fitting approach. The use of a model to fit returns from sea ice regions offers advantages over currently used threshold retracking methods, which are here shown to be sensitive to the combined effect of bandwidth-limited range resolution and surface roughness variations. Laxon et al. (2013) have compared ice thickness results from CryoSat-2 and IceBridge, and found good agreement; however consistent assumptions about the snow depth and density of sea ice were not used in the comparisons. To address this issue, we directly compare ice freeboard and thickness retrievals from the waveform-fitting and threshold tracker methods of CryoSat-2 to Operation IceBridge data using a consistent set of parameterizations. The purpose of the comparison is to highlight the physical basis between differences in the retracking methods. For three IceBridge campaign periods from March 2011 to March 2013, mean differences (CryoSat-2 -IceBridge) of 0.144 and 1.351 m are found between the freeboard and thickness retrievals, respectively, using a 50 % sea ice floe threshold retracker, while mean differences of 0.019 and 0.182 m are found when using the waveform-fitting method. This suggests the waveformfitting technique is capable of better reconciling the sea ice thickness data record from laser and radar altimetry data sets through the usage of consistent physical assumptions.

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).…”

“…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.…”

Utilisation of CryoSat-2 SAR altimeter in operational ice charting

“…We also studied the feasibility of another waveform charasteristic suggested by Kurtz et al (2014). This is the ratio of early tail to peak power (denoted here ETPP):…”

“…SSD has been used, in conjunction with PP, in lead detection, e.g. by Laxon et al (2013), Ricker et al (2014), and Kurtz et al (2014).…”

“…In our classification experiments we chose to experiment with the two features suggested by Kurtz et al (2014). The ratio of late tail to peak power is defined as…”