Abstract
Abstract. We develop a physical model capable of simulating the mean echo power of CryoSat-2 SAR- and SARIn-mode 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 waveform-fitting 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.
Highlights
Remote sensing records of Arctic sea ice thickness span five decades and have shown nearly a 2-fold decrease in mean winter thickness (Kwok and Rothrock, 2009), while observations over the past three decades have shown a 17.2 % decade−1 decline in the areal coverage of multiyear ice (Comiso, 2012)
The focus of this study is to develop a new method for the retrieval of sea ice freeboard from CryoSat-2 data
We demonstrate that this method is consistent with independent measurements from airborne laser and radar altimetry data sets from NASA’s Operation IceBridge mission to retrieve sea ice freeboard and thickness
Summary
Remote sensing records of Arctic sea ice thickness span five decades and have shown nearly a 2-fold decrease in mean winter thickness (Kwok and Rothrock, 2009), while observations over the past three decades have shown a 17.2 % decade−1 decline in the areal coverage of multiyear ice (Comiso, 2012). The interrelated decline in sea ice thickness and multiyear ice coverage is tied to declining trends in ice age and survivability (Maslanik et al, 2007; Maslanik et al, 2011) These changes have significant impacts on the climate, with a notable aspect of declining sea ice cover being linked to the observed higher than global average increase in Arctic surface air temperatures, a phenomenon known as Arctic amplification (Serreze et al, 2009). In the context of this study, we define freeboard as the height of the sea ice layer above the sea surface This large discrepancy underscores the need to establish a set of consistent physical constants for use in the retrieval of sea ice thickness from satellite radar and laser altimetry data.
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