Effect of Snow Salinity on CryoSat‐2 Arctic First‐Year Sea Ice Freeboard Measurements

Abstract

The European Space Agency's CryoSat‐2 satellite mission provides radar altimeter data that are used to derive estimates of sea ice thickness and volume. These data are crucial to understanding recent variability and changes in Arctic sea ice. Sea ice thickness retrievals at the CryoSat‐2 frequency require accurate measurements of sea ice freeboard, assumed to be attainable when the main radar scattering horizon is at the snow/sea ice interface. Using an extensive snow thermophysical property dataset from late winter conditions in the Canadian Arctic, we examine the role of saline snow on first‐year sea ice (FYI), with respect to its effect on the location of the main radar scattering horizon, its ability to decrease radar penetration depth, and its impact on FYI thickness estimates. Based on the dielectric properties of saline snow commonly found on FYI, we quantify the vertical shift in the main scattering horizon. This is found to be approximately 0.07 m. We propose a thickness‐dependent snow salinity correction factor for FYI freeboard estimates. This significantly reduces CryoSat‐2 FYI retrieval error. Relative error reductions of ~11% are found for an ice thickness of 0.95 m and ~25% for 0.7 m. Our method also helps to close the uncertainty gap between SMOS and CryoSat‐2 thin ice thickness retrievals. Our results indicate that snow salinity should be considered for FYI freeboard estimates.

Retrieval of snow freeboard of Antarctic sea ice using waveform fitting of CryoSat-2 returns

“…When using Ku-band altimetry for retrievals of freeboard and thickness, the largest source of uncertainty comes from the snow on sea ice. Uncertainty in the depth, salinity, and vertical structure can impact ranging and freeboard calculation (Armitage and Ridout, 2015;Ricker et al, 2015;Nandan et al, 2017). In order to counteract this uncertainty and improve the knowledge of the scattering effects of a snow layer on sea ice, our work aims to utilize Ku-band altimetry from CryoSat-2 to retrieve the elevation of the air-snow interface and subsequently the snow freeboard.…”

Three years of sea ice freeboard, snow depth, and ice thickness of the Weddell Sea from Operation IceBridge and CryoSat-2

“…In a review article of snow on Antarctic sea ice, Massom et al (2001) report that as a result of capillary suction of brine and flooding, high salinities (> 10 psu) occur up to about 0.1 m in the snow column, but mainly in the basal layer 0-5 cm layer above the ice surface. A recent analysis by Nandan et al (2017) indicate that saline snow above the snow-ice interface on Arctic sea ice (observed on fast ice in the Canadian Arctic Archipelago) may indeed mask the contribution of scattering of 5 the snow-ice interface to the radar return by effectively reducing the penetration into the snow volume, hence affecting thickness estimates using radar freeboards. While the processes associated with saline snow may be different in the Southern Ocean ice cover (perhaps saline snow associated with flooding as noted by Willatt et al (2010) and (Massom et al, 2001)), the results suggest a source of bias in the radar returns worthy of further investigation.…”

Responses to Anonymous Referee #1

“…With averaging, the spot separation is ∼ 1 m along track at an altitude of ∼ 500 m and an air speed of ∼ 250 kts (the nominal flight parameters for all OIB sea ice surveys). The size of the average footprint is ∼ 5-10 m, and the spacing between the processed radar profiles is ∼ 5 m. The reader is referred to the published literature for a more detailed description of the radar system (e.g., Panzer et al, 2013) and of the data characteristics (e.g., Kwok et al, 2011). Snow depth is calculated using a simplified version of the retrieval procedure described by Kwok and Maksym (2014) that has the capability to compensate for effects due to residual system side lobes in the returns (Kwok and Haas, 2015).…”

“…In a review article of snow on Antarctic sea ice, Massom et al (2001) report that as a result of capillary suction of brine and flooding, high salinities (> 10 psu) occur up to about 0.1 m in the snow column, but mainly in the basal layer 0-5 cm above the ice surface. A recent analysis by Nandan et al (2017) indicates that saline snow above the snow-ice interface on Arctic sea ice (observed on fast ice in the Canadian Arctic Archipelago) may indeed mask the contribution of scattering of the snow-ice interface to the radar return by effectively reducing the penetration into the snow volume, hence affecting thickness estimates using radar freeboards. While the processes associated with saline snow may be different in the Southern Ocean ice cover (perhaps saline snow associated with flooding as noted by Willatt , 2010, andMassom et al, 2001), the results suggest a source of bias in the radar returns worthy of further investigation.…”

“…A recent analysis by Nandan et al (2017) indicates that saline snow above the snow-ice interface on Arctic sea ice (observed on fast ice in the Canadian Arctic Archipelago) may indeed mask the contribution of scattering of the snow-ice interface to the radar return by effectively reducing the penetration into the snow volume, hence affecting thickness estimates using radar freeboards. While the processes associated with saline snow may be different in the Southern Ocean ice cover (perhaps saline snow associated with flooding as noted by Willatt , 2010, andMassom et al, 2001), the results suggest a source of bias in the radar returns worthy of further investigation. The consequence is an overestimation of total and ice freeboards ( h f , h f , h fi , h fi in the equations above) and therefore the sea ice thickness using Eqs.…”