Abstract
Uncertainty in snow properties impacts the accuracy of Arctic sea ice thickness estimates from radar altimetry. On first-year sea ice (FYI), spatiotemporal variations in snow properties can cause the Ku-band main radar scattering horizon to appear above the snow/sea ice interface. This can increase the estimated sea ice freeboard by several centimeters, leading to FYI thickness overestimations. This article examines the expected changes in Ku-band main scattering horizon and its impact on FYI thickness estimates, with variations in snow temperature, salinity, and density derived from ten naturally occurring Arctic FYI Cases encompassing saline/nonsaline, warm/cold, simple/complexly layered snow (4–45 cm) overlying FYI (48–170 cm). Using a semi-empirical modeling approach, snow properties from these Cases are used to derive layer-wise brine volume and dielectric constant estimates, to simulate the Ku-band main scattering horizon and delays in radar propagation speed. Differences between modeled and observed FYI thickness are calculated to assess sources of error. Under both cold and warm conditions, saline snow covers are shown to shift the main scattering horizon above from the snow/sea ice interface, causing thickness retrieval errors. Overestimates in FYI thicknesses of up to 65% are found for warm, saline snow overlaying thin sea ice. Our simulations exhibited a distinct shift in the main scattering horizon when the snow layer densities became greater than 440 kg/m3, especially under warmer snow conditions. Our simulations suggest a mean Ku-band propagation delay for snow of 39%, which is higher than 25%, suggested in previous studies.
Highlights
AND BACKGROUNDR ADAR altimeters such as the ERS-1/2 RA, ENVISAT RA-2, CryoSat-2, and Sentinel-3A/B operating at Kuband frequencies have been and are used to estimate sea ice freeboard, the vertical distance between the local sea level and the snow/ice interface of floating sea ice [1]–[7]
It is assumed that Ku-band microwaves attain complete penetration through dry, cold, and homogeneous snow, and returns predominantly originate from the snow/sea ice interface [1], [3], [10]–[12]
These studies acknowledge that the presence of highly dense compacted snow layers and/or ice lenses may cause a vertical upward shift in the radar main scattering horizon toward the air/snow interface owing to complex surface and volume scattering mechanisms occurring within the snow volume
Summary
AND BACKGROUNDR ADAR altimeters such as the ERS-1/2 RA, ENVISAT RA-2, CryoSat-2, and Sentinel-3A/B operating at Kuband frequencies have been and are used to estimate sea ice freeboard, the vertical distance between the local sea level and the snow/ice interface of floating sea ice [1]–[7]. It is assumed that Ku-band microwaves attain complete penetration through dry, cold, and homogeneous snow, and returns predominantly originate from the snow/sea ice interface [1], [3], [10]–[12] These studies acknowledge that the presence of highly dense compacted snow layers and/or ice lenses may cause a vertical upward shift in the radar main scattering horizon toward the air/snow interface owing to complex surface and volume scattering mechanisms occurring within the snow volume. This shift leads to a misrepresentation of the sea ice freeboard [3, Fig. 4] and inaccurate sea ice thickness estimates, with the choice of thresholds in retracker algorithm a factor [4]. Recent studies acknowledge the variable penetration of Ku-band radar into the snow cover owing to snow moisture [12] and sub-footprint and footprint-scale surface roughness variations [12]–[16]
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