Vishnu Nandan

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

The European Space Agencys 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 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.

Multi-frequency microwave backscatter indices from saline snow covers on smooth first-year sea ice

This study inter-compares observed Ku-, X- and C-band microwave backscatter from saline 14 cm, 8 cm, and 4 cm snow covers on smooth first-year sea ice. A surface-borne multi-frequency (Ku-, X- and C-bands) polarimetric microwave scatterometer system is used near-coincident with in situ snow geophysical measurements. The study investigated differences in scatterometer observations for all three frequencies, co-pol ratios, and introduced new dual-frequency ratios to discriminate dominant polarization-dependent frequencies from these snow covers. Preliminary results suggest that, thinnest 4 cm snow cover demonstrate greatest increase in microwave backscatter from all three frequencies, followed by backscatter from thicker 8 cm and 14 cm snow covers. Dual-frequency indices derived for all frequency and polarization combinations suggest greater sensitivity of Ku-band microwaves to snow grain microstructure with increasing snow thicknesses, X-band microwaves to changes in snow salinities with decreasing snow thicknesses. Our results indicate the effect of dielectric loss associated with high salinities throughout all layers of the three snow covers, as the dominant factor affecting microwave penetration and backscatter from all three frequencies.

Ku-, X- and C-Band Microwave Backscatter Indices from Saline Snow Covers on Arctic First-Year Sea Ice

In this study, we inter-compared observed Ku-, X- and C-band microwave backscatter from saline 14 cm, 8 cm, and 4 cm snow covers on smooth first-year sea ice. A Ku-, X- and C-band surface-borne polarimetric microwave scatterometer system was used to measure fully-polarimetric backscatter from the three snow covers, near-coincident with corresponding in situ snow thermophysical measurements. The study investigated differences in co-polarized backscatter observations from the scatterometer system for all three frequencies, modeled penetration depths, utilized co-pol ratios, and introduced dual-frequency ratios to discriminate dominant polarization-dependent frequencies from these snow covers. Results demonstrate that the measured co-polarized backscatter magnitude increased with decreasing snow thickness for all three frequencies, owing to stronger gradients in snow salinity within thinner snow covers. The innovative dual-frequency ratios suggest greater sensitivity of Ku-band microwaves to snow grain size as snow thickness increases and X-band microwaves to snow salinity changes as snow thickness decreases. C-band demonstrated minimal sensitivity to changes in snow salinities. Our results demonstrate the influence of salinity associated dielectric loss, throughout all layers of the three snow covers, as the governing factor affecting microwave backscatter and penetration from all three frequencies. Our “plot-scale” observations using co-polarized backscatter, co-pol ratios and dual-frequency ratios suggest the future potential to up-scale our multi-frequency approach to a “satellite-scale” approach, towards effective development of snow geophysical and thermodynamic retrieval algorithms on smooth first-year sea ice.

Diurnal Scale Controls on C-Band Microwave Backscatter From Snow-Covered First-Year Sea Ice During the Transition From Late Winter to Early Melt

Diurnal observations of coincident in situ physical-, electrical-, and surface-based C-band microwave scattering properties of a 16-cm saline snow cover on smooth, moderately saline, first-year sea ice are presented for the transition period between late winter and early melt. Statistical regression analysis and backscatter modeling are employed to explore the scattering mechanisms within the snowpack and to assess associations between backscatter and snow properties for both periods. Our results demonstrate substantial variation in both measured snow properties and C-band microwave backscatter over the diurnal cycle during the late winter period when the difference between maximum and minimum air and snow surface temperature was approximately 5 °C. No such variation in snow properties and backscatter occurred for the early melt period when our case study exhibited a small diurnal variation (~1°C) in air and snow surface temperature. Statistical and modeled results show significant association between the microwave backscatter and snow properties for the top 8 cm of the snowpack. Basal snow properties do not contribute toward total backscatter in either case. As a result, we are certain that the sea ice surface was a negligible scattering interface during both cases. Correlations between backscatter and snow properties are incidence angle dependent, demonstrating the highest association at 50°. Diurnal backscatter from RADARSAT-2 synthetic aperture radar acquisitions support the influence of varying diurnal snow properties on C-band backscatter, showing a difference of ~4 dB for

Winter Sentinel-1 Backscatter as a Predictor of Spring Arctic Sea Ice Melt Pond Fraction: Melt Pond Fraction Prediction From SAR

{ \sigma }_{\vphantom {R_{j}}{\text {hh}}}^{o}

Multi-frequency polarimetric microwave observations of snow cover on first-year Arctic sea ice

and

Ku-, X- and C-band measured and modeled microwave backscatter from a highly saline snow cover on first-year sea ice

{ \sigma }_{{\text {vv}}}^{o}

Geophysical and atmospheric controls on Ku-, X- and C-band backscatter evolution from a saline snow cover on first-year sea ice from late-winter to pre-early melt

during the late winter period. This difference is reduced to <1 dB for the early melt period.

Multifrequency Microwave Backscatter From a Highly Saline Snow Cover on Smooth First-Year Sea Ice: First-Order Theoretical Modeling

Spring melt pond fraction (fp) has been shown to influence September sea ice extent and, with a growing need to improve melt pond physics in climate and forecast models, observations at large spatial scales are needed. We present a novel technique for estimating fp on sea ice at high spatial resolution from the Sentinel-1 satellite during the winter period leading up to spring melt. A strong correlation (r = -0.85) is found between winter radar backscatter and fp from first-year and multiyear sea ice data collected in the Canadian Arctic Archipelago (CAA) in 2015. Observations made in the CAA in 2016 are used to validate a fp retrieval algorithm, and a fp prediction for the CAA in 2017 is made. The method is effective using the horizontal transmit and receive polarization channel only and shows promise for providing seasonal, pan-Arctic, fp maps for improved understanding of melt pond distributions and forecast model skill.

A spectral mixture analysis approach to quantify Arctic first-year sea ice melt pond fraction using QuickBird and MODIS reflectance data

This study explores the potential of a multi-frequency (Ku-, X- and C-band) scatterometry approach, to understand microwave interactions between teo statistically different snow thickness covers (14cm and 8cm) on first-year Arctic sea ice during the late winter to early-melt season transition. The results show substantial differences in backscatter response from all three frequencies, for both snow covers. Highly-saline snow covers with fluctuating snow geophysical and thermodynamic properties cause these backscatter fluctuations, with contributions from surface and volume scattering from different snow layers and interfaces. C-band exhibited drastic variations in backscatter, especially for the 14cm snow cover, when compared to Ku- and X-band. In the case of 8cm snow cover, all the three frequencies show minimal sensitivity to snow electro-thermo-physical properties. Our results show distinctly different snow thermodynamic processes operating within the different snow layers, essential for snow thickness estimation on first-year sea ice using active microwave remote sensing approaches.