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

Abstract

Abstract: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.

Surface-based Ku- and Ka-band polarimetric radar for sea ice studies

“…It may also be possible to combine Ku-and Ka-bands to simultaneously retrieve both ice thickness and snow depth during winter (Lawrence et al, 2018;Guerreiro et al, 2016). Other studies have additionally suggested the feasibility of combining CS2 with snow freeboard observations from laser altimetry (e.g., ICESat-2) to map pan-Arctic snow depth and ice thickness during the cold season (Kwok and Markus, 2018;Kwok et al, 2020).…”

“…However, several key uncertainties limit the accuracy of the radar-based freeboard retrieval, which then propagate into the freeboard-to-thickness conversion. One important uncertainty pertains to inconsistent knowledge on how far the radar signal penetrates into the overlying snow cover (Nandan et al, 2020;Willatt et al, 2011;. The general assumption is that the radar return primarily originates from the snow-sea ice interface at the and from the air-snow interface at the Ka-band (AltiKa).…”

“…The general assumption is that the radar return primarily originates from the snow-sea ice interface at the and from the air-snow interface at the Ka-band (AltiKa). While this may hold true for cold, dry snow in a laboratory (Beaven et al, 1995), scientific evidence from observations and modeling suggests this assumption may be invalid even for a cold, homogeneous snowpack (Nandan et al, 2020;Willatt et al, 2011;Tonboe et al, 2010). Modeling experiments also reveal that for every millimeter of snow water equivalent (SWE), the effective scattering surface is raised by 2 mm relative to the freeboard (Tonboe, 2017).…”

Snow Thickness Estimation on First-Year Sea Ice from Late Winter Spaceborne Scatterometer Backscatter Variance

“…Previous synthetic aperture radar (SAR) and surface-based scatterometer studies of snow-covered FYI have provided the geophysical basis for development of active microwave based snow on sea ice thickness retrievals [38][39][40][41][42][43][44]. These studies have shown that changes in snow temperature on FYI are manifest as changes in total σ • .…”

Editorial for the Special Issue “Microwave Indices from Active and Passive Sensors for Remote Sensing Applications”

“…As applications devoted to snow monitoring, the co-pol. and dual-frequency ratios at Ku-, X-and C-bands were demonstrated to be adequately sensitive to variations in snow thickness for the Antarctic first year ice [21]. In [22], the combination of active and passive microwave acquisitions allowed characterizing the seasonal behavior of snow properties and retrieving the effective correlation length and the snow water equivalent.…”