Reflectometry measurements have been conducted aboard the German research icebreaker Polarstern during the MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate).Signals of Global Navigation Satellite Systems (GNSS) were recorded using a dedicated GNSS reflectometry receiver for retrieval of sea-ice reflectivity. The primary goal is a reflectometry-based monitoring of sea ice as part of the Arctic climate study. The data set presented here covers the expedition's first leg (late Sep. to mid Dec. 2019) in the Siberian Sector of the central Arctic (at about 82 • N to 87 • N). Daily profiles of reflectivity are retrieved for satellite elevations < 45 • . In agreement with model prediction the results show best reflectivity contrast (about 5 dB between compact pack-ice and lower ice concentrations) for observations at left-handed circular polarization and elevation angles of 10 • to 20 • . A daily resolved time series of sea-ice relative permittivity is inverted from the left-handed data.In general, the level of inversion results is at the lower limit of sea-ice values (rel. permittivity of 3 and below), potentially indicating an influence of incoherent volume scattering. Occasional increase of relative permittivity is attributed to the presence of water. Sea-ice profiles show anomalies that are confirmed by enhanced model prediction (slab reflection). A long-term comparison of prediction and retrieved profiles indicates anomalies' dependence on ice thickness and temperature.
<table><tbody><tr><td> <p>Sea ice is a crucial parameter of the Earth&#8217;s climate system. Its high albedo compared to water and its insulating effect between ocean and atmosphere influences the oceans&#8217; radiation budget significantly. The importance of monitoring sea-ice properties arises from the high variability of sea ice induced by seasonal change and global warming. GNSS reflectometry can contribute to global monitoring of sea ice with high potential to extend the spatio-temporal coverage of today&#8217;s observation techniques. Properties like ice salinity, temperature, thickness and snow cover can affect the signal reflection. The MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate) gave us the opportunity to conduct reflectometry measurements under different sea-ice conditions in the central Arctic. A dedicated setup was mounted, in close cooperation with the Alfred-Wegener-Institute (AWI), on the German research icebreaker Polarstern that drifted for one year with the Arctic sea ice.<br><br>We present results from data recorded between autumn 2019 and spring 2020. The ship drifted in this period from the Siberian Sector of the Arctic (October 2019), over the central Arctic (November 2019 until May 2020) towards Fram Strait and Svalbard (reached in June 2020). Profiles of sea-ice reflectivity over elevation angle (range: 1&#176; to 45&#176;) are derived with daily resolution considering reflection data recorded at left-handed (LH) and right-handed (RH) circular polarization. Respective predictions of reflectivity are based on reflection models of bulk sea ice or a sea-ice slab. The latter allows to include the effect of signal penetration down to the underlying water. Results of comparison between LH profiles and bulk model confirm a reflectivity decrease (about 10 dB) when surrounding open water areas is reduced (by freezing) and the ship drifts in compact sea ice.<br><br>Further results comprise estimates of sea-ice permittivity from mid-elevation range reflectivity (10&#176; to 30&#176;). The median of estimated permittivity 2.4 (period of compact sea ice) lies in the expected range of reported old ice type (mostly second-year ice). The retrieved reflectivity in the low-elevation range (1&#176; to 10&#176;) give strong indication of signal penetration into the dominating second-year ice with influence of sea ice temperature and thickness. We conclude that sea-ice characterization in future can profit form GNSS reflectometry observations. The on-going study is currently extended to the further evolution of Arctic sea ice during winter and spring period of the MOSAiC expedition.</p> </td> </tr></tbody></table>
<p>In current times of a changing global climate, a special interest is focused on the<br>large-scale recording of sea ice. Among the existing remote sensing methods, bi-<br>statically reflected signals of Global Navigation Satellite Systems (GNSS) could<br>play an important role in fulfilling the task. Within this project, sensitivity of<br>GNSS signal reflections to sea ice properties like its occurrence, sea ice thick-<br>ness (SIT) and sea concentration (SIC) is evaluated. When getting older, sea<br>ice tends go get thicker. Because of decreasing salinity, i.e. less permittivity,<br>as well as relatively higher surface roughness of older ice, it can be assumed<br>that reflected signal strength decreases with increasing SIT. The reflection data<br>used were recorded in the years 2015 and 2016 by the TechDemoSat-1 (TDS-1)<br>satellite over the Arctic and Antarctic. It includes a down-looking antenna for<br>the reflected as well as an up-looking antenna dedicated to receive the direct sig-<br>nal. The raw data, provided by the manufacturer SSTL, were pre-processed by<br>IEEC/ICE-CSIC to derive georeferenced signal power values. The reflectivity<br>was estimated by comparing the power of the up- and down-looking links. The<br>project focuses on the signal link budget to apply necessary corrections. For this<br>reason, the receiver antenna gain as well as the Free-Space Path Loss (FSPL)<br>were calculated and applied for reflectivity correction. Differences of nadir and<br>zenith antenna FSPL and gain show influence of up to 6 dB and &#8722;9 dB to 9 dB<br>respectively on the recorded signal strength. All retrieved reflectivity values are<br>compared to model predictions based on Fresnel coefficients but also to avail-<br>able ancillary truth data of other remote sensing missions to identify possible<br>patterns: SIT relations are investigated using Level-2 data of the Soil Moisture<br>and Ocean Salinity (SMOS) satellite. The SIC comparison was done with an<br>AMSR-2 product. The results show sensitivity of the reflectivity value to both<br>SIT and SIC simultaneously, whereby the surface roughness is also likely to<br>have an influence. This on-going study aims at the consolidation of retrieval<br>algorithms for sea-ice observation. The resolution of different ice types and the<br>retrieval of SIT and SIC based on satellite data is a challenge for future work<br>in this respect.</p>
<p>The dielectric properties of sea ice differ significantly from the open-water surface when we consider the L-band frequency range of GNSS signals. In contrast to water, the signal&#8217;s penetration into sea ice can reach several decimeters depending on properties like salinity, temperature and thickness. Exploiting these different dielectric properties is a key to use GNSS for sea-ice remote sensing. For this purpose, GNSS reflectometry measurements have been conducted over the Arctic Ocean during the MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate). A combined receiver setup was used that allows the here described reflectometry study and another study for atmosphere sounding. The setup was mounted, in close cooperation with the Alfred-Wegener-Institute (AWI), on the German research icebreaker Polarstern that drifted during nine months of the expedition with the Arctic sea ice.</p><p>Here, an initial study is presented that focuses on the expedition&#8217;s first leg in autumn 2019 when the ship started drifting at about 85&#176;N to 87&#176;N in the Siberian Sector of the Arctic. Profiles of sea-ice reflectivity are derived with daily resolution considering reflection data recorded at left-handed (LH) and right-handed (RH) circular polarization. Respective model predictions of reflectivity are assuming a sea-ice bulk medium or a sea-ice slab. The later allows to include the effect of signal penetration down to the underlying water. Results of comparison between LH profiles and bulk model confirm the reflectivity contrast (about 10 dB) between sea ice and water. The particularly low level of LH reflectivity in the late observation period (December 2019) indicates the presence of low-saline multiyear (MY) ice. A bias due to snow accumulating on the ice surface may occur. A snow-extended reflection model, driven by additional snow data, can help in future for clarification.</p><p>Anomalies of observed reflectivity with respect to bulk model predictions are especially obvious at lowest elevation angles. According to the model, the slope of profiles at low elevations is about 1.0 to 1.2 dB/&#176;. The observation shows significantly lower values (< 0.5 dB/&#176;) including negative slopes. A comparison of LH results with the ice slab model provides clarification. The anomalies are induced by signal penetration leading to interference pattern of reflections from the ice&#8217;s surface and bottom. Slope retrievals quantify the anomaly and allow a coarse estimation of the mean sea-ice temperature (about -10&#176;C in December 2019) based on the slab model predictions. Further investigations are needed to better understand sea-ice reflectivity at RH polarization. RH profiles show a response to sea ice and features at low elevation angles that cannot be explained by current reflection models.</p><p>As a conclusion, GNSS reflectometry is sensitive to dielectric sea-ice properties. Estimates of ice type/salinity and temperature are reported based on LH observation data. These findings will be exploited to further strengthen the application of GNSS signals for sea-ice remote sensing. Future studies on GNSS observations from ships and satellites are anticipated.</p>
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