Abstract. Accurate satellite measurements of the thickness of Antarctic sea ice are urgently needed but pose a particular challenge. The Antarctic data presented here were produced using a method to derive the sea ice thickness from 1.4 GHz brightness temperatures previously developed for the Arctic, with only modified auxiliary data. The ability to observe the thickness of thin sea ice using this method is limited to cold conditions, meaning it is only reasonable during the freezing period, typically March to October. The Soil Moisture and Ocean Salinity (SMOS) level-3 sea ice thickness product contains estimates of the sea ice thickness and its uncertainty up to a thickness of about 1 m. The sea ice thickness is provided as a daily average on a polar stereographic projection grid with a sample resolution of 12.5 km, while the SMOS brightness temperature data used have a footprint size of about 35–40 km in diameter. Data from SMOS have been available since 2010, and the mission's operation has been extended to continue until at least the end of 2025. Here we compare two versions of the SMOS Antarctic sea ice thickness product which are based on different level-1 input data (v3.2 based on SMOS L1C v620 and v3.3 based on SMOS L1C 724). A validation is performed to generate a first baseline reference for future improvements of the retrieval algorithm and synergies with other sensors. Sea ice thickness measurements to validate the SMOS product are particularly rare in Antarctica, especially during the winter season and for the valid range of thicknesses. From the available validation measurements, we selected datasets from the Weddell Sea that have varying degrees of representativeness: Helicopter-based EM Bird (HEM), Surface and Under-Ice Trawl (SUIT), and stationary Upward-Looking Sonars (ULS). While the helicopter can measure hundreds of kilometres, SUIT's use is limited to distances of a few kilometres and thus only captures a small fraction of an SMOS footprint. Compared to SMOS, the ULS are point measurements and multi-year time series are necessary to enable a statistically representative comparison. Only four of the ULS moorings have a temporal overlap with SMOS in the year 2010. Based on selected averaged HEM flights and monthly ULS climatologies, we find a small mean difference (bias) of less than 10 cm and a root mean square deviation of about 20 cm with a correlation coefficient R > 0.9 for the valid sea ice thickness range between 0 and about 1 m. The SMOS sea ice thickness showed an underestimate of about 40 cm with respect to the less representative SUIT validation data in the marginal ice zone. Compared with sea ice thickness outside the valid range, we find that SMOS strongly underestimates the real values, which underlines the need for combination with other sensors such as altimeters. In summary, the overall validity of the SMOS sea ice thickness for thin sea ice up to a thickness of about 1 m has been demonstrated through validation with multiple datasets. To ensure the quality of the SMOS product, an independent regional sea ice extent index was used for control. We found that the new version, v3.3, is slightly improved in terms of completeness, indicating fewer missing data. However, it is worth noting that the general characteristics of both datasets are very similar, also with the same limitations. Archived data are available in the PANGAEA repository at https://doi.org/10.1594/PANGAEA.934732 (Tian-Kunze and Kaleschke, 2021) and operationally at https://doi.org/10.57780/sm1-5ebe10b (European Space Agency, 2023).

中文翻译：

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SMOS 得出的南极薄海冰厚度：威德尔海的数据描述和验证


摘要。迫切需要对南极海冰厚度进行准确的卫星测量，但这也是一个特殊的挑战。这里提供的南极数据是使用一种从先前为北极开发的 1.4GHz 亮度温度推导出海冰厚度的方法生成的，仅修改了辅助数据。使用这种方法观察薄海冰厚度的能力仅限于寒冷条件，这意味着它仅在冰冻时期（通常是三月至十月）才合理。土壤湿度和海洋盐度 (SMOS) 3 级海冰厚度产品包含海冰厚度的估计值及其厚度可达约 1 m 的不确定性。海冰厚度以极地立体投影网格上的日平均值形式提供，样本分辨率为 12.5 公里，而使用的 SMOS 亮度温度数据的足迹大小约为直径 35-40 公里。 SMOS 的数据自 2010 年起就已提供，并且任务的运行已延长到至少持续到 2025 年底。在这里，我们比较了 SMOS 南极海冰厚度产品的两个版本，它们基于不同的 1 级输入数据（ v3.2 基于 SMOS L1C v620，v3.3 基于 SMOS L1C 724）。进行验证以生成第一个基线参考，以供将来改进检索算法以及与其他传感器的协同作用。用于验证 SMOS 产品的海冰厚度测量在南极洲尤其罕见，特别是在冬季和厚度的有效范围内。 从可用的验证测量中，我们选择了来自威德尔海的具有不同程度代表性的数据集：基于直升机的电磁鸟（HEM）、水面和冰下拖网（SUIT）以及固定上视声纳（ULS）。虽然直升机可以测量数百公里，但 SUIT 的使用仅限于几公里的距离，因此只能捕获 SMOS 足迹的一小部分。与 SMOS 相比，ULS 是点测量，需要多年时间序列才能进行具有统计代表性的比较。 2010 年，只有四个 ULS 系泊点与 SMOS 存在时间重叠。根据选定的平均 HEM 航班和每月 ULS 气候，我们发现小于 10 厘米的小平均差（偏差）和约 10 厘米的均方根偏差20 cm，有效海冰厚度范围在 0 至 1 m 之间，相关系数 R > 0.9。相对于边缘冰区代表性较差的 SUIT 验证数据，SMOS 海冰厚度被低估了约 40 厘米。与有效范围外的海冰厚度相比，我们发现 SMOS 严重低估了真实值，这强调了与高度计等其他传感器结合的需要。总之，SMOS海冰厚度对于厚度不超过1米左右的薄海冰的整体有效性已经通过多个数据集的验证得到了证明。为了保证SMOS产品的质量，采用独立的区域海冰范围指数进行控制。我们发现新版本v3.3在完整性方面略有提高，表明缺失数据更少。 然而，值得注意的是，两个数据集的一般特征非常相似，也具有相同的局限性。存档数据可在 PANGEA 存储库中获取，网址为 https://doi.org/10.1594/PANGAEA.934732（Tian-Kunze 和 Kaleschke，2021 年），操作数据可在 https://doi.org/10.57780/sm1-5ebe10b（European Space机构，2023 年）。