Monitoring bedfast ice and ice phenology in lakes of the Lena River Delta using TerraSAR-X backscatter and coherence time series

Thermokarst lakes and ponds are major elements of permafrost landscapes, occupying up to 40% of the land area in some Arctic regions. Shallow lakes freeze to the bed, thus preventing permafrost thaw underneath them and limiting the length of the period with greenhouse gas production in the unfrozen lake sediments. Radar remote sensing permits to distinguish lakes with bedfast ice due to the difference in backscatter intensities from bedfast and floating ice. This study investigates the potential of a unique time series of three-year repeat-pass TerraSAR-X (TSX) imagery with high temporal (11 days) and spatial (10 m) resolution for monitoring bedfast ice as well as ice phenology of lakes in the zone of continuous permafrost in the Lena River Delta, Siberia. TSX backscatter intensity is shown to be an excellent tool for monitoring floating versus bedfast lake ice as well as ice phenology. TSX-derived timing of ice grounding and the ice growth model CLIMo are used to retrieve the ice thicknesses of the bedfast ice at points where in situ ice thickness measurements were available. Comparison shows good agreement in the year of field measurements. Additionally, for the first time, an 11-day sequential interferometric coherence time series is analyzed as a supplementary approach for the bedfast ice monitoring. The coherence time series detects most of the ice grounding as well as spring snow/ice melt onset. Overall, the results show the great value of TSX time series for monitoring Arctic lake ice and provide a basis for various applications: for instance, derivation of shallow lakes bathymetry, evaluation of winter water resources and locating fish winter habitat as well as estimation of taliks extent in permafrost.热融喀斯特湖（Thermkarst lakes）与塘堰是多年冻土（permafrost）景观的核心组成要素，在部分北极区域可占据高达40%的陆域面积。浅层湖泊会完全封冻至湖床，由此可阻止湖下多年冻土融化，并限制未冻结湖底沉积物中温室气体产生的持续时长。雷达遥感可通过底固冰（bedfast ice）与浮冰的后向散射强度差异，区分出发育底固冰的湖泊。本研究针对西伯利亚勒拿河三角洲连续多年冻土区的湖泊，探索了一套独特的三年重复过境TerraSAR-X（TSX）影像时间序列的应用潜力——该影像具备高时间分辨率（11天）与空间分辨率（10米），可用于监测底固冰以及湖泊冰物候（ice phenology）。研究表明，TSX后向散射强度是监测湖泊浮冰与底固冰，以及冰物候的高效手段。本研究利用TSX反演得到的冰体封冻时机，结合冰体生长模型CLIMo，在具备原位冰厚测量数据的点位处，反演底固冰的厚度。对比结果显示，该反演结果与野外实测年份的冰厚数据吻合度极佳。此外，本研究首次将11天连续干涉相干性时间序列作为底固冰监测的补充手段开展分析。该相干性时间序列可识别出绝大多数冰体封冻事件，以及春季积雪/融冰的起始时间。整体而言，本研究结果证实了TSX时间序列在北极湖泊冰体监测中的重要应用价值，并为诸多场景提供了研究基础：例如浅层湖泊水深反演、冬季水资源评估、鱼类越冬栖息地定位，以及多年冻土中融土核（taliks）的范围估算。