Monitoring bedfast ice 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 land area in some vast thermokarst affected Arctic regions (Grosse et al., 2013). These waterbodies contribute significantly to the exchange of energy and greenhouse gases (GHG) between the land surface and the atmosphere. Deeper lakes (> 2 m) remain typically unfrozen beneath floating ice during winter, leading to the formation of continuously thawed bottom sediments (talik) which facilitate microbial activity and GHG production throughout the year (Langer et al., 2015). Shallower lakes (< 2 m) experience complete freezing down to the bottom which prevents talik formation and strongly limits the length of the GHG production period. Thus, distinguishing floating lake ice from grounded ice is crucial for evaluating the thermal and geobiochemical state of tundra landscapes. Radar remote sensing provides a powerful tool for accomplishing this task through the ability of radar signals to partially penetrate into or through the lake ice. Mostly, a method based on differences between backscatter signatures is used. In case of floating ice, radar signal backscatters from the ice-water interface and return is high in case of rough ice bottom, which is likely for lake ice (Atwood et al., 2015). In case of grounded ice, radar signal penetrates through the ice layer and transmits to the frozen lake bottom due to insufficient dielectric contrast between ice and frozen sediments, resulting in low backscatter. Although the method has been known since 1970’s, the potential of new generation satellite X-band radar imagery such as TerraSAR-X has yet to be evaluated. Interferometric coherence is a parameter which defines the degree of correlation between two radar signals and to a great extent depends on changes in backscattering surface occurred between two radar acquisitions. Interferometric coherence time series are used as a different approach for distinguishing floating lake ice from grounded ice. Coherence is low when the lake ice is growing, because backscattering surface (ice-water interface) changes with every acquisition. Coherence increases significantly when the ice reaches the lake bottom, creating permanent backscattering surface. Our study is based on TerraSAR-X data for the 2012-2013 and 2013-2014 winter seasons. We investigated the use of TerraSAR-X backscatter and interferometric coherence time series with high spatial (3 m) and temporal (11 days) resolution for the detection of grounded ice, for monitoring ice growth stages, for the detection of timing of ice grounding and furthermore, for the derivation of the ice thickness in the number of shallow and deep thermokarst lakes in the Lena River Delta. Ice thickness and water depth measurements obtained from these lakes during a field program in the spring of 2015 are used for ground validation. Grosse G, Jones B, Arp C. 2013. Thermokarst lakes, drainage, and drained basins. In Treatise on Geomorphology. Shroder JF (ed). Elsevier: Amsterdam; Vol. 8: 325–353. Langer M, Westermann S, Anthony KM, Wischnewski K, Boike J. 2015. Frozen ponds: production and storage of methane during the Arctic winter in a lowland tundra landscape in northern Siberia, Lena River Delta. Biogeosciences 12, 977-990. DOI: 10.5194/bg-12-977-2015. Atwood D, Gunn G, Roussi C, Wu J, Duguay C, Sarabandi K. 2015. Microwave backscatter from Arctic lake ice and polarimetric implications. IEEE Transactions on Geoscience and Remote Sensing 53(11): 5972-5982. DOI: 10.1109/TGRS.2015.2429917