1. Introduction
Thermokarst lakes and ponds are abundant and characteristic landscape features of the Arctic lowland permafrost regions on both the Eurasian and North American continents. Estimates of their areal coverage span a wide range and are scale dependent [
1,
2,
3], but all agree that they can occupy a significant proportion of the land area in high latitude regions (up to 40% in some areas [
3]). Arctic water bodies play a crucial role in land-atmosphere exchanges of greenhouses gases and energy fluxes [
4,
5,
6,
7], making them highly important for global climate change science. The seasonal ice cover can reach thicknesses of up to 2 m over an ice season of up to 8–9 months which alters greenhouse gas fluxes, as well as biological productivity within these water bodies. The ice-cover duration is also an important indicator of climate variability and knowledge of lake ice phenology from freeze onset to water-clear-of-ice is important in particular for regional climate change assessments [
8,
9].
Shallow water bodies which have depths of less than the maximum ice thickness eventually experience grounding to the bottom (i.e., bedfast ice) during the winter time. After ice grounding, such water bodies significantly decrease their contribution to energy and gas fluxes as the biological activity is suppressed within the frozen sediments [
10,
11]. Deeper water bodies where unfrozen water remains beneath the ice cover during the entire winter period favor the development of a talik (permanently unfrozen inclusion within permafrost). Therefore, distinguishing grounded from floating ice areas in high latitude lakes is essential for permafrost science from physical, chemical and biological aspects. Changes in lake ice regime from a floating to a grounded state and vice versa, due to water level change or ice thickness variations, can also be used as an indicator of climate variability [
12,
13]. Variations in the fraction of ice that freezes to bed can also be a result of lake drainage or lake expansion indicating landscape transformations caused by local impacts of climate change [
14].
Monitoring lake ice phenology and the fraction of bedfast ice using ground observations is not possible on a regular basis and over larger areas in remote Arctic territories. Remote sensing can provide frequent and spatially representative information on the ice regime of Arctic lakes. Optical remote sensing in the Arctic is limited due to polar night and often persistent cloud cover [
15,
16]. Active microwave radar signals, on the other hand, penetrate through cloud cover and allow for systematic monitoring of lake ice phenology [
17,
18,
19,
20]. Moreover, the difference in radar backscatter intensities between grounded (bedfast) and floating lake ice allows for mapping of these areas and estimation of the timing of grounding. The application of radar technology for this purpose began in the 1970s when airborne radar systems were used to acquire images over Alaskan lakes [
21,
22,
23]. These early studies were limited to the visual (qualitative) interpretation of imagery since no calibrated digital data were available. In the new era of spaceborne Synthetic Aperture Radar (SAR) in the 1990s, a number of studies utilized calibrated ERS-1 data (C-band) and the quantitative analysis of backscatter intensity for different periods during ice growth [
24,
25,
26]. However, the relatively low spatial resolution of SAR data used in these studies (240 m pixel size) confined the analysis to relatively large lakes, omitting the abundant number of water bodies smaller than the resolution. Advancements in technology have allowed for lake ice studies using higher spatial resolution (100 m or better) data from C-band RADARSAT-1, ERS-1 and ENVISAT ASAR [
12,
13,
27,
28,
29]. One of the main constraints for SAR-based investigations to date has been the insufficient temporal resolution of time series, so that acquisitions obtained with different incidence angles or a multi-sensor approach must be employed [
20,
27]. Monitoring lake ice conditions using X-band SAR data has mostly been limited to the early airborne studies mentioned above. The potential of data from new generation X-band SAR, such as TSX with its spatial resolution up to 3 m, is far from fully investigated. Using a single high-resolution Spotlight TSX image, Jones et al. [
30] showed its excellent suitability for distinguishing bedfast from floating lake ice. Sobiech and Dirking [
31] used TSX imagery for the classification of ice and open water fractions during the ice decay period in lakes of the Lena River Delta. To our best knowledge, the latter study is also the only example of SAR-based investigation of lake ice conditions in the Siberian Arctic.
SAR-derived timing of ice grounding in combination with bathymetry information can be used as a proxy for the estimation of ice growth [
28,
29]. Another approach is to use a numerical ice growth model: SAR-derived date of ice grounding can be assigned to a simulated ice thickness on that date, which, in turn, provides information on the depth of a lake (i.e., bathymetry) [
26,
32].
In this study, we investigate the potential of high temporal resolution (11-day) TSX backscatter intensity time series for monitoring ice phenology and ice grounding during three years on lakes in the Lena River Delta, Siberia. In addition, for the first time, we produce a sequential interferometric coherence time series for detection of ice grounding and compare it to the backscatter intensity time series. Using TSX-derived timing of ice grounding and the numerical lake ice model CLIMo, we retrieve the thickness of bedfast ice and evaluate the results against in situ measured ice thicknesses.
3. Study Area
The Lena River Delta (73°N, 126°E) in Siberia occupies an area of about 30,000 km² and is located in the zone of continuous permafrost that reaches depths of up to 600 m [
43]. The region belongs to the typical Arctic tundra ecozone. Water bodies of different size cover about 20% of the delta’s land area [
44]. The climate of the Lena River Delta area is characterized by extremely cold, long winters and short, cool summers. Boike et al. [
45] described the recent climatic characteristics of the region based on regular measurements on Samoylov Island in the southern part of the delta (
Figure 1) during the period from 1998 to 2011. The annual mean air temperature was −12.5 °C with February mean temperatures of −33.1 °C and July mean temperatures of 10.1 °C. Rainfall usually took more than half of annual precipitation with a mean of 125 mm. Snow accumulation usually began in October and snow melt typically started in the second half of May and lasted until early June. The snow depth featured high spatial heterogeneity due to microtopographic features (i.e., polygonal relief) and snow redistribution by wind, but typically did not exceed several decimeters.
In this study, we focus on the southern part of the delta, particularly on Kurungnakh-Sise (or Kurungnakh) Island (
Figure 1). The main part of the island consists of Pleistocene Ice Complex deposits (also known as Yedoma), which are underlain by fluvial sands and covered by a thin Holocene layer. The island has a maximum elevation of 55 m a.s.l. On its generally flat surface, deep thermokarst lakes and basins have made incisions as a result of permafrost degradation that started about 12 ka ago. About 7.5% and 38% of the island area is covered by thermokarst lakes and basins, respectively. Only 16% of the total basin area is occupied by remaining lakes, indicating past and current drainage processes [
46].