Description: Northward shift of the treeline is expected circum-Arctic and has been observed in a number of locations in response to Arctic warming. The transitional zone between forest and tundra is, therefore, a vulnerable region that requires systematic monitoring. Currently, radar remote sensing is hardly employed in the treeline zone. The unique constellation of the TanDEM-X satellites with its bistatic mode and unprecedented spatial resolution opens new opportunities for monitoring of the treeline zone. We focus on an area near the Trail Valley Creek research basin in the east of the Mackenzie Delta in the Northwest Territories, Canada. The area lies at the northern edge of the treeline zone. Erect vegetation there is characterised by deciduous shrubs up to 3 m in height and isolated patches of sparse coniferous forest. We evaluate the potential of TanDEM-X bistatic data to characterise the structural properties of the forest patches. The TanDEM-X data were acquired during the TanDEM-X Science Phase in 2015, when the effective baseline was large and constant (approximately 540 m). We employ interferometric coherence from multitemporal bistatic pairs and compare it with standard vegetation metrics obtained from airborne LiDAR data. The full-waveform airborne LiDAR data were captured in September 2016, covering an area of about 20 km x 6 km with a point density of approximately 5 points per square meter. LiDAR metrics include vegetation height percentiles and vegetation ratio. The preliminary analysis shows a high agreement between TanDEM-X bistatic coherence and LiDAR vegetation metrics. The relation between coherence and LiDAR metrics, averaged for each forest patch, yields in a strong inverse correlation, varying from -0.81 to -0.88 for different LiDAR metrics. On sub- atch scale, spatial patterns of coherence and LiDAR metrics also show high inverse correspondence. Thus, a pixel-by-pixel comparison gives a first-shot correlation between tree height 99 percentile and coherence from -0.45 to -0.63 for different forest patches. Taking into account the global coverage of multiple bistatic TanDEM-X data acquired for the global digital elevation model, our results provide a basis for the quantification of the treeline properties circum-Arctic.

Description: In permafrost areas, seasonal freeze-thaw cycles of active layer result in upward and downward movements of the ground. Additionally, relatively uniform thawing of the ice-rich layer at the permafrost table, contributing to net long-term surface lowering, was reported for some Arctic locations. We use a simple method to quantify surface lowering (subsidence) and uplift in a yedoma area of the Lena River Delta, Siberian Arctic, using reference rods installed deeply in permafrost. The seasonal subsidence was 1.7 ±1.5 cm in the cold summer of 2013 and 4.8 ± 2 cm in the warm summer of 2014. Furthermore, we measured a pronounced multi-year net subsidence of 9.3 ± 5.7 cm from spring 2013 to the end of summer 2017. Additionally, we observed a high spatial variability of subsidence of up to 6 cm across a sub-meter horizontal scale. This variability limits the usage of a pointwise measurement for a validation of spatially extensive remote sensing products. In summer 2013, we accompanied our field measurements with Differential Synthetic Aperture Radar Interferometry (DInSAR) on repeat-pass TerraSAR-X (TSX) data over the same study area. Interferometry was strongly affected by a fast phase coherence loss, atmospheric artifacts, and possibly the choice of reference point. A cumulative ground displacement map, built from a continuous interferogram stack, did not reveal a meaningful signal on the upland but showed a distinct subsidence of up to 2 cm in most of the thermokarst basins. There, the spatial pattern of displacement corresponded well with relative surface wetness identified with the near infra-red band of a high-resolution optical image. Our study suggests that (i) although X-band SAR has serious limitations for ground movement monitoring in permafrost landscapes, it can provide valuable information for specific environments like thermokarst basins, and (ii) due to the high sub-pixel spatial variability of ground movements, a validation scheme needs to be developed and implemented for future DInSAR studies in permafrost environments.

Description: Low-land permafrost areas are subject to intense freeze-thaw cycles and characterized by remarkable surface displacement. We used Sentinel-1 SAR interferometry (InSAR) in order to analyse the summer surface displacement over four spots in the Arctic and Antarctica since 2015. Choosing floodplain or outcrop areas as the reference for the InSAR relative deformation measurements, we found maximum subsidence of about 3 to 10 cm during the thawing season with generally high spatial variability. Sentinel-1 time-series of interferograms with 6–12 day time intervals highlight that subsidence is often occurring rather quickly within roughly one month in early summer. Intercomparison of summer subsidence from Sentinel-1 in 2017 with TerraSAR-X in 2013 over part of the Lena River Delta (Russia) shows a high spatial agreement between both SAR systems. A comparison with in-situ measurements for the summer of 2014 over the Lena River Delta indicates a pronounced downward movement of several centimetres in both cases but does not reveal a spatial correspondence between InSAR and local in-situ measurements. For the reconstruction of longer time-series of deformation, yearly Sentinel-1 interferograms from the end of the summer were considered. However, in order to infer an effective subsidence of the surface through melting of excess ice layers over multi-annual scales with Sentinel-1, a longer observation time period is necessary.