Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Permafrost is a subsurface phenomenon that cannot be directly monitored with satellite remote sensing. A variety of indirect approaches are currently being developed which aim to measure permafrost-related processes and environmental variables. Results of these studies aid the planning of future satellite missions which will allow large-scale permafrost monitoring. This thesis contributes to this ongoing effort by assessing the potential of repeat-pass TerraSAR-X (TSX) time series for permafrost-related applications. For the first time, multi-year Synthetic Aperture Radar (SAR) data with high temporal (11 days) and spatial (3 m) resolution was analysed for a region characterized by continuous permafrost in the Siberian Arctic. Extensive in situ data was collected during three summer and winter expeditions to validate and interpret remote sensing results. Three case studies were carried out: (i) the detection of land surface changes (e.g. ground freezing and thawing, surface wetness variations, snow cover onset and melt); (ii) monitoring bedfast lake ice and ice phenology (freeze-up, melt onset, break-up); and (iii) differential SAR interferometry (DInSAR) for thaw subsidence monitoring. For the first two case studies, time series of both backscatter intensity and 11-day interferometric coherence (i.e. a measure of phase stability between two SAR images) were investigated. Backscatter intensity was generally shown to be insensitive to the land surface changes but responded to events that occurred at the time of TSX acquisition (rain, snow shower, melt/freeze crust on snow). Interferometric coherence decreased dramatically across the entire image upon snow cover onset and melt, permitting the possible use of coherence for the monitoring of these events. Backscatter intensity was found to be an excellent tool for the detection and monitoring of bedfast lake ice due in part to improved temporal resolution compared to previously used SAR systems. Ice phenology was mostly well tracked with backscatter intensity. Interferometric coherence was found to be sensitive to the lake ice grounding and to the onset of surface melt on the lakes with bedfast ice. The investigation of coherence was a useful preparative step for the following DInSAR analysis. For the third case study, coherent 11-day and 22-day interferograms were available only for one summer of the two-year TSX time series. The cumulative DInSAR displacement strongly underestimated the subsidence observed on the ground. In situ observations revealed high variability of subsidence, which likely caused errors in phase unwrapping. Conventional DInSAR processing might therefore not be suitable for the accurate representation of permafrost thaw subsidence. This study highlights the importance of field measurements for the quantification of thaw subsidence with DInSAR, which were mostly omitted in the previous studies. All in all, this thesis shows the limitations and potential of TSX time series to spatially and temporally monitor permafrost. It thus provides an important contribution to the methodological development of a long-term permafrost monitoring scheme.