Investigation on the Dynamics of a Greenland Glacier using TanDEM-X Dual-Pol SAR Data

Kurzfassung

Being extremely sensitive to temperature variations, glaciers and ice sheets are good indicators of climate change. Therefore, the understanding of their dynamics is of fundamental importance for the scientific community. The emergence of airborne and satellite remote sensing techniques has led to unprecedented improvements of glaciological observations in term of spatial coverage, temporal and spatial resolution. In this context, synthetic aperture radars (SAR) represent a powerful tool. Thanks to their high resolution, wide coverage, and the capability to operate independently from the sunlight and in any weather conditions, SARs have become the most used techniques, replacing the more traditional monitoring methods, such as in-situ measurements. SARs employ microwave frequencies which are able to penetrate into dry snow and ice, potentially allowing to infer information about subsurface layers. This makes SAR a suitable tool for the retrieval of a number of glaciological parameters, such as glacier facies extent, snow and firn grain size, accumulation rate and internal layering, which are still hard to obtain with other remote sensing techniques. Particular attention has been given to polarimetric SAR (PolSAR) and interferometric SAR (InSAR), which represent extensions of conventional SAR. The first one exploits electromagnetic waves with different polarizations to gain sensitivity to geometrical and dielectric properties of the scatterers. The second exploits SAR acquisition pairs to derive information about the vertical distribution of the scatterers. However, despite this great potential, it is not entirely clear how electromagnetic waves interact with a complex glacier scenario, where a large number of factors can influence the backscattered signal. The objective of this thesis is to investigate the sensitivity of dual-polarisation (HH and VV) X-band SAR time series to seasonal and inter-annual glacier dynamics, such as surface melt/freeze cycle, snow accumulation, ice zone extent and inter-annual variations, by analyzing several polarimetric and interferometric observables. The work is based on a 3-year (2011-2014) time series of SAR data acquired by the German X-band satellite Mission TanDEM-X over the Helheim glacier, one of the largest outlet glaciers of the Greenland ice sheet. A first part of the study addresses the estimation and interpretation of the selected (InSAR and PolSAR) observables for the identification of the dominant scattering mechanisms and their variations over time. Results showed that freeze/melt cycles can be clearly tracked with all parameters, which also pointed out changes in the scattering mechanisms between the accumulation and melting season. During winter, all indicators had a very constant behavior and indicated that the signal is dominated by volume scattering as the SAR signal easily penetrates the shallow snow cover and interact with subsurface features. In contrast, summer acquisitions showed an increase of surface scattering generated at the wet glacier surface. Finally, late spring and autumn scenes pointed out a gradual Transition from wet to dry conditions and vice versa, confirming the high sensitivity of SAR to glacier surface conditions. Of great interest is the case of the co-polarisation phase difference, which showed large variations during the accumulation season, when all other parameters were very stable over time. Such behavior has been successfully linked to snow accumulation, as confirmed by available ground measurements. In contrast, little information could be retrieved concerning interannual dynamics, mainly concerning differences in snow accumulation. In the second part of the study, the potential of polarimetry for the identification of glacier zones has been addressed. For this, the behavior of the PolSAR indicators w.r.t. elevation was investigated. It was found that the backscattering coefficient has the most potential, as it showed in several cases a clear change of behavior in a well-defined altitude range. This has been identified as possible Equilibrium line altitude of the Helheim glacier, even though no validation could be carried out. On the other hand, the analysis confirmed the limited usefulness of most of the other PolSAR observables mainly due to the limited penetration capability of X-band. Therefore, it is concluded that lower frequency (e.g. C- and L-band) are more suitable for glacier zones observation.