Mapping Greenland ice sheet velocities at high temporal resolution using satellite based imagery

In this thesis, I develop and demonstrate a system for monitoring fluctuations in the speed of Greenland ice sheet outlet glaciers with high temporal frequency from imagery acquired by a range of satellite missions. This work is motivated by an ambition to utilise a new era of operational satellites to better understand how environmental changes are affecting the flow and mass of Greenland’s outlet glaciers. First, I exploited the systematic and frequent acquisition schedule of the Sentinel-1 satellite constellation to track weekly variations in the speed of four fast-flowing, marine-terminating glaciers - Jakobshavn Isbræ, Petermann Glacier, Zachariæ Isstrøm and Nioghalvfjerdsfjorden - between 2015–2017. By combining the Sentinel-1 data with an eight-year time-series derived from TerraSAR-X, I produced a decadal record of variations in glacier flow. On a technical level, I was able to demonstrate the value of Sentinel-1’s 6-day revisit time for glaciology, because it leads to an increase in the degree of correlation between consecutive images and also to improved tracking of movement near to the glacier calving fronts. On a scientific level, I was able to demonstrate that a strong correlation exists between iceberg calving events and glacier speedup, and to show for the first time that Jakobshavn Isbræ has begun to slow down. Next, I assessed the capability of the Sentinel-1 constellation to detect and chart seasonal changes in the speed of five slow-flowing glaciers situated in a 14,000 km2 land-terminating sector of central-west Greenland. These new measurements offer significantly improved spatial and temporal resolution when compared to previous missions, in all seasons. I was able to show that there are marked differences in the degree of seasonal speedup of the five glaciers – with summertime increases in ice flow ranging from 21 to 49 % - reinforcing the need for comprehensive monitoring and the challenges of making regional extrapolations. Thanks to the high temporal frequency afforded by Sentinel-1, I was also able to document for the first time the detailed spatial pattern of speedup persistence, and to show that short- lived peaks of melting match transient spikes in glacier velocity. Finally, I explored the added value and complementarity of the Sentinel-2 multi- spectral instrument (MSI) for tracking ice motion. I was able to combine measurements acquired by Sentinel-1 and Sentinel-2 to detect short-term changes in iceberg drift, iceberg calving, ice motion, and supraglacial lake area at Jakobshavn Isbræ. I also showed that measurements of glacier flow determined from both satellites are in good agreement, and that the spatial coverage they afford is greatest in opposing seasons, illustrating the promise of Sentinel-2 for glaciology.