Integrating Sentinel-2 and Sentinel-1 Imagery to Analyze Glacier Velocity Variability of Six Outlet Glaciers in Greenland

Monitoring glacier flow velocity on the Greenland Ice Sheet is crucial for understanding mass balance and assessing its impact on sea level rise. However, the present high temporal resolution velocity primarily derived from Sentinel-1 SAR data, often exhibit gaps during melt seasons due to daily freeze-thaw cycles on the ice surface. 

Here, we constructed a 6-day velocity time series from 2017 to 2021 for six outlet glaciers to comprehensively capture their velocity variations by combining Sentinel-1 and -2 data, including Petermann Glacier, Jakobshavn Isbræ, Helheim Glacier, Kangerlussuaq Glacier, Nioghalvfjerdsfjorden Glacier, and Zachariæ Isstrøm Glacier. The offset-tracking technique was applied to derive initial velocity time series from SAR and optical data separately, pairing each image with its three subsequent acquisitions. A least squares method based on connected components then calculates the time series for Sentinel-1 and Sentinel-2 separately, which were then fused using a weighted least squares method, with weights determined by RMSEs. 

Sentinel-2 data effectively filled the summer gaps of the glacier velocity time series that only generated with Sentinel-1 imagery (such as NSIDC-0766), improving the coverage rates by over 30% in summer. The filled gaps concentrated in the elevation range of 600-1400 meter for Petermann Glacier, Nioghalvfjerdsfjorden Glacier, and Zachariæ Isstrøm Glacier, while for Jakobshavn Isbræ, it was most prominent between 1000-1800 meter. The coverage increase for Helheim Glacier and Kangerlussuaq Glacier is most significant in the elevation of 1500-2000 meter. These improvements are primarily observed in the radar glacier zones of wet snow zone and the percolation zone, where daily freeze-thaw more frequently occurred, leading to decoherent of its surface backscattering. In contrast, improvements are less pronounced at the dry snow zone where no thawing occurs and ice crevasses distributed glacier terminus with abundant features for offset-tracking.

At the groundline, Petermann exhibited relatively stable flow, ranging from 1.17 to 1.20 km/yr. Jakobshavn Isbræ showed significant variability, peaking at 4.39 km/yr in 2019 before declining to 2.52 km/yr in 2021. Helheim displayed lower velocities, ranging from 0.15 to 0.26 km/yr, while Kangerlussuaq maintained consistently high flow rates of 4.16 to 4.57 km/yr. Zachariæ Isstrøm demonstrated a steady increase from 0.68 to 0.74 km/yr, and Nioghalvfjerdsfjorden showed minor variations, ranging from 1.14 to 1.17 km/yr. The velocity map gap filled by Sentinel-2 revealed quicker flow rates during the summer months, especially for Jakobshavn Isbræ, Kangerlussuaq, and Zachariæ Isstrøm , reaching up to 1.0 m/day, indicating a lower estimation of the glacier mass loss with the flux gate method. As for other outlet glaciers, Petermann Glacier, Nioghalvfjerdsfjorden Glacier and Helheim Glacier, the underestimation of velocity using only Sentinel-1 velocity time series was more pronounced further from the glacier terminus. Precision analysis shows the Sentinel-1 offset-tracking precision is approximately 10 times better than that of Sentinel-2, emphasizing the importance of weighted fusion when combining the datasets.