Three different glacier surges at a spot: What satellites observe and what not

In the Karakoram, dozens of glacier surges occurred in the past two decades, making the region one of the global hotspots. Detailed analyses of dense time series from available optical and radar satellite images revealed a wide range of surge behaviours in this region: from slow advances characterized by slow ice flow over periods longer than a decade to short, pulse-like advances with high velocity over one or two years.

In this study, we present an analysis of three glaciers that are currently surging in the same region of the central Karakoram: North Chongtar, South Chongtar and an unnamed glacier referred to as NN9. A full suite of optical and SAR satellite sensors and digital elevation models (DEMs) are used to (1) obtain comprehensive information about the evolution of the surges between 2000 and 2021 and (2) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring small glaciers in steep terrain. 

The analysis for (1) reveals a contrasting evolution of advances rates and flow velocities for the three glaciers, while the elevation change pattern is more similar. South Chongtar Glacier showed advance rates of more than 10 km y^-1, velocities up to 30 m d^-1 and surface elevations raised by 200 m. In comparison, the three times smaller North Chongtar Glacier has a slow and almost-linear increase of advance rates (up to 500 m y^-1), flow velocities below 1 m d^-1 and elevation increases of up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 km y^-1, but showing the typical surface lowering higher up only recently. It seems that, despite a similar climatic setting, different surge mechanisms are at play in this region and that the surge mechanisms can change in the course of a single surge. 

On topic (2) we found that sensor performance is dependent on glacier characteristics (size, flow velocity, amplitude of changes). In particular velocities derived from Sentinel-1 performed poorly on small, narrow glaciers in steep environment. The comparison of elevation changes revealed that all considered DEMs have a sufficient accuracy to detect the mass transfer during the surges and that elevations from ICESat-2 ATL06 data fit neatly.