Supplementary material to "Three different glacier surges at a spot: What satellites observe and what not"

Abstract. In the Karakoram, dozens of glacier surges occurred in the past two decades, making the region one of its global hot spots. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over one or two years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same region but differ strongly in surge behaviour. A full suite of satellite sensors and digital elevation models (DEMs) from different sources are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 km y−1, velocities up to 30 m d−1 and surface elevations increased by 200 m. In contrast, the neighbouring and three times smaller North Chongtar Glacier had a slow and near linear increase of advance rates (up to 500 m y−1), flow velocities below 1 m d−1 and elevation increases 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. It seems that, despite a similar climatic setting, different surge mechanisms are at play and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges and elevations from ICESat-2 ATL06 data fit neatly. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes.