Deriving a time series of 3D glacier motion to investigate interactions of a large mountain glacial system with its glacial lake: Use of Synthetic Aperture Radar Pixel Offset-Small Baseline Subset technique

We investigated the interactions of Lake Merzbacher with the Southern Inylchek Glacier (Central Tien Shan) using the Synthetic Aperture Radar (SAR) Pixel Offset-Small Baseline Subset (PO-SBAS) to derive a time series of three-dimensional (3D) glacier motion. The measurements of 3D glacier velocity were ∼17% more precise than a previous study that did not use the SBAS estimation. The velocities of the glacier were up to 58 cm/day east, 70 cm/day north, and 113 cm/day vertically. Combining these data with Landsat images indicated that movement of the glacier is sensitive to changes of Lake Merzbacher. Specifically, the entry of more lake water into the glacier during the ablation season increased englacial ablation due to thermal erosion. Moreover, ice calving begins when the lake water gradually lifts the ice dam. Calving can cause greater loss of glacier mass than normal ablation. Trying to replenish the front mass loss, the distributary accelerates and the mass loss further intensifies. A time series of the vertical velocity indicates that the glacier tongue has a huge englacial cavity. We suggest that the lake outburst is directly related to the crack of this cavity. Bursting of the lake triggers a mini-surge at the glacier tongue. The vertical velocity at the ice dam was ∼+60 cm/day before the lake outburst, and ∼−113 cm/day afterwards. After drainage of the lake, flow velocities at the distributary, do not sharply decrease because pre-drainage mass loss needs to be replenished by fast flow. Based on comparisons with previous measurements, our results indicate that the lake had an increasing influence on the glacier from 2005 to 2009. This study demonstrates that a time series of 3D glacier motion based on the PO-SBAS technique is effective for assessing the dynamics of a mountain glacial system and interactions with its glacial lake.