Observations and models of englacial deformation during supraglacial lake drainage

The hydrofracture-driven drainage of supraglacial lakes rapidly introduces large volumes of meltwater to the ice-sheet bed, influencing ice-sheet dynamics on multiple timescales. Immediate ice deformation mainly arises from three sources: the opening of the hydrofracture crack, separation of the ice from the bed, and additional slip at the bed. An understanding of the ice response to drainage requires knowledge of these spatially and temporally varying sources, ideally constrained by observations obtained both on the ice surface and within the ice column. Previous work examining ice dynamics during supraglacial lake drainage relies on ice-surface observations only: aerial and satellite imagery, Global Navigation Satellite System (GNSS) data, and pressure-sensor records from draining lakes. We deployed three autonomous phase-sensitive radio echo sounders (ApRES) near a set of three supraglacial lakes at ~950 m elevation, in the mid-ablation zone of the western Greenland Ice Sheet, to record englacial deformation during lake drainage. The ApRES stations were embedded within a geophysical network including GNSS stations, air-temperature sensors, and a lake pressure logger, and were configured to make repeat measurements every 15 minutes from May 2022 to September 2023. In 2022, two of the lakes adjacent to the ApRES stations drained abruptly via hydrofracture, exhibiting characteristics of inter-lake static-stress triggering; in 2023, all three lakes drained in a similar manner. We demonstrate the capability of the ApRES system to provide estimates of the time-varying change in englacial vertical strain rate that accompanies hydrofracture-driven lake drainage, despite the short durations of the drainages and the wet and variable ice surface that is inevitable during the melt season. At station locations ~1 km away from the hydrofracture cracks, we observe vertical strain rates of magnitude up to ~1 yr^-1 during lake drainages, averaged over the top 500 m of ice and over 15 minutes; background vertical strain rates have magnitudes of ~10^-3 yr^-1 at these locations. As a first step towards incorporating these englacial observations of deformation as constraints on an inverse problem to obtain the spatial and temporal history of the deformation source, we compare the englacial observations to predictions from a source model constructed using only GNSS data. Following previous work, we use an elastic dislocation model and invert the GNSS data to obtain time- and space-varying estimates of the opening of the hydrofracture crack, opening at the ice-bed interface, and excess slip at the bed during lake drainage. We then use this model to predict changes in strain in the ice under the ApRES stations, and compare the resulting timeseries with our observations. We evaluate the additional sensitivity provided by our englacial observations to the deformation source.