Sentinel-1 based time series of ice slab extent reveals regional divergence in ice slab evolution

The growth of ice slabs influences both the spatial extent and the rate at which the Greenland Ice Sheet’s wet snow zone transitions from storing meltwater in firn, to exporting it as surface runoff. These changes reorganize meltwater storage and flow pathways, shaping the ice sheet’s contribution to sea level rise. In this work, we combine Sentinel-1 (S1) C-band satellite radar mosaics with Operation IceBridge (OIB) airborne radar profiles to produce a decade long time series of ice slab expansion in Greenland. To efficiently process the large S1 data volume, we use Google Earth Engine to compile Interferometric Wide and Extra Wide Swath data acquired during boreal winters (1 November – 30 March) from 2015 to 2025, producing annual co-polarized (HH) and cross-polarized (HV) backscatter mosaics that are multilooked and linearly corrected to a common incidence angle. We then use logistic regression to optimize ice slab detection thresholds and to quantify classification uncertainty.

Our time series reveals pronounced differences in the rate of ice slab expansion between northern and southwest Greenland. Isolated OIB radargrams also suggest marked differences in ice slab geometry between these two regions. In northern Greenland, thick downflow ice slabs transition abruptly into laterally extensive, thin ice slabs that extend tens of kilometers upslope into the percolation zone. In contrast, ice slabs in southwest Greenland either remain thick at their upflow fronts or, when thin, occur deeper in the firn column and are rapidly buried by subsequent accumulation events. To capture these contrasting ice slab front geometries, we develop a refined classification scheme to map thick and thin ice slabs across the Greenland Ice Sheet using Sentinel-1 backscatter thresholds. The observed spatial and temporal patterns point to regions where atmospheric forcings and percolation zone firn conditions have restricted meltwater infiltration depth, accelerating shallow ice slab growth and altering the near-surface hydrologic regime.