Mapping Firn Saturation Over Greenland Using NASA's Soil Moisture Active Passive Satellite

Mapping the spatial extent of recently identified englacial hydrological features (i.e., ice slabs and perennial firn aquifers) formed by meters-thick water-saturated firn layers over the percolation facies of the Greenland Ice Sheet using L-band microwave radiometry has recently been demonstrated. However, these initial maps are binary, and do not provide a parameter to estimate the spatial variability in the thickness and volumetric fraction of meltwater stored within the firn pore space. Here, we exploit enhanced-resolution vertical-polarization L-band brightness temperature (T-V(B)) imagery (2015-2019) generated using observations collected over Greenland by NASA's Soil Moisture Active Passive (SNAP) satellite and a simple two-layer L-band brightness temperature model. We map water-saturated firn layers via a firn saturation parameter, and interpret our results together with ice slab and perennial firn aquifer spatial extents, estimates of snow accumulation simulated via the Regional Atmospheric Climate Model (RACMOp2.3), and airborne radar surveys collected via NASA's Operation IceBridge (OIB) campaigns. We find that variable firn saturation parameter values are mapped in lower snow accumulation ice slab areas in western, northern, and northeastern Greenland, where firn is colder and water-saturated firn layers seasonally refreeze as solid-ice. Higher firn saturation parameter values are mapped in higher snow accumulation perennial firn aquifer areas in southeastern, southern, and northwestern Greenland, where firn is near the melting point, and meters-thick water-saturated firn layers exist. Our results have implications for identifying expansive englacial reservoirs that store significant volumes of meltwater in locations that are vulnerable to meltwater-induced hydrofracturing and accelerated outlet glacier flow year-round.

Automated summary

Using L-band microwave radiometry, the spatial extent of englacial hydrological features in the Greenland Ice Sheet, such as ice slabs and perennial firn aquifers, has been mapped. A combination of enhanced-resolution imagery and a two-layer brightness temperature model is used to map water-saturated firn layers and analyze the variability in saturation parameter values. The results highlight the differences in firn saturation between areas with low and high snow accumulation. These findings are important for identifying large reservoirs of meltwater that are vulnerable to hydrofracturing and accelerated glacier flow.