An empirical algorithm to map perennial firn aquifers, ice slabs, and perched firn aquifers within the Greenland Ice Sheet using satellite L-band microwave radiometry
Authors: Miller Julie Z. , Culberg Riley , Long David G. , Shuman Christopher A. , Schroeder Dustin M. , Brodzik Mary J. 
Published: April 2021

Abstract. Perennial firn aquifers are subsurface meltwater reservoirs formed from a water-saturated firn layer. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly-continuous ice layers that form on spatial scales of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting, but in areas where snow accumulation is at least ~25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be inferred from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth. Here, for the first time, we use enhanced-resolution vertically-polarized L-band brightness temperature (TBV) imagery (2015–2019) generated using observations collected over the GrIS by NASA’s Soil Moisture Active Passive (SMAP) satellite to map both perennial firn aquifer and ice slab areas as a continuous system over the percolation facies. We also map “perched” firn aquifer areas, which we define as areas where shallow water-saturated firn layers transiently form on top of buried ice slabs, or other semi-impermeable layers within the snow and firn. An empirical algorithm previously developed to map the extent of Greenland’s perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves is recalibrated to also map the extent of ice slab and perched firn aquifer areas using airborne ice-penetrating radar surveys collected by NASA’s Operation Ice Bridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over ~64,000 km2, ice slab areas extended over ~76,000 km2, and perched firn aquifer areas extended over ~37,000 km2. Combined together, these three sub-facies are the equivalent of ~24 % of the percolation facies of the GrIS. As Greenland’s climate continues to warm, and seasonal surface melting increases in extent, intensity, and duration, quantifying the possible rapid expansion of each of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice sheet-wide variability in englacial firn hydrology resulting in meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation run-off that can impact the mass balance and stability of the GrIS.