Abstract
Abstract. Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. 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 can also form on spatial scales as large as tens 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 identified 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 (TVB) imagery (2015–2019) generated using observations collected over the GrIS by NASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use 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. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA's Operation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64 000 km2, and ice slab areas extended over 76 000 km2. Combined together, these sub-facies are the equivalent of 24 % of the percolation facies of the GrIS. As Greenland's climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice-sheet-wide variability in englacial hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.
Highlights
The recent launches of several satellite L-band microwave radiometry missions by NASA (Aquarius mission, Le Vine et al, 2007; Soil Moisture Active Passive (SMAP) mission, Entekhabi et al, 2010) and ESA (Soil Moisture and Ocean Salinity (SMOS), Kerr et al, 2001) have provided a new Earth-observation tool capable of detecting meltwater stored tens of meters to kilometers beneath the ice sheet surface. Jezek et al (2015) recently demonstrated that in the highelevation (3500 m a.s.l.) dry snow facies of the Antarctic Ice Sheet, meltwater stored in subglacial Lake Vostok can be detected as deep as 4 km beneath the ice sheet surface
Miller et al (2020) cited significant uncertainty in the SMAP-derived perennial firn aquifer extent as a result of the lack of a distinct temporal L-band signature delineating the boundary between perennial firn aquifer areas and adjacent percolation facies areas
For the first time, we have demonstrated the novel use of the L-band microwave radiometer on NASA’s SMAP satellite for mapping perennial firn aquifers and ice slabs together as a continuous englacial hydrological system over the percolation facies of the Greenland Ice Sheet (GrIS)
Summary
The recent launches of several satellite L-band microwave radiometry missions by NASA (Aquarius mission, Le Vine et al, 2007; Soil Moisture Active Passive (SMAP) mission, Entekhabi et al, 2010) and ESA (Soil Moisture and Ocean Salinity (SMOS), Kerr et al, 2001) have provided a new Earth-observation tool capable of detecting meltwater stored tens of meters to kilometers beneath the ice sheet surface. Jezek et al (2015) recently demonstrated that in the highelevation (3500 m a.s.l.) dry snow facies of the Antarctic Ice Sheet, meltwater stored in subglacial Lake Vostok can be detected as deep as 4 km beneath the ice sheet surface. Upwelling L-band emission from the radiometrically warm bedrock underlying the subglacial lakes is effectively blocked by high reflectivity and attenuation at the interface between the bedrock and the overlying lake bottom This results in a lower observed microwave brightness temperature (T B) at the ice sheet surface as compared to other dry snow facies areas where bedrock contributes to L-band emission depth-integrated over the entire ice sheet thickness. Perennial firn aquifers have been identified via field expeditions (Forster et al, 2014), airborne ice-penetrating radar surveys (Miège et al, 2016), and satellite microwave sensors (Brangers et al, 2020; Miller et al, 2020) in the lower-elevation (< 2000 m a.s.l.) percolation facies of the Greenland Ice Sheet (GrIS) at depths from between 1 and 40 m beneath the ice sheet surface. Upwelling L-band emission from deeper glacial ice and the underlying bedrock is effectively blocked
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