An empirical algorithm to map perennial firn aquifers and ice slabs 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: January 2022

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.