Brief communication: Mapping Greenland's perennial firn aquifers using enhanced-resolution L-band brightness temperature image time series

Julie Z. Miller,Mary J. Brodzik,Ted A. Scambos,Kenneth C. Jezek,Joel T. Johnson,Christopher A. Shuman,Lora S. Koenig,David G. Long

doi:10.5194/tc-14-2809-2020

Abstract

Abstract. Enhanced-resolution L-band brightness temperature (TB) image time series generated from observations collected over the Greenland Ice Sheet by NASA's Soil Moisture Active Passive (SMAP) satellite are used to map Greenland's perennial firn aquifers from space. Exponentially decreasing L-band TB signatures are correlated with perennial firn aquifer areas identified via the Center for Remote Sensing of Ice Sheets (CReSIS) Multi-Channel Coherent Radar Depth Sounder (MCoRDS) that was flown by NASA's Operation IceBridge (OIB) campaign. An empirical algorithm to map extent is developed by fitting these signatures to a set of sigmoidal curves. During the spring of 2016, perennial firn aquifer areas are found to extend over ∼66 000 km2.

Highlights

Firn is a porous layer of recrystallized snow near the surface of a glacier or an ice sheet
The L-band TB-derived perennial firn aquifer extent is generally consistent with previous C-band (5.3 GHz) satellite radar scatterometer-derived extents mapped using the Advanced SCATterometer (ASCAT) on the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Operational A (MetOp-A) satellite (2009–2016, ∼ 52 000–153 000 km2; Miller, 2019) and the Active Microwave Instrument in scatterometer mode (ESCAT) on the ESA’s European Remote Sensing (ERS) satellite series (1992–2001, ∼ 37 000–64 000 km2; Miller, 2019) as well as with the C-band (5.4 GHz) synthetic aperture radar-derived extent mapped using the synthetic aperture radar on the ESA’s Sentinel-1 satellite (2014–2019, 54 000 km2; Brangers et al, 2020)
We have derived an empirical algorithm by analyzing spatiotemporal differences in exponentially decreasing TB signatures over Greenland’s percolation facies

Summary

Introduction

Firn is a porous layer of recrystallized snow near the surface of a glacier or an ice sheet. Given sufficiently high surface melting, firn aquifers form, or recharge, in the percolation facies as a result of vertical and lateral percolation of meltwater into the pore space of firn layers overlying impermeable ice layers. Greenland’s firn aquifers store meltwater seasonally, intermittently or perennially, depending on location and climate. They range from shallow water-saturated firn layers that perch on top of near-surface ice layers to deeper water-saturated firn layers that extend from the ice sheet surface to the firn–ice transition. Simulations using a simple firn model suggest that high snowfall thermally insulates water-saturated firn layers, allowing meltwater to be stored in liquid form throughout the freezing season (i.e., the time between surface freeze-up to melt onset) if the overlying snow layer is sufficiently deep (Munneke et al, 2014)

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Conclusion