Abstract
AbstractFirn compaction models inform mass-balance estimates and paleo-climate reconstructions, but current models introduce key uncertainties. For example, models disagree on the dependence of density and compaction on accumulation rate. Observations of compaction to test these models are rare, partly because in situ methods for measuring englacial strain are time-consuming and expensive. Moreover, shallow measurements may confound strain due to compaction with strain due to ice-sheet flow. We show that phase-sensitive radio-echo sounder (pRES) systems, typically deployed to measure sub-shelf melting or ice-sheet deformation, can be used to measure firn compaction and test firn models. We present two complementary methods for extracting compaction information from pRES data, along with a method for comparing compaction models to pRES observations. The methods make different assumptions about the density structure and vary in their need for independent density measurements. Compaction profiles computed from pRES data collected on three ice rises in West Antarctica are largely consistent with measured densities and a physics-based model. With their minimal logistic requirements, new pRES systems, such as autonomous pRES, could be inexpensively deployed to monitor firn compaction more widely. Existing phase-sensitive radar data may contain compaction information even when surveys targeted other processes.
Highlights
Firn is partially metamorphosed snow that has survived at least 1 year on an ice-sheet or glacier surface
Helsen and others (2008) suggest that surface-height changes associated with the impact of temperature and accumulation on firn densification were the same order of magnitude as those observed by altimetry over East Antarctica between 1980 and 2004
Using conservation of mass and an exponential expression to describe density, this simple inverse method produced compaction rates that broadly agree with the results of Method 1 and modeled compaction rates, even in the presence of the erroneous vertical variations in phase-sensitive radio-echo sounder (pRES)-derived velocities seen at some Skytrain Ice Rise (SIR) locations
Summary
Firn is partially metamorphosed snow that has survived at least 1 year on an ice-sheet or glacier surface. Helsen and others (2008) suggest that surface-height changes associated with the impact of temperature and accumulation on firn densification were the same order of magnitude as those observed by altimetry over East Antarctica between 1980 and 2004. Smith and others (2020) modeled changes in the densification rate on the order of surface height changes measured by ICESat-2 in 2018–19 in some Antarctic drainage basins (their Fig. S8). These densification rate changes are transient; Ligtenberg and others (2014) suggest that using a steady-state firn densification model can underestimate Antarctic mass loss by up to 23% compared to a time-dependent model
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