Firn Compaction in Greenland and Antarctica: A Novel Automated Method Using Repeat Airborne Observations of Stratigraphy

Accurately quantifying the rate at which snow compacts into firn and glacial ice, known as firn compaction, is important for converting volume changes detected through satellite altimetry to ice mass changes, which determines an ice sheet’s contribution to sea level change. Measuring firn compaction in situ requires tracking firn layers as they are compacted and buried by new snow through time; therefore, these measurements are challenging to make and very sparse across Greenland and Antarctica. Multiple firn compaction models have been developed, but without widespread observations for comparison, the models cannot yet be thoroughly validated. However, the University of Kansas snow radar onboard NASA’s Operation IceBridge airborne field campaigns offers a unique perspective to analyze the snow and firn stratigraphy through its decade-long, near annual campaigns. By tracking strong radar reflections, which we refer to as layers, and comparing their downward progression and separation over years, it is possible to calculate firn compaction. However, manually identifying and tracking layers for each repeated flight is very time-consuming and subject to human error. In this study, we present an automated technique to measure firn compaction by comparing the radar return power vertical profiles (or layer stratigraphy) that are collocated in space but separated in time by at least one year. We apply various vertical compaction and burial scenarios to the older layer stratigraphy using an empirical firn compaction model. We determine the combination of total column compaction and burial that best fits the newer layer stratigraphy. The process effectively tracks various stratigraphic horizons through vertical space and time, and given that we use airborne radar, allows us to do so over 1000s of km of tracks. A compaction rate dataset of this level of coverage does not yet exist for either Greenland or Antarctica, which when completed, will provide unparalleled observational constraint of firn compaction rates over Earth’s ice sheets.