Operational System for Estimating Compaction of Arctic Glacial Firn and Surface Mass Balance

NASA and other agencies are invested in obtaining measurements of Greenland Ice Sheet surface elevation with the objective of constraining the ice sheet's contribution to present and future sea level rise. However, a major limitation of repeat altimetry measurements, such as air and space-based surveys, is that variations in the density of surface accumulation and the firn layer must be constrained in order to relate changes in ice thickness to the overall ice sheet mass imbalance. The temporal variability of accumulation thickness and firn compaction rate is poorly constrained, particularly in regions subject to surface melt and refreezing. Additionally, recent warming and extreme melt events may be substantially changing the density of the firn, affecting altimeter measurements. This knowledge gap about snow accumulation and firn density, therefore, is a critical challenge to the ICESat-2 mission objectives. Our in-situ sensor packages, dubbed the Compaction Reconnaissance of Arctic Glacial Snow (CRAGS), will aid the deconvolution of surface change observations, with the goal of providing an operational system in concert with the launch of ICESat-2. The CRAGS prototype was deployed in April 2014 at 69.0754N, -45.6603E, and two fully-equipped systems were deployed in April 2015 at 69.0952N, -46.4446E and 69.0527N, -44.4281E. CRAGS is a tower system equipped with three instruments for measuring snow accumulation: a strain sensor, a sonar, and a snow scale. These systems will be taking measurements for at least two more years, but they are designed to last longer with limited maintenance. The use of a modular tower design allows for them to be extended higher for many years of observations. This method will supplement air and space-based observations, providing the polar science community with valuable ground-truth data regarding the state of the interior of the Greenland ice sheet.