Investigations of physical processes in polar firn through modeling and field measurements

Thesis (Ph.D.)--University of Washington, 2018Evolution of firn on the polar ice sheets is important for several applications in glaciology, including interpreting climate records from ice cores and correcting ice-sheet-elevation change measurements from altimetry for changes in the amount of air in firn. I have developed the Community Firn Model (CFM), a modular model framework that can simulate numerous physical processes in firn, including densification, air transport, heat diffusion, and grain growth. In this dissertation, I use the CFM and measurements from Greenland to investigate firn densification, gas transport in firn, and uncertainty within firn models. First, I have coupled a firn-air model to a firn-densification model. I use this new model to investigate air transport in the lock-in zone and show that the lock-in zone could be created by impermeable layers, or it could be caused by differences between the advective and diffusive timescales of air transport. I also show that δ^15 N and δ^40 Ar data from the GISP2 ice core can be better explained than previously by including transient firn thickening during rapid climate-change events. Additionally, I explore the effects that impermeable layers in the firn have on gas records. I conclude that including transient firn evolution is essential to modeling gas transport correctly during climate changes, but limited knowledge of the microstructure of firn limits our ability to model firn-air transport accurately in the past. Next, I intercompare 11 firn-densification models and test their sensitivities to initial and boundary conditions at Summit, Greenland. I show that there is no ‘best’ firn model, and that there are numerous sources of uncertainty in firn models. The uncertainty in predicting the depth-integrated porosity of the firn is ~10%, and the uncertainty in predicting the depth and age of the firn-ice transition is greater than 10%. Uncertainties in surface boundary conditions, including the climate forcing and surface density, as well as model parameterizations contribute to model uncertainties. Finally, I present firn-compaction data from Greenland and compare firn-densification-model predictions to those data. I use the firn-compaction data to derive a new firn-densification equation for Summit. I show that performance of firn densification models varies by site and by depth in the firn. At Summit, several models can predict the compaction rate to within 10%, but at EastGRIP, all models have an RMS error greater than 20%. My results indicate that the models do not yet represent physical processes correctly; I identify future research that is needed to help improve the models