Christianson, KnutHorlings, Annika2023-08-142023-08-142023-08-142023Horlings_washington_0250E_25742.pdfhttp://hdl.handle.net/1773/50053Thesis (Ph.D.)--University of Washington, 2023Understanding processes in the snow and firn are important for studies of ice dynamics, ice-sheet mass balance, glacial hydrology, and ice-core interpretation. In this dissertation, I use a combination of modeling, geophysical methods, and analyses of regional climate model output and reanalysis to study the evolution of firn compaction, firn hydrology, and snow accumulation on alpine glaciers and ice sheets in response to changes in ice dynamics and climate. The first study is devoted to investigating one process that is neglected in firn-air content estimates derived from firn-compaction models: enhanced layer thinning due to horizontal divergence. In this study, I implemented a new scheme into the Community Firn Model, a modular model framework that can simulate numerous physical processes in firn. Modeling results showed that horizontal divergence can reduce local firn-air content by up to 41% and may contribute to 16% of surface lowering for some dynamic fast-flowing regions in West Antarctica. I find that omitting transient horizontal divergence in estimates of firn-air content leads to an overestimation of ice loss via satellite-altimetry methods in regions of dynamic ice flow. The second study characterizes the extent of four firn aquifers in the Helheim, Ikertivaq, and Køge Bugt glacier basins of southeast Greenland using airborne ice-penetrating radar data from 1993 to 2018. All four firn aquifers first appear and/or show decadal-scale inland expansion during this time period. A relative energy budget calculation using regional climate model output shows that these aquifer expansions are driven by decreasing cold content in the firn since the late 1990s and a recently increasing number of high-melt years. High-melt years are projected to increase on the Greenland Ice Sheet and may contribute to the continued inland expansion of firn aquifers, impacting the ice sheet's surface mass balance and hydrological controls on ice dynamics. The third study is devoted to examining the recent snow-accumulation rates at Hercules Dome, Antarctica, a prospective ice-core site. Only one observation of the snow-accumulation rate exists and accuracy of climate models remains limited there, meriting further study. I use ground-based very high frequency ground-penetrating radar collected during the 2019-2020 and 2022-2023 austral summers to construct the first spatially resolved snow-accumulation dataset over Hercules Dome. The 420-year, 326-year, and 225-year time-averaged accumulation rates are similar and range from 0.10 to 0.15 m/yr ice equivalent across the divide, with greater accumulation to the grid north (EPSG:3031) toward the Filchner-Ronne sector. Analyses of regional climate-model output and climate reanalysis show that the snow-accumulation pattern is likely a result of an orographic effect on the regional scale and wind redistribution on the local scale. The snow-accumulation variability across the divide will be important to consider for future ice-core science at Hercules Dome. This dissertation improves our understanding of a variety of polar near-surface processes that are integral in assessing ice-sheet mass balance in the past, present, and the future.application/pdfen-USCC BY-NCGeophysicsEarth and space sciencesThesis