Advancing Observational and Modeling Capabilities of Meltwater – Firn Interactions in Ice Sheet Percolation Zones
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Authors
McDowell, Ian Ellis
Issue Date
2024
Type
Dissertation
Language
en_US
Keywords
firn , hydrology , ice sheets , modeling , observations
Alternative Title
Abstract
Approximately 90% of the Greenland Ice Sheet and 99% of the Antarctic Ice Sheet are covered with firn, the intermediate material between fresh snow and glacial ice. The firn layer consists of interconnected, air-filled pore spaces and ranges from several to hundreds of meters thick. Many glaciological research applications require an understanding of firn structure, including monitoring changes in ice sheet mass balance, interpreting ice core paleoclimate records, and understanding ice sheet hydrology. Constraining how firn structure affects englacial hydrology is critical to quantifying firn’s meltwater storage capacity. This is especially crucial given that the Greenland and Antarctic ice sheets are the largest potential contributors to future sea level rise, but the rate and timing of their contribution are highly uncertain. Meltwater retention in porous firn delays runoff from ice sheets and mitigates mass loss. Therefore, understanding how liquid water originating from surface melt interacts with firn structure as it flows through open pore space is necessary for determining the fate of meltwater on ice sheets. While macrostructural properties such as firn density or porosity are relatively easy to measure in the field or laboratory settings, grain-scale properties that greatly influence firn’s hydraulic properties are morechallenging to describe. In this dissertation, I use a combination of laboratory observations, field data, climate reanalyses, and modeling to understand the interaction between macroscale and microscale firn properties and meltwater flow. I statistically demonstrate that firn macrostructure (density) is a poor indicator of firn microstructure (Chapter 2). I use grain size data from firn cores collected in Greenland and modeling to reveal dynamic feedback mechanisms between meltwater percolation and firn microstructure (Chapter 3). Because measuring firn grain size is tedious, time-consuming, and possibly subjective, I adapt a near-infrared hyperspectral imaging system in a cold laboratory to systematically measure grain size and develop ice layer distributions from firn cores (Chapter 4). Lastly, I develop a premelting parameterization in a one-dimensional firn hydrology model that can be used to explain observations of subfreezing liquid water in firn (Chapter 5). Together, these studies improve our understanding of meltwater fate and transport in firn on ice sheets, which is integral to assess the potential future sea level contribution from ice sheets.