Jun Saito

Year of Award

2024

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Joel T. Harper

Commitee Members

Toby W. Meierbachtol, Jesse V. Johnson, Joel M. Brown, Neil F. Humphrey

Keywords

Elevation change, Firn, Greenland Ice sheet, Heat flux, Remote sensing, Temperature

Abstract

In this dissertation, I use in situ and satellite based observational data combined with numerical model output to examine various problems which are important to the surface mass balance of the Greenland Ice Sheet. In Chapter 1, I present the research background relevant to Chapters 2, 3 and 4, highlighting the contribution of ice mass loss from the Greenland Ice Sheet to sea level rise and examining shifts in its mass loss mechanisms. In Chapter 2, I conducted a comprehensive assessment of multi-decadal elevation changes on land-terminating glaciers in West Greenland, utilizing diverse remote sensing data sources. While previous regional assessments relied on modeled surface mass balance, my approach incorporated a variable digital elevation model, providing a more temporally and spatially variable estimation of surface elevation changes. To enhance accuracy, I employed co-registration methods for each digital elevation model, mitigating systematic biases related to terrain height and slope. Additionally, I integrated regional climate model and ice velocity data to elucidate regional disparities in elevation changes. The findings suggest that changing ice flow can impact ice surface elevation on land-terminating glaciers, beyond the influence of surface mass balance. In Chapter 3, I investigated understudied aspects of the temperature field within the firn layer of the Greenland Ice Sheet. Utilizing the first observed temperature time-series to depths of 30-96 m across a 35 km transect, I analyze the thermal structure during the high-melt summer of 2019. This chapter highlights a pronounced gradient in the thermal structure of the firn layer, characterized by increased heat uptake and deeper heat penetration at lower elevations. A 1D numerical model further enhances our understanding, emphasizing consistent conductive heat gain despite changes in elevation. Latent heat transfer due to refreezing meltwater emerges as the primary source of heat gain. This study offers unique insights into the time evolution of the thermal structure of the ice sheet in a warmer climate.

In Chapter 4, I investigated winter reflux from the firn layer based on observed temperature time-series and model simulations using the Community Firn Model. Winter reflux to the atmosphere is highly time/space variable, primarily driven by the winter surface temperature and the quantity of heat stored heat from previous summer. Greater influx during summer promotes higher reflux during winter and there tends to be a near annual balance between the two large fluxes. However, model simulations show that reflux diminishes to a maximum as uptake increases. Observations from the extreme melt year of 2023 show a strong summer/winter imbalance. The high melt intensified a wetting front within the subsurface of the firn layer, delaying the release of stored heat back to the atmosphere. As summer and winter temperatures rise in the future, more heat is likely to remain trapped within the firn layers.

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