Improving estimates of ice sheet elevation change derived from AltiKa and CryoSat-2 satellite radar altimetry

While satellite Ku-band (13.5 GHz) radar altimetry has been used since the 1990s to track changes in the Greenland and Antarctic ice sheets' shape, the launch of AltiKa in 2013 provided the first opportunity to use data from higher frequency Ka-band (36 GHz) and compare it to contemporaneous Ku-band CryoSat-2 data. In this thesis, I develop novel methods and datasets, based on the processing of Ku-band CryoSat-2 and Ka-band AltiKa data, to improve our ability to detect and interpret trends in elevation change from satellite radar altimetry.

First, I produced an assessment of higher-frequency Ka-band AltiKa data in West Antarctica. By developing a new slope correction algorithm and applying a least-square model fit to AltiKa surface elevation measurements, I demonstrated that AltiKa detects trends in surface elevation in good agreement with coincident Ku-band CryoSat-2 and airborne laser data within 0.6 ± 2.4 cm/yr and 0.1 ± 0.1 cm/yr, respectively, showing that trends in penetration are minor in this region. Using this new dataset, I showed that surface lowering at Pine Island Glacier has fallen by 9% since the 2000s, while at Thwaites Glacier it has risen by 43%.

Next, I examined the impact of surface melting on firn stratigraphy and radar penetration in West Central Greenland by using a combination of airborne radar data, in-situ firn density measurements, and firn densification models. I showed that surface melt strongly affects the degree of radar penetration into the firn, with the largest fluctuations recorded after the extreme melt event of 2012, which caused a 6.2 ± 2.4 m reduction in Ku-band radar penetration. I further assessed different methods to mitigate the effect of fluctuations in radar penetration on surface heights and showed that using threshold retracking algorithms results in surface heights to within 14 cm from coincident airborne laser data. In addition, I showed that over this transect, Ka-band radar penetration is half that of coincident Ku-band data.

Finally, I used a decade of CryoSat-2 data to study the imbalance of the Northwest sector of the Greenland Ice Sheet and showed that the margins of this region are rapidly thinning at an average rate of 42.7 ± 0.9 cm/yr. I derived mass balance within 73 individual glacier drainage basins of this region, showing that the Northwest sector lost a total of 386.0 ± 3.7 Gt of ice between July 2010 and July 2019 with all glacier basins losing mass. I compared this new altimetry-based mass balance estimate to independent estimates from the gravimetry and mass budget techniques and found that, while the altimetry estimate is the least negative, differences between techniques vary regionally, with the mass budget and gravimetry exhibiting higher and lower ice losses, respectively.