Locating Ice Sheet Grounding Lines Using Satellite Radar Interferometry and Altimetry Anna Elizabeth Hogg Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Earth and Environment Centre For Polar Observation and Modelling (CPOM) August, 2015 The candidate confirms that the work submitted is his/her own and that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. The right of Anna Elizabeth Hogg to be identified as Author of this work has been asserted by her in accordance with the Copyright, Designs and Patents Act 1988. © 2015 The University of Leeds and Anna Elizabeth Hogg Acknowledgements This work was funded by a UK Natural Environment Research Council’s (NERC) National Centre for Earth Observation (NCEO) PhD studentship, hosted jointly by the Centre for Polar Observation and Modelling (CPOM) at the University of Leeds and at the Directorate of Earth Observation Programmes, ESA-ESRIN. Firstly, I thank my supervisor Andy Shepherd for his support, advice and encouragement over the last few years. The work contained in this thesis has been greatly improved by his contribution and I look forward to continuing to work together in the future. I am also extremely grateful to have received support from Martyn Chipperfield, Marcus Engdahl and Mark Doherty during the course my PhD. Thanks also go to the many colleagues and friends who have kept me entertained, and excited about science throughout my PhD. In particular Kate, Debbie, Mal, Steve, David, Hannah and Aud have endured more than most. Finally thanks to my family and to Ali for always supporting me. Abstract In this thesis, I use synthetic aperture radar (SAR) and radar altimeter data to make new observations of Antarctic and Greenland ice sheet grounding lines. I use ERS SAR data acquired between 1992 and 2011 to map the Petermann Glacier grounding line on 17 occasions using quadruple difference interferometric SAR (QDInSAR). Over the 19-year period, the grounding line position varied by 0.5 km, on average, with no significant trend over time. Although tidal forcing explains a fraction (34 %) of the movement, localised variations in the glacier thickness could explain it all were they to alter the glaciers hydrostatic balance as they advect downstream – a hitherto unconsidered possibility that would reduce the accuracy with which changes in grounding line position can be detected. Next, I developed a new technique for detecting grounding lines using differential range direction offset tracking (DRDOT) in incoherent SAR data. I then applied this technique to a sequence of 11 TerraSAR-X images acquired in 2009 over Petermann Glacier. The DRDOT technique is able to reproduce the shape and location of the grounding line with an estimated lateral precision of 0.8 km and, although this is 30 times poorer than QDInSAR, provides a complementary method given the paucity of coherent SAR data. Finally, I developed another new method for detecting the grounding line as the break in ice sheet surface slope computed from CryoSat-2 elevation measurements. I then applied this technique to map grounding lines in the sectors of Antarctica buttressed by the Filchner- Ronne, Ekström, Larsen-C, and Amundsen Sea ice shelves. The technique is able to map the grounding line to within 4.5 km, on average, and, although this is far poorer than either QDInSAR or DRDOT, it is computationally efficient and can succeed where SAR-based methods fail, offering an additional complementary approach. Contents Acknowledgements ................................................................................................................................. 3 Abstract ................................................................................................................................................... 4 Acronyms .......................................................................................................................................... 10 Arctic Ocean Tidal Forward Model ....................................................................................................... 10 Chapter 1 ............................................................................................................................................... 11 Introduction and Background ........................................................................................................... 11 1.1 Importance of Grounding Lines ........................................................................................ 11 1.2 Grounding Line Migration ................................................................................................. 14 1.3 Review of Existing Methods for Detecting Grounding Lines ............................................ 18 1.4 Aim .................................................................................................................................... 26 1.5 Objectives .......................................................................................................................... 27 1.6 Thesis Structure ................................................................................................................ 27 Chapter 2 ............................................................................................................................................... 28 Methods ................................................................................................................................................ 28 2.1 Methods Overview .................................................................................................................. 28 2.2 Overview of the QDInSAR Method ......................................................................................... 29 2.3 Overview of the DRDOT Method ............................................................................................ 43 2.4 Overview of the CryoSat-2 Method ........................................................................................ 44 Chapter 3 ............................................................................................................................................... 46 Grounding line migration from 1992 to 2011 on Petermann Glacier, North West Greenland ........ 46 3.1. Abstract .................................................................................................................................. 46 3.2. Introduction ........................................................................................................................... 47 3.3. Petermann Glacier Study Area ............................................................................................... 48 3.4. Data and Methods ................................................................................................................. 51 3.5. Results and Discussion ........................................................................................................... 56 3.6. Drivers of Grounding Line Migration ..................................................................................... 63 3.7. Conclusions ............................................................................................................................ 69 Chapter 4 ............................................................................................................................................... 71 Differential Range Direction Offset Tracking (DRDOT): A Technique For Measuring the Ice Sheet Grounding Line in Incoherent Synthetic Aperture Radar Data ......................................................... 71 4.1 Abstract ................................................................................................................................... 71 1 4.2 Introduction ............................................................................................................................ 72 4.3 Study Area ............................................................................................................................... 74 4.4 Data ......................................................................................................................................... 75 4.5 Ice Speed Methods and Results .............................................................................................. 78 4.6 Grounding Line Methods and Results ..................................................................................... 85 4.7 Conclusions ............................................................................................................................. 94 Chapter 5 ............................................................................................................................................... 95 Locating the Antarctic Ice Sheet Break In Surface Slope Using CryoSat-2 ........................................ 95 5.1 Abstract ................................................................................................................................... 95 5.2 Introduction ............................................................................................................................ 95 5.3 CryoSat-2 Data ........................................................................................................................ 98 5.4 Study Areas