PhD Defence Renée Mie Fredensborg Hansen

Principal supervisor:

Senior researcher Henriette Skourup

Co-supervisors:

Professor Knut Vilhelm Høyland, NTNU
Associate professor Eero Rinne, UNIS
Professor René Forsberg, DTU Space

Examiners:

Professor Louise Sandberg Sørensen, DTU Space
Professor Christian Haas, Alfred Wegener Institute (AWI), Germany
Assistant professor Rosemary Willatt, UCL Department of Earth Sciences, UK

Chairperson at defence:

Senior researcher Mai Winstrup, DTU Space

Summary

Advancements in Polar Altimetry: Understanding Sea Ice Dynamics and Snow Depth through Multi-Frequency Observations
Sea ice and its snow cover are critical to Earth's climate, influencing numerous environmental processes. This research explores how snow and sea ice features—like ridges, cracks, and complex snow conditions—affect multi-frequency altimetry, focusing on both air- and spaceborne systems. These features, shaped by winds and weather, complicate remote sensing, as ice and snow heterogeneity distort the electromagnetic signals used for observation. This thesis employs three key approaches: (a) detecting localized deformation features with high-resolution laser altimetry, (b) analyzing dual-frequency satellite observations to estimate snow depth, and (c) comparing air- and spaceborne altimetry over the same sea ice.

Key findings include the derivation of sea ice ridges statistics in the Barents Sea and Fram Strait using high-resolution surface samplings from ICESat-2, with notable discrepancies when comparing these results to upward-looking sonars. These findings highlight the importance of using similar reference levels and minimum-detection-thresholds to accurately represent the distribution of ridges. The first orbital snow depth estimates from the recently aligned near-coincident ICESat-2 and CryoSat-2 (CRYO2ICE) tracks reveal the potential for snow depth assessment over Arctic sea ice, especially important for the upcoming high-priority dual-frequency polar altimetry mission, CRISTAL, despite challenges in radar penetration during synoptic events. An Antarctic case study using an unprecedented combination of multi-frequency airborne altimeters showed significant microwave scattering at both the air-snow and the snow-air interfaces. These results challenge traditional assumptions about main scattering horizons and retrieval methods currently applied for airborne altimetry, which are crucial to accurately consider when translating to satellite scales.

This research provides insights relevant for refining altimetry techniques to better account for localized sea ice deformation and snowpack complexity. The findings highlight critical limitations in aligning air- and spaceborne altimetry observations. These insights can enhance the accuracy of sea ice monitoring and aid in understanding discrepancies between ground, air, and space altimetry
observations.