PhD Thesis Defense: Nicholas Wright

Monday, November 18, 2019, 3:30–4:30pm

Rm 202, Cummings Hall

"Novel Algorithms to Analyze Remotely Sensed Optical Imagery revealsNew Behavior of Sea Ice Melt Ponds"

Abstract

Meltwater that pools on the surface of Arctic sea ice enhances solar absorption and accelerates further ice melt. The impact of melt ponds on energy absorption is controlled primarily through their influence on ice albedo, which is, in turn, governed in large part by the ponds’ spatial coverage. This work seeks to observe the spatial coverage of melt ponds across the Arctic basin from remotely sensed optical datasets. To achieve this, an open source system was created to provide a standardized, automated, and reproducible technique for processing high-resolution optical imagery of sea ice. The method classifies surface coverage into three main categories: Snow and bare ice, melt ponds and submerged ice, and open water. The method was demonstrated on imagery from four sensor platforms and on imagery spanning from spring thaw to fall freeze-up. The classification accuracy exceeds 96% in most tests. This information is then used to inform, improve, and test spectral unmixing techniques that seek to determine melt pond coverage from more widely available, but lower resolution, optical satellite imagery (e.g. MODIS). A new machine learning approach was created that improves accuracy over spectral unmixing and can contribute to improved efforts to validate melt pond models or understand trends in pond coverage. Nevertheless, significant challenges to retrieving melt pond fractions from low-resolution optical imagery were encountered and carefully documented. Finally, the high-resolution processing tools developed here were deployed to the supercomputer cluster at the NASA Ames Research Center to process hundreds of thousands of optical images acquired along NASA Operation IceBridge flight tracks. These new surface fraction results were then analyzed to investigate the behavior of meltwater on first-year ice in comparison to multiyear ice. Observations show that first-year ice does not ubiquitously have a higher melt pond fraction than multiyear ice under the same forcing conditions, contrary to established knowledge in the sea ice community. We discover and document a larger possible spread of pond fractions on first-year ice leading to both high and low pond coverage, in contrast to the uniform melt evolution that has been previously observed on multiyear ice floes.

Thesis Committee

For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.