Sea Ice Freeboard and Thickness Retrievals Over Ross Sea, Antarctica




Tian, Liuxi

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In the context of global warming, the Ross Sea’s increased sea ice extent and duration during the last few decades are in contrast with the decreases observed in the Arctic from passive microwave satellite data, though in the last four years the trend has changed with a sharp decline in extent that first occurred in late 2016. But the changes in sea ice thickness, volume, and production in this region is still unclear. Without knowing the changes of sea ice thickness over Ross Sea, it is hard to evaluate how sea ice cover is responding to changing climate in this region. This dissertation contains three essays and aims to use the airborne altimetry data (IcePod 2016-2017 and IceBridge 2013) and spaceborne data (ICESat 2003-2008) to retrieve sea ice freeboard and thickness over Ross Sea, Antarctica.

The first essay is to detect leads and sea surface tie shots within leads by combining DMS images and reflectivity of Airborne Topographic Mapper L1B data and to compute total freeboard and ice thickness from the 2013 NASA Operation IceBridge data in the Ross Sea. The estimated mean sea ice thickness values are found to be in the 0.48-0.99 m range. Along the N-S track, sea ice was thinner southward rather than northward of the fluxgate, resulting in two peaks of modal thickness: 0.35 m (south) and 0.7 m (north). This supports that new ice produced in coastal polynyas is transported northward by katabatic winds off the ice shelf. The second essay is to investigate the patterns of interannual variations of total freeboard (sea ice thickness) in the Ross Sea by comparing the IcePod 2016-2017 data with both the IceBridge 2013 and the ICESat 2003-2008 data by using lowest elevation method, where a percentage of the lowest elevations over a particular length scale has been used as the local sea level. We find that, compared to ICESat years, sea ice near the coast was thicker and sea ice offshore was thinner in the more recent IceBridge / IcePod years. The result also shows that, in general, sea ice was thicker in 2017 as compared to 2013 or 2016 (mean total freeboard is 0.02 m-0.55 m higher). The third essay is to assess the effect of different segment scale and percentage scale on the lowest elevation method by using the IceBridge dataset, which provides a unique dataset with the acquisition of laser altimeter measurements with coincidently high spatial resolution optical images that allow for an independent sea level validation. The results show that the segment-length scales are more influential to the sea level retrievals than the percentage scales. The 1 km segment length and 0.1 % percentage scales get the best estimation overall for all tracks examined.


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Environmental Science