Mapping of large magnitude discontinuous sea ice motion
SIGSPATIAL Special
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The changes in the global climatic conditions are believed to be intimately connected to the dynamics, thickness, and extent of the sea ice in the Arctic and Antarctic. Given the importance of these geophysical phenomena, researchers have undertaken many studies to ascertain the changes that are occurring in sea ice. With the availability of high-spatial resolution, and all-weather Synthetic Aperture Radar (SAR) sensors, it is now possible to complement point measurements taken on the ice, with measurements from a much larger geophysical scale (500∼1000km). This also provides a non-intrusive method to track sea ice, which is an important component in understanding the sea ice mass balance. This work extends the body of knowledge on sea ice motion tracking in three specific directions. The first is in the development of a computationally efficient, high-resolution motion tracking system at the geo-spatial mesoscale (1 km2 - 100 km2). Using this motion tracking algorithm, it is possible to estimate differential motion at a resolution of ∼400m within a locally referenced coordinate system. Unlike Pan-Arctic products that track sea ice motion at a standard resolution of 3∼5km, this motion tracking system is able to estimate local dynamics at a much finer resolution. This system thus provides a possible mechanism to complement existing large-scale motion tracking efforts with a fine resolution local motion. As with any computational techniques, the robustness of this motion tracking system has also been quantified using synthetic and real data. The synthetic data was generated using a parametric vector field, where the average error was measured in the presence of various types of noise models. In order to measure the accuracy against real data, the in-situ GPS buoys from the “Sea ice Experiment - Dynamic Nature of the Arctic” (SEDNA) ice camp were used. The estimates from the motion tracking system are found to be statistically comparable with the ground truth GPS measurements, with an average error that is ≤ 0.06cm s–1. The second direction of this work focuses on the extension of the motion tracking technique to handle motion at close proximity to discontinuous regions. This work primarily stems from the requirement to identify and track discontinuous zones across large (basin) scales. This component is developed as a modified Expectation-Maximization (EM) framework to analyze motion near discontinuities such as leads, cracks and ridges. Using this framework, local particle streamlines are used to compute a plausible flow at discontinuities, thereby predicting the motion more accurately than obtained from the original motion tracking system. This theoretical framework is validated by manually tracking discontinuous features and comparing the manual estimates against the streamline algorithm. The streamline regularization showed a marked improvement (reduction in the average vector error by 60m) in comparison to the original motion tracking algorithm, especially at discontinuities. Finally, this work also focuses on the development of a vector field interpolation technique. This technique allows vector field characteristics to be incorporated into the interpolation via local streamline approximations. Results indicate that this algorithm is comparable to the bilinear interpolation technique when interpolating vector fields under limited noise levels. However, in the presence of noise, this vector field oriented algorithm tends to improve the accuracy of the interpolation. All the three directions allow motion to be estimated at a high resolution in a simultaneously efficient and robust manner. With the observed changes in global climate, sparked by variations in the sea ice thickness and extent, this system could be potentially used to merge the “temporally rich” GPS measurements with the “spatially rich”measurements from satellite images. It is my hope that many of the techniques developed here might be further improved and the full-fledged product might be freely distributed to sea ice researchers around the world.