Study Tests STARRS Imaging of Short-Lived Small-Scale Dispersion on Ocean’s Surface
– AUGUST 15, 2019
Researchers described field methods and observations using the Ship-Tethered Aerostat Remote Sensing System (STARSS) to better understand how buoyant material moves and disperses on the ocean’s surface. Using algorithms based on geo-rectified imagery, the STARSS successfully quantified small-scale and high-frequency surface dispersion in an open-ocean environment and improved observational diffusivity estimates in the 3 to 40 meters scale. The system captured anisotropic dispersion along a density front and distinguished bamboo plates from ephemeral features like sun glitter and whitecaps. This study is the first observational attempt to simultaneously resolve surface ocean Lagrangian dispersion at oceanic boundary layer scales and submesoscales in an offshore, deep-ocean setting. These methods can be replicated and integrated into existing and future field campaigns using sensors and aerial platforms that satisfy the STARSS requirements outlined in the study.
The researchers published their findings in Frontiers in Marine Science: Surface ocean dispersion observations from the ship-tethered aerostat remote sensing system.
After Deepwater Horizon, it became evident that improved transport and dispersion forecasts at the air-sea interface (a difficult region to measure) and at large spatial ranges (meters to kilometers) and temporal ranges (hours to months) were needed. Recent observational campaigns that focused on submesoscale transport and mixing have improved velocity estimates, data assimilating models, and understanding of turbulence (PNAS Poje et al., 2014, JTEC Berta et al., 2015, JGR Oceans Jacobs et al., 2015, and PNAS D’Asaro et al., 2018); however, relatively few in situ studies have quantified near-surface velocities at oceanic boundary layer scales. Traditional ocean observation tools (e.g., drifters, ships, and satellites) have limited ability to measure velocities at the air-sea interface and resolve dispersion at these scales.
“Observing ocean transport and mixing at the surface of the ocean is difficult, especially when you’re far from land where there are no fixed reference points,” explained study author Dan Carlson. “The difficulty increases when you want to observe short-lived (seconds to hours) motion at small spatial scales (meters to hundreds of meters), but these environmental conditions and scales are very relevant to the drift and spread of small patches of oil.”
Although aerostats have been used for over a hundred years and particle tracking for decades, this study employed a unique combination of high-resolution aerial imagery, image analysis techniques, and biodegradable drifters. This study’s team conducted ten STARRS experiments during the Lagrangian Submesoscale Experiment (LASER), which simultaneously deployed 1,000 surface drifters approximately 130 km southeast of the Mississippi River Delta. The STARSS was equipped to collect high-resolution imagery (8,688 x 5,792 pixels) with ~300×200 m field of view and obtained images every 15 seconds over nearly four hours.
The team deployed bamboo plates on an offshore density front and imaged them using STARSS. Then they geo-rectified the images using GPS and an inertial navigation system (INS combines latitude, longitude, altitude, pitch, roll, and heading) and minimized movement between frames using a relative rectification technique. After rectifying the bamboo plates’ positions, they quantified scale-dependent dispersion based on relative dispersion, relative diffusivity, and velocity structure functions. The team used surface drifter trajectories to connect small-scale features to the submesoscale, and the range of diffusivities (10-4 m2s-1 to 0.4 m2s-1) agreed with other observations at similar spatiotemporal scales.
“Our measurements show that features like density fronts can introduce a preferred direction of motion even at small spatial scales and short temporal scales,” said Carlson. “These motions are currently difficult to reproduce in numerical ocean models, as the model resolution is larger than the scales of the mixing. These kinds of studies push us into new scientific frontiers and guide our efforts to produce more reliable ocean and oil spill models that offer the best guidance possible in the event of future spills.”
The authors noted that the main drawbacks of STARSS were INS performance and the aerostat’s size. However, the aerostat offered a safe and stable aerial platform that was relatively simple to operate. They recommended that integrating an unmanned aerial system (UAS) imaging and communications systems into a STARSS-like system could provide the convenience of plug-and-play hardware and software, which a smaller aerostat could accommodate.
Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at doi:10.7266/N7280608 and doi:10.7266/n7-8h5w-nn91.
The study’s authors are Daniel F. Carlson, Tamay Ozgokmen, Guillaume Novelli, Cedric Guigand, Henry Chang, Baylor Fox-Kemper, Jean Mensa, Sanchit Mehta, Erick Fredj, Helga Huntley, A.D. Kirwan Jr., Maristella Berta, Mike Rebozo, Milan Curcic, Ed Ryan, Bjorn Lund, Brian Haus, Jeroen Molemaker, Cameron Hunt, Shuyi Chen, Laura Bracken, and Jochen Horstmann.
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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment II (CARTHE II).
The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit https://gulfresearchinitiative.org/.
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