N242-088 TITLE: Low-cost Floats for Observing Interior Ocean Flows

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Network Systems-of-Systems; Integrated Sensing and Cyber; Microelectronics

OBJECTIVE: Develop and demonstrate a low-cost, expendable ocean float that can be deployed in large numbers along with a concept to track the floats to observe interior ocean velocities through the reconstruction of Lagrangian pathways.

DESCRIPTION: Velocities in the interior of the ocean are difficult to observe, with Acoustic Doppler Current Profilers (ADCPs) and current meters representing the primary sources of observation. These techniques largely provide ocean velocities in an Eulerian reference frame. From the perspective of constraining numerical ocean models via observations, trajectories of interior ocean flow in a Lagrangian reference frame provide a more stringent measure of the fluid streamfunction, and data assimilation techniques that use Lagrangian information have proven to be very effective in ocean prediction. However, most Lagrangian information in the ocean comes only at the surface from drifting buoys. Observing the interior trajectories of the ocean has traditionally been more difficult, though there is a history of success using acoustic tracking to follow floats in the water column (see SOFAR, RAFOS, or COOL floats as examples). Modern manufacturing techniques, acoustic modem technologies, and advancements in low-power electronics and sensing may enable significant advancements in Lagrangian float design, tracking, and sensing capability. These Lagrangian sensing techniques may be particularly useful in regions where surface operations are difficult, such as the Arctic Ocean where sea ice cover impedes the use of oceanographic vessels to collect subsurface ocean velocity using traditional ADCP techniques.

The proposed observing capability would enable the characterization of the interior ocean streamfunction by deploying a large number of neutrally-buoyant in situ floats that would follow the ocean currents in the upper water column (from the surface to perhaps 500 meters deep), along with a concept to track the floats for up to a month to reveal submesoscale ocean flow features in a regional area (up to 105 square kilometers). Disposable Floats should be designed to have a specific reconfigurable density, without a requirement for active buoyancy control due to the expected significant increase in cost and complexity this would require. Additional oceanographic sensors (e.g., temperature, salinity) could be integrated into the floats, depending on float complexity, the overall tracking concept proposed, and cost of integration. It is up to the proposer to determine which options to include, providing cost considerations.

The Phase II effort will require an at sea demonstration of at least five prototype floats along with the proposed tracking system.

PHASE I: Design and develop a concept for a distributed sensing network of modular low-cost drifting floats that can be used to reconstruct flow trajectories for the interior ocean, including prototype float development and analysis of the predicted sensing performance of a full deployment of 100 floats over a representative ocean region. Develop a Phase II Plan.

PHASE II: Produce initial prototypes of floats along with the proposed sensing system, and test them at sea in a sufficiently complex maritime environment to demonstrate the capability of the system to characterize interior ocean streamfunction. Include the assimilation of data collected by the proposed system into numerical ocean models as part of the demonstration, with improved ocean characterization and prediction by the models as a critical metric.

PHASE III DUAL USE APPLICATIONS: Finalize float design and incorporate additional sensor payloads, if achievable. Commercial applications include oceanographic research (physical, chemical, and biological), effluent management and water quality monitoring, and use in coastal and open-ocean observing systems.

REFERENCES:

1. Rossby, H.T., Levine, E.R., and Connors, D.N. "The isopycnal Swallow float: a simple device for tracking water parcels in the ocean." Progress in Oceanography, Eos, Transactions of the American Geophysical Union, Conference Abstract; Issue 63/3, 1982, p110. https://www.sciencedirect.com/science/article/pii/0079661185900254
2. Rudnick, D., Costa, D., Johnson, K., Lee, C., and Timmermans, M.-L. eds. "Observing the ocean with Autonomous and Lagrangian Platforms and Sensors (ALPS): The role of ALPS in sustained observing systems." Oceanography, 16:31-36. https://doi.org/10.5670/oceanog.2003.06
3. Molcard, A., Griffa, A., and Özgökmen, T. "Lagrangian data assimilation in multilayer primitive equation models." American Meteorological Society, 2005. Journal of Atmospheric and Oceanic Technology., Volume 22, 70–83. https://journals.ametsoc.org/view/journals/atot/22/1/jtech-1686_1.xml

KEYWORDS: Oceanographic Sensing; Lagrangian Data Assimilation; Trajectory Analysis; Dynamical Systems Theory; Ocean Velocity; Interior Ocean Observing; Acoustic Tracking; Ocean Floats; Isopycnal

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