Feature ArticleUnderstanding Radar Clutter To Predict Wave Behavior
By Edward Lundquist
Senior Science Writer
A research effort involving a variety of oceanographic platforms off the coast of California is trying to measure waves and predict them.
The Office of Naval Researchís (ONR) High-Resolution Air-Sea Interaction Departmental Research Initiative (Hi-Res DRI) is collecting data to better measure, understand and model the interaction between wind and waves at the sea surface, which will help interpret ship-based radar returns to ensure safer naval operations in higher sea states.
Following research and planning, as well as a series of pilot experiments over several years, the main Hi-Res field experiment was held off Bodega Bay in Northern California in June 2010. At the center of the experiment was RP FLIP, a U.S. Navy-owned research platform operated by the Marine Physical Laboratory at Scripps Institution of Oceanography, which anchored with a three-point mooring about 15 nautical miles off the coast of California in 200 meters of water. The endeavor also involved moored and drifting buoys, research vessels and aircraft, each employing a range of instruments and sensors supported by high-frequency coastal radar data and satellite imagery.
Also participating in Hi-Res was the RV Sproul, a coastal research vessel owned by the State of California and operated by Scripps as part of the University-National Oceanographic Laboratory System fleet. A twin-engine fixed-wing aircraft, a DHC-6 Twin Otter from the Naval Postgraduate Schoolís Center for Interdisciplinary Remotely-Piloted Aircraft Studies, was also among the array of assets devoted to this program.
Overall, scientists from 12 institutions in the United States, NATO and Europe are participating in Hi-Res, combining their skills to collaborate in addressing the scientific and technical questions raised by the program.
Carl Friehe of the University of California (UC), Irvine, Ken Melville of the Scripps Institution of Oceanography at UC San Diego and Dick Yue of the Massachusetts Institute of Technology are the co-chief scientists acting under the sponsorship of Dr. Linwood Vincent at ONR.
Marine Atmospheric Boundary Layer
The Hi-Res program is measuring and modeling the surface waves and the atmospheric boundary layer over the ocean to better understand phase-resolved surface-wave processes and the interaction between the atmosphere and the ocean. In addition to the wave and radar measurements, the wind forcing of the marine atmospheric boundary layer (MABL) was measured and modeled to improve the surface wind stress inputs to models of wave generation and ocean currents.
Hi-Res DRI aims to utilize the marine radars typically used for navigation and piloting to also retrieve the phase-resolved surface wave field (PRSWF). In addition, the Hi-Res DRI is working to validate numerical surface wave models to predict the evolution of the PRSWF and then seeing how surface wave processes affect the MABL.
Wave motion at the sea surface may seem like a relatively simple oceanographic phenomenon. In reality, it is quite complex. Winds, currents (including tidal currents), ocean fronts and other factors all affect the surface wave field—how high the waves are, how long they are, how they spread in space and how they dissipate, mainly through breaking.
Embracing the Clutter
Whereas many radar operators curse the sea return—or clutter—that obscures a clear image on their displays, the Hi-Res experiment actually embraces this clutter as having the clues to being able to solve the problem of measuring and predicting the wave field in real time. Hi-Res inverts the clutter, which is then used as an input to predictive wave models. While any shipboard radar can measure waves, the radar must be adjusted to be able to detect them so that they can be analyzed.
As access to foreign ports and bases has become more problematic, the Navy has developed the concept of seabasing, where joint military or humanitarian relief capabilities can be aggregated and deployed from a sea base. One of the largest impediments to the success of seabasing is the difficulty of ship-to-ship transfers of heavy cargo in the open ocean and, in general, open-ocean cargo transfer operations in all but the most benign sea states.
Many events that take place at sea, even in the nearshore environment, involve launching and recovering boats, unmanned vehicles and aircraft, or transferring personnel and cargo between vessels. Anyone who has ever climbed a Jacobís ladder from a small boat, such as a rigid-hulled inflatable boat, to get aboard a ship—even in a moderate sea—knows that timing is everything. One second it is a smooth hop from the boat to the ladder to the ship. A second later could put someone in the drink. Similarly, launching or landing a helicopter on a flight deck that is pitching and rolling in heavy seas may be impossible outside a safe operating envelope.
Therefore the Navy wants to be able to predict wave-induced ship motion so as to be able to time a critical operation—even by a matter of seconds or minutes—to have a safer and more successful outcome, whether itís the launch or recovery of a small boat or the ship-to-ship transfer of heavy cargo. To continue this article please click here.
U.S. Navy Capt. Edward Lundquist (retired), of Arlington, Virginia, is a principal science writer for MCR LLC. He writes for naval, maritime and defense publications.