To surfers, breaking waves represent the thrill and challenge at the core of their sport. To scientists who study interactions between the air and the sea, breaking waves represent one of the most vital air-sea exchange mechanisms. A research study led by Professor Ken Melville of Scripps Institution of Oceanography at the University of California, San Diego, has provided unprecedented insight into the dynamics involved in breaking waves.
The study, the cover story of the May 2 issue of the London-based journal Nature, advances the science of important processes associated with breaking waves, including limiting the heights of ocean waves, the generation of ocean currents, and mixing of the ocean surface.
"One of the most important functions of breaking waves is to transfer momentum from the wind to ocean currents," said Melville, a professor of oceanography in the Marine Physical Laboratory and Physical Oceanography Research Division at Scripps. "Wave breaking also is vital to air-sea exchange processes such as heat and gas transfer, which may have a profound effect on weather and climate."
In the past, wave breaking has been tracked by so-called "whitecap coverage," in which still or video imagery was used to statically identify ocean whitecaps and the corresponding surface coverage by breaking waves. But those measurements suffered because they failed to account for the motion of the breaking waves, an aspect critical for understanding an array of air-sea interactions.
For the new study (part of the Shoaling Waves Experiment, or "SHOWEX"), Melville and Peter Matusov (also of Scripps Institution) used high-tech instrumentation aboard a light aircraft to obtain detailed image sequences of breaking waves. The equipment included an airborne video system and differential global positioning system (GPS) technology to precisely characterize breaking processes.
The results of the study not only demonstrate that wave breaking can be accurately measured using remote imaging techniques, they help describe aspects of wave growth and decay in unique detail. The information also will be used by scientists to improve descriptions of marine aerosol production and heat and gas transfer.
"With the data we’ve taken in the field, plus laboratory measurements, we’ll be able to do much to improve models of aerosol generation," said Melville.
The Nature study sets the stage for two new projects beginning this summer that will attempt to probe characteristics of hurricanes. In the first, with Melville and Scripps’s Eric Terrill, a new imaging system will be flown aboard a National Oceanic and Atmospheric Administration (NOAA) P3 "Hurricane Hunter" aircraft. Data from this project (part of the Office of Naval Research Coupled Boundary Layers/Air-Sea Transfers, or "CBLAST" experiment), to be obtained over the next three years, will be used to measure hurricane-driven whitecaps and ocean spray in an effort to understand their role in hurricane dynamics and thermodynamics.
The second project, led by Terrill and Melville, will use modified autonomous ocean profilers to measure the influence of hurricanes on the thermal structure and mixing of the upper ocean, and the air-sea interface. The profilers, originally developed by Scripps Professor Russ Davis, will be deployed by aircraft ahead of hurricanes and transmit data via satellite.
"The improved knowledge of air-sea interaction in hurricanes from both of these projects will lead to improved numerical predictions of hurricane winds, waves, storm surges, and hurricane landfall," said Melville. "The potential monitoring and predictive aspects of these projects are most important as the continuing development and growth of coastal communities requires longer warning and evacuation times to minimize the loss of life and property from hurricanes."
Funding for the Nature study was provided through grants from the Office of Naval Research (physical oceanography) and the National Science Foundation (ocean sciences). Melville also acknowledges the unique capabilities of NOAA’s Long-EZ aircraft and Tim Crawford’s "fearless flying" during the project.
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