Understanding how the air and sea interact and affect each other during hurricane conditions is crucial in predicting the storm track, its intensity, storm surges, and ocean wave fields. When scientists create computer models to help them assess the parameters of a hurricane, they must take into account not only the atmospheric conditions of the storm, but also the conditions in the ocean, including the age and the frequency of waves.
In the current issue of the Journal of the Atmospheric Sciences, University of Rhode Island physical oceanographers Il-Ju Moon, Isaac Ginis and Tetsu Hara have published two companion papers that investigate the how surface waves and wind affect the dynamics of growing seas and complex seas under extreme hurricane conditions using a combination of computer models. Other collaborators on the project include Stephen Belcher, Department of Meteorology, University of Reading, Berkshire, England, and Hendrik Tolman, the NOAA National Center for Environmental Prediction Environmental Modeling Center, Camp Springs, MD.
The team of scientists combined three computer models to ascertain their results. The NOAA WAVEWATCH III ocean surface wave model accounts for wind input, wave-wave interaction and dissipation due to whitecapping, and wave-bottom interaction. The equilibrium spectrum model, created by Hara and Belcher, estimates the effect of the wind on the ocean by taking into account the stress caused by the waves. The wave boundary layer model, also created by Hara and Belcher, explicitly calculates the near-surface wind profile, as well as the surface drag created by the waves. In the first study, the combined model predicted the effect of the wind-wave interaction by calculating how the waves contribute to the dynamics of a mature and growing sea. The second study followed the same approach, but focused on the effect of surface waves on air-sea exchange in extreme complex seas forced by tropical cyclones.
The scientists found a new characterization of the effect of surface waves on air-sea momentum under hurricane wind forcing. The size and location of the waves as well as the wind speed and direction and their impact on the other create a variety of conditions that can affect the track and intensity of a hurricane. The research team determined that the coupling of a surface wave model with a hurricane model is necessary for accurate predictions of track and intensity. This finding is significant because the wind-wave interaction is presently ignored by hurricane prediction models.
"There have been impressive strides taken in the quality of hurricane track forecasting over the last 10 years mainly due to improved computer models," said Ginis. "However, there appears to be still limited skill in predicting storm intensity changes. In light of the fundamental role the air-sea interaction processes play in supplying energy to the hurricane, our results seem to be promising for major improvements in hurricane intensity forecasting."
The combined models used in this project have helped scientists to further understand the interaction of the atmosphere and the ocean by introducing parameters that describe the ocean waves under high wind conditions, including during a tropical cyclone. Additional factors, such as the effect of breaking waves and sea spray, may also play an important role in air-sea interaction and momentum, but the team of scientists predicts that adding parameters will only further confirm their results.
The URI Graduate School of Oceanography is one of the country's largest marine science education programs, and one of the world's foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as harmful algal blooms, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Institute for Archaeological Oceanography, the Pell Marine Science Library, and the National Sea Grant Library.
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