In the surface layer over the ocean the Monin–Obukhov similarity theory is often applied to construct vertical profiles of pressure, temperature, humidity, and wind speed. In this context, the rough boundary layer is derived from empirical relations where ocean wave characteristics are neglected. For seas where wind speed is less than ~ 10 m s−1 there is excellent agreement for both meteorological and microwave propagation theory and measurements. However, recent evidence indicates that even small waves perturb these profiles. It is, therefore, hypothesized that mechanical forcing by sea waves is responsible for modifying scalar profiles in the lowest portion of the surface layer, thereby reducing the effects of evaporation ducting on microwave signal propagation. This hypothesis, that a rough sea surface modifies the evaporation duct, was the primary motivation for the Rough Evaporation Duct (RED) experiment.

RED was conducted off of the Hawaiian Island of Oahu from late August to mid-September 2001. The Scripps Institution of Oceanography Research Platform Floating Instrument Platform, moored about 10 km off the northeast coast of Oahu, hosted the primary meteorological sensor suites and the transmitters for both the microwave and the infrared propagation links. Two land sites were instrumented—one with microwave receivers and the other with an infrared receiver—two buoys were deployed, a small boat was instrumented, and two aircraft flew various tracks to sense both sea and atmospheric conditions.

Through meteorological and propagation measurements, RED achieved a number of its objectives. First, although we did not experience the desired conditions of simultaneous high seas, high winds, and large surface gradients of temperature and humidity necessary to significantly affect the evaporation duct, observations verify that waves do modify the scalars within the air–sea surface layer. Second, an intriguing and controversial result is the lack of agreement of the scalar profile constants with those typically observed over land. Finally, as expected for the conditions encountered during RED (trade wind, moderate seas, unstable), we show that the Monin–Obukhov similarity theory, combined with high-quality meteorological measurements, can be used by propagation models to accurately predict microwave signal levels.

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Atmospheric Propagation Branch, Space and Naval Warfare Systems Center San Diego, San Diego, California

School of the Environment, University of Leeds, Leeds, United Kingdom

Remote Sensing Division, Naval Research Laboratory, Washington, DC

Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

TNO Physics and Electronics Laboratory, The Hague, Netherlands

Department of Atmospheric Sciences, University of Washington, Seattle, Washington

Department of Meteorology, Naval Postgraduate School, Monterey, California

Defense Research and Development Canada, Val-Belair, Quebec, Canada

Department of Mechanical Engineering, and Department of Earth System Science, University of California, Irvine, Irvine, California

Department of Earth and Planetary Sciences, John Hopkins University, Baltimore, Maryland

Marine Meteorology Division, Naval Research Laboratory, Monterey, California

Electrical and Computer Engineering Department, University of Massachusetts—Amherst, Amherst, Massachusetts

Scripps Institution of Oceanography, University of California, San Diego, Lajolla, California