Rough Evaporation Duct (RED) Experiment

R/F Flip

Figure 1. Floating Instrumentation Platform (FLIP)

Prediction of electromagnetic (EM) signal propagation over a wind-roughened sea relies on a thorough knowledge of its interaction with the sea surface, the mean profiles of pressure, temperature, humidity, wind and their turbulent fluctuations. Yet, within the marine surface layer, these mechanisms are not sufficiently understood nor has satisfactory data been taken to validate empirical models. The RED experiment provides the first data for validation of both meteorological and EM propagation models in the marine surface layer for rough surface conditions.

Over the ocean, similarity theory is often applied to construct mean profiles of temperature, humidity, and wind in the surface layer. For smooth seas, there is good agreement between theory and measurements, both meteorological and EM.  However, recent evidence indicates that even small waves perturb pressure, temperature, humidity and wind profiles throughout the surface layer. 

Earth science is interested in wave effects on the marine surface layer as the fluxes of pressure, temperature, humidity and wind form the basis for radiation power budget calculations. As more than two thirds of the earth is covered by water, understanding the radiation balance between the marine surface layer and the free atmosphere is critical for atmospheric research. Hence, both naval operations and earth science research can benefit from a study of wave interaction with the marine surface layer.  

The interaction between the ocean and the atmosphere is a subject actively studied in order to improve weather forecasts. In particular, the air-sea exchange of momentum and kinetic energy is responsible for generating currents and surface waves. The air pressure fluctuations in the marine atmospheric boundary layer (i.e. the first 20 meters above the water surface) carry and embody this mechanical air-sea interaction and their measurement is of primary interest for meteorologists and oceanographers. Only limited number of previous experiments have provided data on these fluctuations. To detect these fluctuations during our experiment, we selected Paroscientific instruments for their sensitivity and reliability.

MET3A and 202BG

Figure 2. Paroscientific Instrument Locations on FLIP

The RED experiment was conducted offshore of Oahu, HI, from mid-August to mid-September 2001. The Hawaiian Islands in late summer are ideal for RED because climatology shows this area and timeframe to have the highest joint probability of strong winds and intense evaporation ducts (about 20 percent of the time winds are greater than 10 m s-1 with duct heights exceeding 15 m). Floating Instrumentation Platform (FLIP) was crucial to the success of RED. Moored about 10 km off of the NE portion of Oahu , FLIP hosted the primary meteorological sensor suites and served as one terminus for the electromagnetic propagation links.

The principal tasks of this experiment were:

1.Determine the extent to which ocean surface roughness modifies both extinction and the distribution of EM refractivity in the marine wave, surface, and boundary layers.

2.Evaluate and validate new parameterizations, accounting for surface roughness, of meteorological quantities and aerosol distributions in the marine wave, surface, and boundary layers.

The pressure measurements were part of a data acquisition system involving about 40 instruments deployed at six levels along a vertical mast. Data were collected continuously for two weeks. The Paroscientific instruments we used were two Met3A barometers and two 202BG bi-directional gauge pressure transducers, deployed at levels 2 and 3 on the mast. A Met3A and a 202BG formed a pair producing complementary measurements: the Met3A registered the total atmospheric pressure, while the more sensitive 202BG accurately detected the fluctuations. The 202BGs had one port connected to a reference pressure reservoir, while the other was open to the ambient pressure through the inlet port of the Met3A barometers. The high-performance pressure port of the Met3A eliminated dynamic pressure fluctuations due to winds. The inevitable distortions of the 202BG data resulting from the reference pressure reservoir, were later fully compensated by applying a digital filter to the 202BG signals.

The Paroscientific instruments worked flawlessly throughout the experiment. They met all of our expectations and produced a high quality data set. The measurements delivered good insight into the structure and dynamics of the marine atmospheric boundary layer. In particular, the pressure data brought clear information about the signature of the ocean surface waves in the air, which is responsible for wind-to-waves energy transfer.

The Rough Evaporation Duct project is supported by the Office of Naval Research and is organized and overseen by SPAWAR, San Diego. Researchers from SPAWAR, University of California, Johns Hopkins University, and others, participated in the project.

Author: This article was submitted by
SPAWAR and Johns Hopkins University