RED experiment: an assessment of boundary layer effects in a trade winds regime on microwave and infrared propagation over the sea, The

Includes bibliographical references (pages 1364-1365).The Rough Evaporation Duct experiment aimed to see if the effects of ocean waves account for errors in modeling the ranges at which radar and infrared can detect low-flying targets

Infrared Propagation in the marine atmospheric surface layer: extinction and refraction

9.8The near-surface portion of the marine atmospheric surface layer is a dynamic propagation environment for optical and infrared (IR) signals. Particularly eminent are the effects of strong vertical refractivity gradients and localized aerosol gradients. The type and concentration of aerosols and gases in the intervening atmosphere result in a degradation of the IR and visible (VIS) signals. For a number of different viewing angles close to the horizon the atmospheric transmittance is determined by the absorption by atmospheric gases and by the absorption and scattering by aerosols. A longrange goal of the studies of the performance of IR/VIS systems is to obtain an understanding of the effects generated by the mixture of different meteorological conditions, locations (over ocean or over land), and solar position. Typically the atmospheric effects result in three primary distortions: a) extinction, which results from absorption and scattering by aerosols and molecules, b) refraction, which results from the collective bending of the beam, and c) scintillation, which results from incoherent scattering. The above distortions affect various types of systems, i.e., IRST (IR Search & Track), and wavelengths, such as laser at IR and visible frequencies, and radar in radio frequencies

Recent results on modeling the refractive-index structure parameter over the ocean surface using bulk methods

Infrared scintillation measurements were obtained along a 7.2 km path over San Diego Bay, concurrently with mean meteorological and turbulence measurements obtained from a buoy located along the path. Bulk estimates and turbulence measurements of Cn 2 were computed from the buoy data and compared with the optical scintillation-derived Cn 2 values. Similar to the results of previous experiments, the bulk Cn 2 estimates agreed well with both the scintillation and turbulence measurements in unstable conditions, increasingly underestimated Cn 2 as conditions approached neutral, and agreed less well with scintillation and turbulence Cn 2 values in stable conditions. The mean differences between bulk Cn 2 estimates and both the turbulence and scintillation measurements when conditions were not near-neutral exhibited an air-sea temperature difference and wind speed dependence, possibly indicating that the forms of the empirical stability functions used by the bulk model are incorrect. The turbulent Cn 2 measurements from the buoy showed excellent agreement with the scintillation values in unstable conditions, but had surprisingly large differences in weakly stable conditions. This disagreement may be related to the fact that humidity fluctuations begin to increasingly influence refractive index fluctuations when the air-sea temperature difference is small and are not properly taken into account by the sonic temperature measurements. As the absolute air-sea temperature difference approaches zero the bulk Cn 2 estimates decrease much more rapidly and to much smaller values than either the scintillation or turbulence measurements. Fortunately, in such near-neutral conditions scintillation is usually small enough to have little effect on many optical system applications.This work was funded by the Naval Sea Systems Command, PMS405, Dr. Sadegh Siahatgar, program manager

Near-Surface Scintillation in the Marine Atmospheric Layer During the Red Field Campaign

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Air-sea interaction processes observed from buoy and propagation measurements during the red experiment

In recent years researchers have spent much effort towards gaining an understanding of the complex physical mechanisms through which the atmosphere and ocean interact with each other. This is due to the fact that knowledge of air-sea exchanges is important for a wide range of applications, such as the diverse topics of global climate modeling and near-horizon electromagnetic (EM) wave propagation assessment and prediction. EM propagation through the atmosphere is highly dependent upon the vertical profiles of air temperature and humidity and the horizontal variations in these profiles. It is well known that under most conditions these near-surface scalar profiles depend upon the turbulent air-sea fluxes. Traditional Monin-Obukhov similarity (MOS) theory has been used to successfully predict near-surface profiles over the ocean for most, but not all, stability conditions. It is also becoming increasingly clear that ocean waves influence near-surface profiles, although an understanding of the exact mechanisms through which this occurs and parameterizations to describe these processes so far have remained elusive (e.g. Hare et al. 1997; Hirstov et al. 1998)

Low-altitude infrared propagation in a coastal zone: refraction and scattering

Applied Optics, Volume 41, No. 18, pp. 3706-3724 (20 June 2002)Midwave and long-wave infrared propagation were measured in the marine atmosphere close to the surface of the ocean. Data were collected near San Diego Bay for two weeks in November 1996 over a 15-km horizontal path. The data are interpreted in terms of effects expected from molecules, aerosol particles, and refraction. Aerosol particles are a dominant influence in this coastal zone. They induce a diurnal variation in transmission as their character changes with regular changes in wind direction. A refractive propagation factor calculation is introduced, and it is systematically applied to the model and to the data analysis. It is shown that this refractive propagation factor is a necessary component of a complete near-sea-surface infrared transmission model

The Red Experiment: an assessment of boundary layer effects in a trade winds regime on microwave and infrared propagation over the sea

The article of record as published may be found at http://dx.doi.org/10.1175/BAMS-85-9-1355The Rough Evaporation Duct experiment aimed to see if the effects of ocean waves account for errors in modeling the ranges at which radar and infrared can detect low-flying targets.Office of Naval Research (Code 322M