SSC San Diego San Diego, CA 92152--5001

This report presents an approximate formula for the grazing reflection of a circular source by the rough sea. The formula can be applied, for example, to the reflectivity along the center of a sun glint pattern or to the reflectivity of a missile plume (considered as a circle) as seen from the deck of a ship. In the optical region, the reflectivity of an ocean without gravity waves can be calculated using geometrical optics. The reason is that the wavelength of the radiation being reflected is much less than the radius of curvature of a typical capillary wave facet. The ocean can therefore be modeled as a collection of flat, mirror-like facets whose tilts fluctuate at random under the wind's influence. If the slope of a facet is correct, a ray from a source such as the Sun will be reflected into an optical receiver such as the eye. The problem then reduces to finding the Fresnel reflectivity of a small seawater mirror and counting the relative number of mirrors which, on average, can reflect a ray from the source into the receiver. Of course, Cox and Munk (1954, 1955, and 1956) have already addressed this problem in a famous series of papers. They measured the statistical occurrence of various capillary wave slopes by taking photographs of sun glitter from an airplane. However, their equation for reflected radiance applies to the case where the ocean is observed from above, not to the grazing case. When calculating reflectivity, Cox and Munk assumed that a patch of ocean of area A appears to have an area of A sinY when viewed from the side at a grazing angle, Y. In other words, for that part of their calculation, they assumed that the patch was flat. Because radiance involves the power received per unit solid angle per unit projected area, and because the projected area..

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Sea spray aerosol in the marine coastal atmosphere are important for various reasons. They offer a surface for condensation of chemically reactive species such as HNO3, may play a role in corrosion of structures at sea or ashore, sustain fragile eco-systems, transport pollutants and micro-biological species from the sea surface into the atmosphere where they may cause health effects, etc. They also scatter electro-optical radiation and thus affect the transmission at wavelengths from the UV to the IR. An example of the effect on IR transmission in the coastal region is discussed in this contribution

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This document was prepared for the Office of Naval Research (ONR 322) by the Atmospheric Propagation Branch, Code D858, SSC San Diego. ACKNOWLEDGMENTS Dr. Scott Sandgathe and Dr. Ronald Ferek of the United States Office of Naval Research supported this work. Mr. Stuart G. Gathman generously supplied details of his aerosol models as well as the optical indices and unpublished reports in the aerosol literature. I thank Dr. Warren J. Wiscombe for his Mie subroutine, MIEV0, which is an indispensable part of this work. iii EXECUTIVE SUMMARY OBJECTIVE The objective of this project was to develop a computer program that will predict the optical consequences of the Navy Aerosol Model (NAM). The Navy Aerosol Model describes the marine aerosol close to the surface of the open ocea

Atmospheric effects on low elevation transmission measurements at EOPACE

An analysis is presented showing the effects of refraction, aerosol extinction, and molecular extinction on transmission measurements obtained during the EO Propagation Assessment in Coastal Environments (EOPACE) campaign carried out in San Diego during March and April 1996. Infrared transmission measurements were made over both a 7 km path (mid IR) and a 15 km path (mid JR and far IR) at heights below 10 m above sea level. The average difference between all the measured transmissions and aerosol transmittances over the two paths with results obtained using the JR Boundary Layer Effects Model (IRBLEM) were found to be relatively small, even though the difference for individual measurements can be significant. The effect of molecular transmittance, as calculated using MODTRAN, is found to reduce the transmission by about 35% forthe 7 km path, 72% for the mid JR over the 15 km path, and between 70% and 90% for the far JR over the 15 km path.The effect of aerosol transmittance, as calculated using a variation of the Navy Aerosol Model (NAM), is found to reduce the transmission from 10% to 90% for the mid JR over both the 7 and 15 km paths, and from 10% to 60% for the far JR over the 15 km path. The effect of refractance, the focussing and defocussing of radiation due to atmospheric refraction, on the predicted transmissions is found to account for gains and losses up to 20% for the 7 km path, and gains and losses up to 100% for the 15 km path. Consequently, any JR transmission model for the marine boundary layer (MBL) must properly take into account the effects on the transmission due to molecular extinction, aerosol extinction, and refractance. ©2005 Copyright SPIE - The International Society for Optical Engineering