FY 1985

Unpolarized irradiance reflectances and glitter patterns of random capillary waves on lakes and seas, by Monte Carlo simulation

Preisendorfer, R.W., and C.D. Mobley

NOAA Tech. Memo. ERL PMEL-63, NTIS: PB83-123577, 141 pp (1985)

The downward irradiance reflectance r and the upward irradiance reflectance r of a random air-water surface, formed by capillary waves, are computed as a function of lighting conditions and wind speed by Monte Carlo means for incident unpolarized radiant flux. The possibility of multiple scattering of light rays and of ray-shielding of waves by other waves is included in the calculations. The effects on r of multiple scattering and wave shielding are found to be important for higher wind speeds (10 m/s) and near horizontal light ray angles (70°) of incidence. A simple analytic first-order model of irradiance reflectance, which assumes a binormal distribution of water facet slopes, is tested against the relatively exact Monte Carlo results. Regions are defined in wind-speed and incident-angle space over which the first-order model is acceptable. Plots of the Monte Carlo r are drawn as functions of wind speed and angle of incidence of light rays. r are also found for various continuous radiance distributions simulating overcast skies and upwelling submarine light fields just below the air-water surface. Simulated glitter patterns are displayed as functions of wind speed and angle of incidence of light rays for both reflected and transmitted rays, and for light sources located both above and below the air-water surface. Extensions of the present Monte Carlo procedure to include gravity waves as well as capillary waves are outlined.