ISLAND PERTURBATION TO THE SKY RADIANCE OVER THE OCEAN - SIMULATIONS

Sky-radiance measurements at the sea surface can be used to estimate r adiative properties of aerosols over water. We demonstrate, through Mo nte Carlo simulations, that significant perturbations to sky radiance over the ocean can occur when measurements are carried out with radiom eters located on islands. In particular, we present examples of the in fluence of the physical and optical thicknesses of an aerosol layer, t he azimuth of observation relative to the Sun, the size of the island, the location of the radiometer on the island, and the albedo of the i sland on the magnitude of the perturbation for a circular island of un iform albedo. Relative errors in sky radiance of as high as 39% were f ound in the blue. Simulated (perturbed) sky radiances were combined wi th an algorithm for retrieving the aerosol phase function P(theta), wh ere theta is the scattering angle, and with the single-scattering albe do omega(0), to demonstrate how the perturbation can influence the ret rieved values. It was found that the fractional error in the retrieved values of the product omega(0)P(theta) can be significantly greater t han the fractional error in the sky radiance, because of the effects o f multiple scattering. This underscores the importance of removing the island perturbation before an inversion algorithm is used. A first-or der procedure for removing the island perturbation based on the values of omega(0)P(theta) retrieved from the perturbed sky radiance is prop osed and is found to be effective if the island perturbation is not to o large. A simplified Monte Carlo procedure that is applicable to an i sland of arbitrary shape and albedo distribution is presented. The pro cedure could be used to assess the suitability of a given island as a measurement site, and to provide a first-order correction to actual ex perimental measurements. (C) 1995 Optical Society of America