Optical Closure of Apparent Optical Properties in Coastal Waters off Goa

Abstract

Inherent and apparent optical properties were measured for coastal waters off Goa, a site on the west coast of India using the in-situ measuring profiling optical instruments. The optical closure was obtained using the radiative transfer model, Hydrolight (Sequoia Inc.). Hydrolight simulations were carried out at all stations using “average particle” Petzold phase function and Fournier Forand phase function with optimum bbp/bp value. The optical properties derived from Hydrolight simulations were then compared with the measured optical properties. The widely used “average particle” phase function measured by Petzold 1972 for coastal waters was found to provide poor results for most of the stations while for few others it was found to produce results comparable to Fournier Forand phase function. The mean percentage deviation for water leaving radiance (remote sensing reflectance) simulated using Petzold and Fournier Forand phase function for wavelength 488 nm varied from 3.8 to 240.7 % (3.8 to 240.8 %) and 1.0 to 48.9 % (1.0 % to 49.0 %) respectively. These closure studies are essential in strengthening the confidence in models or algorithms developed for ocean color remote sensing and also in better understanding of the marine optical properties of waters of the study area.