Avoiding Ship-Induced Light-Field Perturbation in the Determination of Oceanic Optical Properties

Abstract

THE DETERMINATION of in-water spectral irra- diance and the subsequent estimation of ocean optical properties is essential for the meaningful understand- ing and predictive modeling of the marine photoenvironment; this in turn is fundamental to the understanding of both biological and physical oceano- graphic processes. Further, the accurate estimation of near-surface optical properties is essential for the surface validation of ocean color satellite sensors. Two primary sources of error in the determination of optical prop- erties at sea are the perturbation of the in-water radiant energy field due to the ship (Gordon, 1985; Smith and Baker, 1986) and the estimation of quantitative cor- rections for atmospheric variability in radiant energy incident on the sea-surface during in-water measure- ments (Smith and Baker, 1984). Here we briefly describe two new optical oceanographic instrument and de- ployment techniques designed to reduce or eliminate these sources of error. The first is an Optical Free Fall Instrument (OFFI) designed to profile the watercolunm away from potential perturbation effects of the ship: the second is an Optical Surface Floating Instrument (OSFI) designed to obtain continuous optical data just below the air-water interface. A cartoon illustrating deployment of these instruments is shown in Fig. 1. Shipboard observations of spectral irradiance and radiance as a function of depth in the water column are commonly obtained by lowering an instrument on a hydrowire on the sunny side of the ship. Typically the instrument is lowered relatively close to the ship, which results in significant perturbation of the mea- sured in-water light field. Using Monte Carlo simula- tions, Gordon (1985) has shown that the bias in deter- mining downwelling irradiance seldom exceeds 1-2% for clear skies with the sun within + 45 ° of the beam of the ship, while the error can exceed 10% if the sun is off the bow or stem. While generally small, this error under clear skies is a function of wavelength. The error for overcast skies with a diffuse light field will be increased by the fraction of the sky obscured by the ship, after accounting for refraction and the Fresnel