Effects of optically shallow bottoms on upwelling radiances: Bidirectional reflectance distribution function effects

Abstract

Radiative transfer simulations were carried out for a variety of measured and modeled benthic bidirectional reflectance distribution functions (BRDFs), incident lighting conditions, bottom depths, and water inherent optical properties. These simulations quantify the errors that occur in predictions of above-surface remote-sensing reflectances and in-water upwelling radiances if non-Lambertian ocean bottoms are replaced by Lambertian bottoms having the same irradiance reflectance. We found that when computing water-leaving radiances, non-Lambertian BRDFs can be replaced by a Lambertian BRDF having the same irradiance reflectance, with errors seldom greater than 10% and often much less, for considerations of above-surface ocean-color remote sensing by Ocean Portable Hyperspectral Imager for Low-Light Spectroscopy (Ocean PHILLS) or similar systems using near-nadir viewing directions. The crucial measurement to make in characterizing the reflectance properties of a benthic surface for such applications is the spectral irradiance reflectance.

Light in shallow waters: A brief research review

“…Although the simulations included the usual assumption of Lambertian ocean floor reflectance and comparisons with field measurements were not considered, the work indicated that the resulting simulations were at least consistent between models using a variety of numerical approaches to solve the radiance-based problem. A comprehensive text on the general subject of oceanic radiative transfer was published the following year (Mobley 1994). In that same year, the first instance of an ocean radiative transfer model validation effort was reported-a simple one-dimensional irradiance model was compared with field measurements (Maritorena et al 1994).…”

“…An approach to replacing the constant reflectance factor with a morphologybased parameterization of a seagrass canopy was reported by Ackleson and Klemas (1986) using a one-dimensional analytical approach to solving the irradiance-based transfer problem; however, as in the previous work, the means to validate the model in the field were not available. Mobley et al (1993) reported an intermodel comparison representing a comprehensive set of environmental scenarios that included the effects of an optically shallow water column. Although the simulations included the usual assumption of Lambertian ocean floor reflectance and comparisons with field measurements were not considered, the work indicated that the resulting simulations were at least consistent between models using a variety of numerical approaches to solve the radiance-based problem.…”

“…Over the course of the past decade, ocean optical investigations have increased in number and scope because of an appreciation for global variability in ocean color as measured from space (Mitchell 1994 and references therein), the development of affordable in situ instruments to measure subsurface light fields and ocean optical properties (Maffione 2001 and references therein), and the development of highly accurate radiance-based radiative transfer models capable of running on standard desk-or laptop computers (Mobley 1994 and references therein). However, with respect to optically shallow environments, the primary advances that have enabled scientists to make quantitative optical observations of the ocean floor (e.g., diver-operated sensors) were only realized in the past 5 yr (Mazel 2001 and references therein).…”

“…All other expressions of surface reflectance can be expressed as a function of the BRDF and the illumination conditions (see Mobley et al [2003] for a more complete discussion of the relationships between different reflectance formulations).…”

“…The ocean radiative transfer problem and, to a lesser extent, the nature of optically important matter in ocean water is well understood in areas where the depth of the ocean floor is greater than the depth of sunlight penetration. In this situation, variability in the subsurface light field under a specified illumination condition and surface wave field (Walker 1994) is largely determined by the distribution of optically important matter dissolved and suspended in the water column, and light intensity generally decreases with depth in accordance with Beer's Law (Jerlov 1976;Kirk 1983;Mobley 1994). In shallow water, where the depth is much less than the potential for light to penetrate, a large fraction of the subsurface light reaches the ocean floor, where portions of the light energy are absorbed, reflected back into the overlying water column, or re-emitted as fluorescence.…”