Search

There has been a large effort to relate the apparent optical properties of ocean water to the inherent optical properties, which are the absorption coefficient a, the scattering coefficient b, and the scattering phase function [Rho]([Theta]). The diffuse attenuation coefficient [k.sub.diff] has most often been considered an apparent optical property. However, [k.sub.diff] can be considered a quasi-inherent property [k.sub.diff][prime] when defined as a steady-state light distribution attenuation coefficient. The Honey-Wilson research empirically relates [k.sub.diff] to a and b. The Honey-Wilson relation most likely applies to a limited range of water types because it does not include dependence on [Rho]([Theta]). A series of Monte Carlo simulations were initiated to calculate [k.sub.diff][prime] in an unstratified water column. The calculations, which reflected open ocean water types, used ranges of the single-scattering albedo [[Omega].sub.0] and the mean forward-scattering angle [[Theta].sub.m] for two analytic phase functions with different shapes. It was found that [k.sub.diff][prime] is nearly independent of the shape of [Rho]([Theta]) and can be easily parameterized in terms of a, b, and [[Theta].sub.m] for 0.11 [less than or equal to] [[Theta].sub.m] [less than or equal to] 0.48 rad and 0.5 [less than or equal to] [[Omega].sub.0] [less than or equal to] 0.95. [k.sub.diff][prime] is an asymptotic quantity; that is, a steady-state distribution is reached only after many scattering lengths.

An algorithm is described and evaluated for determining the absorption and backscattering coefficients a(z) and [b.sub.b](z) from measurements of the nadir-viewing radiance [L.sub.u](z) and downward irradiance [E.sub.d](z). The method, derived from radiative transfer theory, is similar to a previously proposed one for [E.sub.u](z) and [E.sub.d](z), and both methods are demonstrated with numerical simulations and field data. Numerical simulations and a sensitivity analysis show that good estimates of a(z) and [b.sub.b](z) can be obtained if the assumed scattering phase function is approximately correct. In an experiment in Long Island Sound, estimates of a(z) derived with these methods agreed well with those obtained from an in situ reflecting tube instrument.

The graphic presentation of relationships between a, b, R, and micron bar in a previously published nomogram on calculation of inherent optical properties (Aust. J. Mar. Freshwater Res. 32, 533 (1981)) are replaced by explicit mathematical expressions.

A model developed recently by Loisel and Stramski [Appl. Opt. 39, 3001-3011 (2000)] for estimating the spectral absorption a([Lambda]), scattering b([Lambda]), and backscattering [b.sub.b]([Lambda]) coefficients in the upper ocean from the irradiance reflectance just beneath the sea surface R([Lambda], z = [0.sup.-]) and the diffuse attenuation of down-welling irradiance within the surface layer [([K.sub.d]([Lambda])).sub.1] is compared with measurements. Field data for this comparison were collected in different areas including off-shore and near-shore waters off southern California and around Europe. The a([Lambda]) and [b.sub.b]([Lambda]) values predicted by the model in the blue-green spectral region show generally good agreement with measurements that covered a broad range of conditions from clear oligotrophic waters to turbid coastal waters affected by river discharge. The agreement is still good if the model estimates of a([Lambda]) and [b.sub.b([Lambda]) are based on R([Lambda], z = [0.sup.-]) used as the only input to the model available from measurements [as opposed to both R([Lambda], z = [0.sup.-]) and [([K.sub.d]([Lambda])).sub.1] being measured]. This particular mode of operation of the model is relevant to ocean-color remote-sensing applications. In contrast to a([Lambda]) and [b.sub.b]([Lambda]) the comparison between the modeled and the measured b([Lambda]) shows large discrepancies. These discrepancies are most likely attributable to significant variations in the scattering phase function of suspended particulate matter, which were not included in the development of the model. [C] 2001 Optical Society of America OCIS codes: 010.4450, 030.5620, 290.5860, 280.0280.