A RADIATIVE TRANSFER MODEL FOR REMOTE SENSING OF VERTICALLY INHOMOGENEOUS WATERS

In remote sensing of water, it is regularly assumed that the water is vertically homogeneous over the effective depth of penetration of the remote instrument. It appears that this assumption is often valid given many successful applications of remote sensing using this assumption. Yet, there will be conditions for which vertical inhomogeneities will be both significant and detectable. In order to explore the sensitivity of remote observations to vertical inhomogeneities a mathematical model is developed which will illustrate the qualitative aspects of the problem, and which, at the same time, will give reasonably accurate quantitative results.;The connection of the SSI model with two-flow theory is established for the case of deep, homogeneous waters, in fact, for this case the SSI model represents a simplified version of two-flow theory. The simplification consists in ignoring a term which represents the downward scattering of upwelling light. While this term is actually small for most oceanic water types (and even for many coastal and inland waters), it may not be negligible when scattering becomes the dominant mechanism of attenuation.;Given equivalent assumptions, the SSI model yields results that are very close to Monte Carlo calculations for deep, homogeneous waters, shallow, homogeneous waters and optically deep, two-layers waters. Only in the last of these were calculations applied to strongly scattering waters ((omega)(,o )= 0.8), for which the results were almost as good as for the clearer waters ((omega)(,o) < 0.6).;The real strength of the SSI model lies in the relative ease with which results can be obtained for complete spectra (rather than only single wavelengths) and for complex vertical distributions of water properties. This becomes particularly apparent when two- and three-layered systems are considered. The SSI model results compare well to Monte Carlo calculations for a two-layer ocean when the same restrictive assumptions are made. The assumptions are necessary in order to optimize the Monte Carlo calculations. More generalized results are difficult to obtain with Monte Carlo techniques but are easily obtained using the SSI model.;The model is relatively standard in its approach to describing (1) the downwelling light field above the water surface and (2) the transmission and reflection of light at the air-water interface. In contrast, the approach to radiative transfer within the water column is more novel in that it relies entirely on apparent optical properties of the water, an approach that is justified by the regularities in the underwater light field. A major advantage to this approach is that multiple scattering is handled implicitly, resulting in considerable simplification in the final equations.