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A radiative transfer code termed OSOA for the ocean-atmosphere system that is able to predict the total and the polarized signals has been developed. The successive-orders-of-scattering method is used. The air-water interface is modeled as a planar mirror. Four components grouped by their optical properties, pure seawater, phytoplankton, nonchlorophyllose matter, and yellow substances, are included in the water column. Models are validated through comparisons with standard models. The numerical accuracy of the method is better than 2%; high computational efficiency is maintained. The model is used to study the influence of polarization on the detection of suspended matter. Polarizing properties of hydrosols are discussed: phytoplankton cells exhibit weak polarization and small inorganic particles, which are strong backscatterers, contribute appreciably to the polarized signal. Therefore the use of the polarized signal to extract the sediment signature promises good results. Also, polarized radiance could improve characterization of aerosols when open ocean waters are treated.

[1] A field experiment was carried out in summer 2002 on an oceanographic platform near the coast of Crimea, in the Black Sea. For the first time, the spectral volume scattering function (VSF) was measured for a wide range of scattering angles (i.e., from 0.6 to 177.3 degrees) using a recently developed device. Our analysis revealed that the mineral particles are the primary component influencing the scattering and backscattering coefficient in the study area. The good correlation obtained between the backscattering coefficient b bp and the nonalgal particles absorption coefficient showed that the absorption efficiency of the mineral particles is high in the second half of the experiment. The ratio Chla/c p (where Chla is the chlorophyll a concentration and c p is the beam attenuation coefficient) did not correlate with the backscattering ratio and thus could not be used in this experiment as an alternative proxy to estimate the bulk composition of the particles. The spectral variation of b p (the scattering coefficient) and b bp (the backscattering coefficient) was less steep than what can be found in the open ocean waters. That was explained by the influence of the absorption on the scattering process, especially in the blue, as a consequence of the anomalous dispersion. The average backscattering ratiõ b bp varied spectrally within 4%. Nevertheless, a high spectral variability ofb bp (around 30%) was observed suggesting that the use of a flat spectral variation is not accurate in coastal zones.

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correction of the Multi-Spectral Instrument (MSI)-SENTINEL-2 imagery over inland and sea waters from SWIR bands. Remote Sensing of Environment, Elsevier, 2018, 204, pp.308-321 A B S T R A C TRemote sensing of inland and sea waters depends on the quality of the retrieval of the water-leaving radiance from the top-of-atmosphere measurements. The water-leaving radiance can be difficult to observe due to the reflection of direct sunlight on the air-water interface (sunglint) in the direction of the satellite field of view. The viewing geometry of Sentinel-2 satellite (European Space Agency) makes it vulnerable to sunglint contamination. In this paper, an original method is proposed to correct Sentinel-2-like imagery for sunglint contamination. The sunglint contribution is first estimated from the shortwave-infrared (SWIR) part of the spectrum and then extrapolated toward the near-infrared and visible bands. The spectral variation of the sunglint signal is thus revisited for a wide spectral range (from 350 to 2500 nm). The bidirectional reflectance distribution function related to the sunglint is shown to vary by > 28% from the SWIR to the blue bands of Sentinel-2. The application of the proposed algorithm on actual Sentinel-2 data demonstrates that sunglint patterns are satisfactorily removed over the entire images whatever the altitude of the observed target. Comparison with in situ data of water-leaving radiances (AERONET-OC) showed that our proposed algorithm significantly improves the correlation between satellite and in situ data by 55% (i.e., from R 2 = 0.56 to R 2 = 0.87). In addition, the discrepancies between satellite and in situ measurements are reduced by 60%. It is also shown that the aerosol data provided by the Copernicus Atmosphere Monitoring Service (CAMS) can be safely used within the proposed algorithm to correct the Sentinel-2-like satellite data for both sunglint and atmospheric radiances. Improvements of the proposed method potentially rely on simultaneous retrievals of the aerosol optical properties. The proposed method is applicable to any satellite sensor which is able to measure in SWIR spectral bands over aquatic environments.

[1] The inverse problem of ocean color consists in deriving the inherent optical properties (IOP) of marine particles from a reflectance spectrum measured at the sea surface. Such a problem is ill-posed or ambiguous because of the nonuniqueness of the solution; that is, several combinations of IOP values can lead to a unique reflectance spectrum. Currently, great efforts are made in the development of inverse methods to accurately retrieve the IOPs. However, many fewer studies have been devoted to the analysis of the ambiguities, which affect yet the error on the IOPs retrieval. In this paper, the ambiguities related to the ocean color problem in coastal waters are characterized and their implications for inverse modeling are studied. A synthetic data set is created on the basis of radiative transfer modeling. The simulations are constrained using in situ observations and statistical rules to make the data set realistic. The ambiguity rate of remote sensing reflectance (Rrs) spectra is around 90%, thus meaning that the ocean color problem is extremely ambiguous. The influence of the ambiguities on the IOPs retrieval is evaluated. It is demonstrated that the error that is ascribed to the occurrence of ambiguity is equal to the dispersion of all the plausible IOPs solutions. The ambiguity error made on the total absorption coefficient is shown to be greater in highly absorbing water mass. On the other hand, the ambiguity error made on the total backscattering coefficient is higher in turbid scattering waters. Finally, different strategies to reduce the effects of ambiguities are discussed.Citation: Defoin-Platel, M., and M. Chami (2007), How ambiguous is the inverse problem of ocean color in coastal waters?,

[1] The particulate backscattering coefficient b bp is an inherent optical property that plays a central role in studies of ocean color remote sensing. Because of practical difficulties associated with measurements of the volume scattering function (VSF) over the whole backward hemisphere, b bp is currently derived using fixed-angle backscattering sensors and applying a conversion factor for particulate backscattering, referred to as c p . The underlying assumptions of the fixed-angle approach are as follows: (1) in the green band, c p is fairly constant in the angular range 100°-150°and (2) for a fixed scattering angle, c p is wavelength-independent. In this study we investigated the variability of c p based on spectral measurements of the full VSF, both in situ and for algal culture in the laboratory. The in situ data used in our study were acquired in a coastal environment outside of phytoplankton blooms, whereas the laboratory data were representative for phytoplankton bloom conditions in oceanic waters. At 555 nm, c p was found to vary significantly in the angular range 100°-130°, and at 140°, c p was found to be weakly variable in nonblooming waters only. The spectral variability of c p was studied for the first time, and the spectral slopes of c p , measured in situ, were found to vary within ±6%. Under the assumption that c p (140°) is wavelength-independent, the induced error in the estimates of b bp was found to be lower than 10%. The algal culture showed a much higher spectral variability in c p (±20%), which induced an error in the estimates of b bp up to ±25.8%.Citation: Chami, M., E. Marken, J. J. Stamnes, G. Khomenko, and G. Korotaev (2006), Variability of the relationship between the particulate backscattering coefficient and the volume scattering function measured at fixed angles,

The particulate backscattering ratio (b(bp)/b(p)) is a useful indicator of the angular scattering characteristics of natural waters. Recent studies have shown evidence both for and against significant spectral variability in b(bp)/b(p) in the visible domain, but most show significant variability in its magnitude. We present results from a case study in which both backscattering and scattering coefficients were measured at nine wavelengths in a region of UK coastal waters where optical scattering is strongly influenced by inorganic particles and where a wide range of turbidities is found in a small geographic area. Using a new approach based on regression analysis of in situ signals, it is shown that, for this study site, most of the apparent variability in the magnitude of the backscattering ratio can be attributed to measurement uncertainties. Regression analysis suggests that b(bp)/b(p) is wavelength dependent for these mineral-rich waters. This conclusion can only be avoided by positing the existence of undocumented, systematic, wavelength-dependent errors in backscattering measurements made by two independently calibrated sensors. These results are important for radiative transfer simulations in mineral-dominated waters where the backscattering ratio has often been assumed to be spectrally flat. Furthermore, spectral dependence also has profound implications for our understanding of the relationship between b(bp)/b(p) and particle size distributions in coastal waters since the commonly assumed power-law distribution is associated with a spectrally flat particulate backscattering ratio for nonabsorbing particles.