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The sea-surface reflectance factor ρ required for the determination of the water- leaving radiance from above-water radiometric measurements is derived from radiative transfer simulations relying on models of the sky-radiance distribution and sea-surface statistics. This work primarily investigates the impact on ρ of various sky-radiance and sea-surface models. A specific replicability analysis, restricted to the 550 nm wavelength, has been performed with the Monte Carlo code for Ocean Color Simulations (so-called MOX) accounting for the measurement geometry recommended in protocols for the validation of satellite ocean color data and commonly applied for operational measurements. Results indicate that the variability of ρ increases with wind speed and reaches the largest values for sun elevations close to the zenith or approaching the horizon. In particular, a variability up to about 2% is observed for wind speeds below 4 ms−1 and sun zenith angles larger than 20°. Finally, the benchmark of the ρ values from this study with those formally determined with the Hydrolight code and widely utilized by the ocean color community, exhibits systematic differences. The source of these differences is discussed and the implications for field measurements are addressed.

The adjacency radiance field surrounding a small island (i.e., the Lampedusa Island in the Central Mediterranean Sea) was theoretically analyzed to address implications on a hypothetical nearby system vicarious calibration (SVC) infrastructure for satellite ocean color sensors. Simulations, performed in the visible and near-infrared regions for the Ocean Land Color Instrument (OLCI) operated onboard Sentinel-3 satellites, show different patterns of adjacency effects (AE) around the island. In the direction of the reflected sunbeam (i.e., in the north-western region), AE mainly originate by missing glint contributions from the sea surface masked by the island. These AE are mainly negative, decrease with wavelength, and strongly depend on sea surface anisotropy (i.e., sea state) and illumination conditions; this hinders the capability to provide a general unique description of their features. In the remaining marine regions, AE are positive and do not exceed the radiometric sensitivity of OLCI data beyond approximately 14 km from the coast. At shorter distances, uncertainties in satellite radiance due to AE would hence not allow fulfilling requirements for SVC.

This publisher’s note corrects an equation in Appl. Opt. 59, C63 (2020).APOPAI0003-693510.1364/AO.378512

The minimization of adjacency effects (AE) in SeaWiFS primary products at the Aqua Alta Oceanographic Tower (AAOT) was investigated using sample images concurrent with in situ measurements. The validation exercise was performed with the NASA SeaDAS processing scheme ingesting original SeaWiFS data and alternatively SeaWiFS top-of-atmosphere data corrected for AE, and additionally including and excluding the default turbid water (TW) correction algorithm. Results show overestimates of the TW contributions partially compensating for AE. The analysis also suggests that intra-annual biases observed in SeaWiFS radiometric products at the AAOT may result from a misinterpretation of the NIR atmospheric signal as water contribution in data acquired in winter, and from uncompensated AE in data acquired in summer.

An extensive comparison between radiative transfer codes in the atmosphere–ocean system is proposed and performed. The full angular radiance distribution at several optical depths in atmosphere and water is intercompared on a set of idealized problems designed to study codes’ accuracy in modeling separate, specific system features. In-water profiles of upwelling nadir radiance, upwelling and downwelling irradiance are intercompared for a realistic case extracted from an experimental data set. Two models are involved: the FEM numerical algorithm, based on the finite element method, and the P HO-TRAN 3D backward Monte Carlo code. The results show an optimal agreement between the codes under any condition. Codes’ relative differences are always lower than the estimated statistical error on the P HO-TRAN results.

The well-known expression of Cox and Munk for the bi-directional reflectivity of a rough water surface tends to infinity for reflection angles close to the horizontal direction. This creates evident difficulties in applying it in numerical radiative transfer schemes. Detailed analysis of the process of reflection from a non-flat surface shows that the singularity arises from some contradictory implicit assumptions adopted in the theoretical derivation of the bi-directional reflectivity expression. In the present paper, these assumptions are modified in order to provide for consistency and a bi-directional reflectivity expression applicable for all possible reflection angles is derived. An analytical form of Fourier expansion of bi-directional reflectivity in azimuthal variables is also obtained.

A methodology has been developed and applied to accurately quantify and analyze adjacency effects in satellite ocean color data for a set of realistic and representative observation conditions in the northern Adriatic Sea. The study, properly accounting for sea surface reflectance anisotropy, off-nadir views, coastal morphology and atmospheric multiple scattering, include a sensitivity analysis on commonly applied approximations. Results indicate that, within the accuracy limits defined by the radiometric resolution of the ocean color sensors, adjacency effects in coastal waters might be significant at both visible and NIR wavelengths up to several km off the coast. These results additionally highlight a significant dependence on the angle of observation, on the directional reflectance properties of the sea surface, and on the atmospheric multiple scattering., JRC.H.1-Water Resources