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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.

The detectability of adjacency effects (AE) in ocean color remote sensing by SeaWiFS, MODIS-A, MERIS, OLCI, OLI and MSI is theoretically assessed for typical observation conditions up to 36 km offshore (20 km for MSI). The methodology detailed in Bulgarelli et al. (2014) is applied to expand previous investigations to the wide range of terrestrial land covers and water types usually encountered in mid-latitude coastal environments. Simulations fully account for multiple scattering within a stratified atmosphere bounded by a non-uniform reflecting surface, sea surface roughness, sun position and off-nadir sensor view. A harmonized comparison of AE is ensured by adjusting the radiometric sensitivity of each sensor to the same input radiance. Results show that average AE in data from MODIS-A, and from MERIS and OLCI in reduced spatial resolution, are still above the sensor noise level (NL) at 36 km offshore, except for AE caused by green vegetation at the red wavelengths. Conversely, in data from the less sensitive SeaWiFS, OLI and MSI sensors, as well as from MERIS and OLCI in full spatial resolution, sole AE caused by highly reflecting land covers (such as snow, dry vegetation, white sand and concrete) are above the sensor NL throughout the transect, while AE originated from green vegetation and bare soil at visible wavelengths may become lower than NL at close distance from the coast. Such a distance increases with the radiometric resolution of the sensor. It is finally observed that AE are slightly sensitive to the water type only at the blue wavelengths. Notably, for an atmospheric correction scheme inferring the aerosol properties from NIR data, perturbations induced by AE at NIR and visible wavelengths might compensate each other. As a consequence, biases induced by AE on radiometric products (e.g., the water-leaving radiance) are not strictly correlated to the intensity of the reflectance of the nearby land., Highlights • Quantification of adjacency effects for a wide range of coastal environments • Comparison of adjacency effects for data from various ocean color sensors • Relevance of adjacency effects as a function of the SNR of satellite sensors

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.

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

A methodology is presented to quantify the adjacency effects induced in satellite ocean color radiometric data by the presence of land in coastal regions. Specifically, the adjacency radiance, defined as the difference in top-ofatmosphere (TOA) signal when accounting for and when neglecting nearby mainland, is parameterized to decouple the dependence on the optical properties of land and water from those of atmosphere and sea surface, and from measurement geometry. The methodology is applied for a set of realistic satellite observation conditions, along a transect in the Northern Adriatic Sea crossing the Acqua Alta Oceanographic Tower (AAOT, 45.31N, 12.51E) ocean color validation site. The newly developed Novel Adjacency pert Urbation Simulator for Coast Al Areas (NAUSICAA) 3D backward Monte Carlo code and the well established highly accurate plane-parallel FEM radiative transfer code, are used to simulate signal contributions at TOA. Results at relevant ocean color center-wavelengths for the AAOT site i...

The cosine error of in situ seven-channel radiometers designed to measure the in-air downward irradiance for ocean color applications was investigated in the 412-683 nm spectral range with a sample of three instruments. The interchannel variability of cosine errors showed values generally lower than +/-3% below 50 degrees incidence angle with extreme values of approximately 4-20% (absolute) at 50-80 degrees for the channels at 412 and 443 nm. The intrachannel variability, estimated from the standard deviation of the cosine errors of different sensors for each center wavelength, displayed values generally lower than 2% for incidence angles up to 50 degrees and occasionally increasing up to 6% at 80 degrees. Simulations of total downward irradiance measurements, accounting for average angular responses of the investigated radiometers, were made with an accurate radiative transfer code. The estimated errors showed a significant dependence on wavelength, sun zenith, and aerosol optical thickness. For a clear sky maritime atmosphere, these errors displayed values spectrally varying and generally within +/-3%, with extreme values of approximately 4-10% (absolute) at 40-80 degrees sun zenith for the channels at 412 and 443 nm. Schemes for minimizing the cosine errors have also been proposed and discussed.