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.

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 procedure properly accounts for sea surface reflectance anisotropy, off-nadir views, coastal morphology, and atmospheric multiple scattering. The study further includes a sensitivity analysis on commonly applied approximations. Results indicate that, within the accuracy limits defined by the radiometric resolution of ocean color sensors, adjacency effects in coastal waters might be significant at both visible and near-infrared wavelengths up to several kilometers 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.

[1] The aerosol optical properties in the Adriatic Sea are presented using a 9-year time series (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) of automated measurements collected on the Acqua Alta Oceanographic Tower (AAOT) in the northern part of the basin and a coincident satellite record obtained from an atmospheric correction scheme adapted for European seas and applied to the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). At AAOT, the overall averages of aerosol optical thickness t a at 500 nm and Å ngström exponent a are 0.29 ± 0.21 and 1.51 ± 0.34, respectively. The average single scattering albedo varies from 0.957 at 440 nm to 0.910 at 1020 nm. The aerosol size distribution derived by optical data inversion exhibits an increase of the radius of the accumulation mode with t a . From 402 coincident data records, the agreement between satellite and field t a is remarkable, with mean absolute percentage differences of 17-20% in the 412-870 nm spectral range. On the other hand, the satellite record tends to filter out occurrences of high t a . The satellite-derived products are used to analyze the seasonal cycle over the Adriatic basin, showing minima in winter and maxima in summer (t a (500) from 0.06 to 0.23). Then, the results of radiative transfer simulations are combined with the satellite-derived seasonal cycles of t a and cloud fraction to determine the clear-sky aerosol direct radiative effect for the Adriatic Sea. At the surface, the aerosol load results in a monthly average cooling effect ranging from À1 W m À2 in winter up to À9.6 W m À2 in August and a corresponding atmospheric warming from 0.5 to 4.6 W m À2 .

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