Analysis of adjacency effects for Copernicus ocean colour missions

The present report focuses on the uncertainties induced by nearby land in OC observations of coastal regions, summarizing most recent quantifications and analyses. Standard algorithms for the processing of satellite data generally assume an infinite water surface, and hence neglect the presence of the nearby land. As a consequence, the radiance reflected by the land and then scattered by the atmosphere in the field of view of a satellite sensor observing a water target represents a source of perturbations leading to uncertainties in OC products. This phenomenon is called adjacency effects (AE), and always occurs in the presence of a scattering medium overlaying a surface of non-homogeneous reflecting properties. Specific attention is given to AE affecting marine observations by two EOdedicated satellite sensors of the Copernicus Space component: i) the Ocean and Land Colour Instrument (OLCI) on board Sentinel-3, specifically developed to deliver OC observations of the sea; and ii) the MultiSpectral Imagery (MSI) on board Sentinel-2, which, aims at providing high-resolution optical land imagery, but also acquires data up to 20 km offshore. AE are quantified and analyzed for a wide range of typical mid-latitude coastal environments and for specific case studies, i.e., the Aqua Alta Oceanographic Tower (AAOT) validation site located in the Northern Adriatic Sea, included in the Ocean Color component of the Aerosol Robotic Network (AERONET-OC), also considered for vicarious calibrations of marine MSI data; and the marine region surrounding the Lampedusa Island located in the Southern Mediterranean Sea, hosting a validation site, and considered for long-term vicarious calibrations of OLCI data. The study analyzes the relevance of AE in the signal at the sensor with regard to standardized signal-to-noise ratios (SNR). Considerations are also drawn on perturbations induced by AE in satellite radiometric products. The content of this Report builds on the long-standing experience of the JRC on the modeling of OC satellite and in situ observations. This experience counts on the development and decadal utilization of highly accurate radiative transfer models (RTM) for the propagation of the solar radiation in the atmosphere-ocean system. These in-house modeling capabilities (the Advanced Radiative Transfer Models for In-situ and Satellite Ocean color data, ARTEMIS-OC) comprise a plane-parallel numerical RTM based on the finite element method and a three-dimensional (3D) MonteCarlo (MC) code. Overall, this Report summarizes a number of recent investigations led by the JRC on AE in satellite observations of coastal waters. The final objective is to consolidate in a single document theoretical findings and considerations about adjacency perturbations from nearby land in the coastal remote sensing observations performed within the Copernicus Programme. Briefly, the various Chapters summarize: - The general definition and description of the AE, while briefly illustrating the applied modeling technique; - The theoretical quantification of AE for a wide range of typical midlatitude coastal environments. - The theoretical evaluation of AE at the AAOT and Lampedusa validation sites.