Analysis of adjacency effects for Copernicus Ocean Colour Missions

摘要

The Copernicus Programme was established by the European Union (Regulation EU No377/2014) to develop European information services based on satellite Earth Observation (EO) and in situ data. \nAmong the six Copernicus Services, the Copernicus Marine Environment Monitoring Service (CMEMS) and the marine component of the Copernicus Climate Change Service (C3S) both rely on EO data delivered by satellite ocean color (OC) sensors, i.e., primary OC radiometric products (such as the radiance Lw leaving the water body) and Chlorophyll-a concentrations (Chla, a proxy for phytoplankton biomass). These variables, able to provide unique monitoring capabilities of the green marine environment, have been identified by the Global Ocean Observing System (GOOS) as Essential Ocean Variables (EOV) to monitor the health of the oceans, and by the Global Climate Observation System (GCOS) as Essential Climate Variable (ECV) to support the work of the United Nations Framework Convention on Climate Change (UNFCCC).\nECV contributing to the creation of Climate Data Records (CDRs) needs to accomplish high accuracy requirements.\nThis is particularly demanding in coastal water, where the simultaneous presence of non-covarying in-water optically active components (i.e., pigments, colored dissolved organic matter and suspended sediments) and potential contributions from sea-bottom and nearby land leads to rather complex bio-optical properties. As such, while the determination of the optical properties of the open ocean from satellite measurements is nowadays largely established, the remote sensing of coastal waters still represents an open challenge. \nNonetheless, the economical and environmental importance of coastal zones is widely acknowledged: a large portion of the global population lives in coastal areas, whereas coastal marine habitats are extremely sensitive to the impacts of climate variability and change. A specific action for the coordination of enhanced shelf and coastal observations for climate has been indeed designed by the GCOS Implementation Plan (GCOS, 2016) with the aim to define detailed specific observational requirements for an improved understanding, assessment and prediction of the impact of climate in the coastal environment.\nECV high accuracy requirements imply a thorough evaluation of the uncertainties affecting satellite and in situ data, and the procedures applied for the retrieval of OC products from the satellite observations.\nWithin such a framework, 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.\nSpecific attention is given to AE affecting marine observations by two EO-dedicated 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.\nAE 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 c