System Vicarious Calibration for Copernicus Ocean Colour Missions: Updated Requirements and Recommendations for a European Site

The Copernicus Program has been established through the Regulation EU No377/2014 with the objective to ensure long-term and sustained provision of accurate and reliable data on environment and security through dedicated services. Among these, the Copernicus Marine Environment Monitoring Service and the marine component of the Climate Change Service, both rely on satellite ocean colour observations to deliver data on water quality and climate relevant quantities such as chlorophyll-a concentration used as a proxy for phytoplankton biomass. Satellite ocean colour missions require in situ highly accurate radiometric measurements for the indirect calibration (so called System Vicarious Calibration (SVC)) of the space sensor. This process is essential to minimize the combined effects of uncertainties affecting the space sensor calibration and those resulting from the inaccuracy of processing algorithms and models applied for the generation of data products. SVC is thus a fundamental element to maximize the return on investments for the Copernicus Program by delivering to the user science community satellite ocean colour data with accuracy granting achievement of target objectives from applications addressing environmental and climate change issues. The long-term Copernicus Program foresees multiple ocean colour missions (i.e., the Sentinel-3 satellites carrying the Ocean and Land Colour Instrument (OLCI)). The need to ensure the highest accuracy to satellite derived data products contributing to the construction of Climate Data Records (CDRs), suggests the realization, deployment and sustain of a European in situ infrastructure supporting SVC for Sentinel-3 missions, fully independent from similar facilities established and maintained by other space agencies (e.g., that operated in the Pacific Ocean by US agencies). It is emphasized that the need to cope with long-term Copernicus objectives on data accuracy, implies very stringent requirements for the in situ infrastructure and location providing reference measurements for SVC. These requirements, in fact, are much higher than those imposed by SVC for a single mission. The content of this Report, which is a revised version of a previous one (Zibordi et al. 2017), builds on the long-standing experience of the JRC on ocean colour radiometry. This experience counts on decadal field and laboratory measurements performed in support of validation and SVC applications, and additionally on activities comprehensively embracing measurement protocols, instruments characterization and the initiation of autonomous measurement infrastructures. Overall, this Report summarizes a number of recent investigations led by the JRC on SVC requirements for the creation of CDRs. The final objective is to consolidate in a single document the elements essential fJRC.D.2-Water and Marine Resource

System Vicarious Calibration for Copernicus Ocean Colour Missions: Requirements and Recommendations for a European Site

The Copernicus Program has been established through the Regulation EU No377/2014 with the objective to ensure long-term and sustained provision of accurate and reliable data on environment and security through dedicated services. Among these, the Copernicus Marine Environment Monitoring Service and the marine component of the Climate Change Service, both rely on satellite ocean colour observations to deliver data on water quality and climate relevant quantities such as chlorophyll-a concentration used as a proxy for phytoplankton biomass. Satellite ocean colour missions require in situ highly accurate radiometric measurements for the indirect calibration (so called System Vicarious Calibration (SVC)) of the space sensor. This process is essential to minimize the combined effects of uncertainties affecting the space sensor calibration and those resulting from the inaccuracy of processing algorithms and models applied for the generation of data products. SVC is thus a fundamental element to maximize the return on investments for the Copernicus Program by delivering to the user science community satellite ocean colour data with accuracy granting achievement of target objectives from applications addressing environmental and climate change issues. The long-term Copernicus Program foresees multiple ocean colour missions (i.e., the Sentinel-3 satellites carrying the Ocean and Land Colour Instrument (OLCI)). The need to ensure the highest accuracy to satellite derived data products contributing to the construction of Climate Data Records (CDRs), suggests the realization, deployment and sustain of a European in situ infrastructure supporting SVC for Sentinel-3 missions, fully independent from similar facilities established and maintained by other space agencies (e.g., that operated in the Pacific Ocean by US agencies). It is emphasized that the need to cope with long-term Copernicus objectives on data accuracy, implies very stringent requirements for the in situ infrastructure and location providing reference measurements for SVC. These requirements, in fact, are much higher than those imposed by SVC for a single mission. The content of this Report builds on the long-standing experience of the JRC on ocean colour radiometry. This experience counts on decadal field and laboratory measurements performed in support of validation and SVC applications, and additionally on activities comprehensively embracing measurement protocols, instruments characterization and the initiation of autonomous measurement infrastructures. Overall, this Report summarizes a number of recent investigations led by the JRC on SVC requirements for the creation of CDRs. The final objective is to consolidate in a single document the elements essential for the realization of a European SVC infrastructure in support of the Copernicus Program. Briefly, the various Chapters summarize: • General requirements for a long-term SVC infrastructure, which indicate the need for spatially homogenous oceanic optical properties, seasonal stability of marine and atmospheric geophysical quantities, negligible land perturbations, hyperspectral radiometry, and low measurement uncertainties; • Spectral resolution requirements for in situ SVC hyperspectral measurements as a function of bandwidths and center-wavelengths of most advanced satellite sensors, which specify the need for sub-nanometre resolutions to allow for supporting any scheduled satellite ocean color sensor; • Suitable SVC locations in European Seas showing the fitness of regions in the Eastern Mediterranean Sea to satisfy fundamental requirements.JRC.D.2-Water and Marine Resource

Effects of integration time on in-water radiometric profiles

This work investigates the effects of integration time on in-water downward irradiance E-d, upward irradiance E-u and upwelling radiance L-u profile data acquired with free-fall hyperspectral systems. Analyzed quantities are the subsurface value and the diffuse attenuation coefficient derived by applying linear and non-linear regression schemes. Case studies include oligotrophic waters (Case-1), as well as waters dominated by colored dissolved organic matter (CDOM) and non-algal particles (NAP). Assuming a 24-bit digitization, measurements resulting from the accumulation of photons over integration times varying between 8 and 2048ms are evaluated at depths corresponding to: 1) the beginning of each integration interval (FST); 2) the end of each integration interval (LST); 3) the averages of FST and LST values (AVG); and finally 4) the values weighted accounting for the diffuse attenuation coefficient of water (WGT). Statistical figures show that the effects of integration time can bias results well above 5% as a function of the depth definition. Results indicate the validity of the WGT depth definition and the fair applicability of the AVG one. Instead, both the FST and LST depths should not be adopted since they may introduce pronounced biases in E-u and L-u regression products for highly absorbing waters. Finally, the study reconfirms the relevance of combining multiple radiometric casts into a single profile to increase precision of regression products. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Analysis of adjacency effects for Copernicus Ocean Colour Missions

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. Among 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). ECV contributing to the creation of Climate Data Records (CDRs) needs to accomplish high accuracy requirements. This 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. Nonetheless, 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. ECV 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. Within 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. Specific 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. 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 mid-latitude coastal environments. • The theoretical evaluation of AE at the AAOT and Lampedusa validation sites.JRC.D.2-Water and Marine Resource

Bio-optical Algorithms for European Seas: Performance and Applicability of Neural-Net Inversion Schemes

The report presents and discusses the application of Multi Layer Perceptron (MLP) neural networks to derive Chlorophyll-a concentration (Chl-a), absorption of the yellow substance at 412 nm (ays(412)) and concentration of the total suspended matter (TSM) from remote sensing reflectance Rrs values. MLPs were developed on the basis of data collected within the framework of the Coastal Atmosphere and Sea Time Series (CoASTS) and Bio-Optical mapping of Marine Properties (BiOMaP) programs carried out by the Institute for Environment and Sustainability (IES), JRC of E.C., Italy. Investigated oceanographic regions include the Eastern Mediterranean Sea, the Ligurian Sea, the Northern Adriatic Sea, the Western Black Sea, the English Channel and the Baltic Sea. The study verifies the applicability of MLPs to retrieve ocean color data products in each basin. For instance, the highest accuracy in retrieving Chl-a has been found in the Easter Mediterranean Sea and the Ligurian Sea (14 and 25 %, respectively). In the case of ays(412), the MLP is the most performing in the waters of the English Channel and the Baltic Sea (14 and 13%). Instead, the TSM retrieval is the most accurate in the Black Sea and at the Acqua Alta Oceanographic Tower (14 and 19%). To enhance mission specific ocean color resuls, MLP coefficients are also computed applying band-shift corrections to produce Rrs spectra at wavelengths matching those of SeaWiFS, MODIS and MERIS. Resulting tables of MLP parameters are reported to permit independent applications of neural networks presented in this analysis.JRC.H.3-Global environement monitorin

In situ determination of the remote sensing reflectance: an inter-comparison

Inter-comparison of data products from simultaneous measurements performed with independent systems and methods is a viable approach to assess the consistency of data and additionally to investigate uncertainties. Within such a context the inter-comparison called Assessment of In Situ Radiometric Capabilities for Coastal Water Remote Sensing Applications (ARC), was carried out at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea to explore the accuracy of in situ data products from various in- and above-water optical systems and methods. Measurements were performed under almost ideal conditions including: a stable deployment platform, clear sky, relatively low sun zenith angles and moderately low sea state. Additionally, all optical sensors involved in the experiment were inter-calibrated through a post-field absolute radiometric calibration performed with the same standards and methods. Inter-compared data products include: spectral water-leaving radiance Lw,above-water downward irradiance Ed(0+) and remote sensing reflectance Rrs. Data products from the various measurement systems/methods were directly compared to those from a single reference system/method. Results for Rrs indicate spectrally averaged values of relative differences comprised between -1 and +6%, while spectrally averaged absolute values of relative differences vary from approximately 6% for the above-water systems/methods to 9% for buoy-based systems/methods. The good agreement between Rrs spectral relative differences and estimates of combined uncertainties of the inter-compared systems/methods is noteworthy.JRC.H.1-Water Resource

Regional Bio-optical Relationships and Algorithms for the Adriatic Sea, the Baltic Sea and the English Channel/North Sea Suitable for Ocean Colour Sensors

Regional bio-optical relationships and empirical algorithms were developed on the basis of measurements collected during the CoASTS 1995-2005 bio-optical time-series in the northern coastal Adriatic Sea as well as during ship campaigns performed in coastal regions of the Adriatic Sea, the Baltic Sea and the English Channel/North Sea between 2000 and 2005. The empirical algorithms aim at the retrieval from ocean colour data of the Chlorophyll a and Total Suspended Matter concentrations, of the absorption coefficient of the Coloured Dissolved Organic Matter, of the diffuse attenuation coefficient of downwelling irradiance and of the Secchi depth. Bio-optical relationships relating the marine optically significant components to their absorption or scattering properties are also presented for the investigated coastal areas.JRC.H.3-Global environement monitorin

Ocean Colour Calibration and Validation: The JRC contribution to Copernicus

Copernicus Sentinel-3 missions, including the ongoing Sentinel-3A and -3B and the future Sentinel-3C and -3D, offer an unprecedented opportunity for long-term ocean colour observations to support global environmental and climate investigations. Nevertheless, any ocean colour mission incorporates calibration and validation activities essential for the indirect calibration of the space sensor and the validation of data products. These calibration and validation activities are largely centered on the production of highly accurate in situ reference measurements relying on state of the art measurement methods and instrumentation. Since the start of the operational ocean colour missions in 1997, the JRC sustained the required calibration and validation activities by developing unique expertise and setting up specific measurement programs and infrastructures. This expertise, measurement programs and infrastructures, currently support the Copernicus ocean colour calibration and validation tasks through the delivery and exploitation of in situ reference data essential for the quality control of satellite data products. This Technical Report aims at providing: i. a general introduction to the ocean colour paradigm; ii. an extended synopsis of requirements and strategies for satellite ocean colour missions with a detailed focus on the JRC experimental activities carried out during the last decades; and finally iii. a discussion supporting the need for a sustained support of the JRC laboratory and field measurement programs assisting the production and exploitation of in situ reference data for the validation of Sentinel-3 ocean colour products. The Report, mostly through section 2, should naturally satisfy readers interested in appraising the specific JRC activities performed to support ocean colour calibration and validation. The same Report through sections 1 and 3, should also satisfy the need for more essential information supporting the need for sustaining the JRC ocean colour validation activities currently embedded in the Copernicus Earth Observation program of major relevance for global marine and climate investigations.JRC.D.2-Water and Marine Resource

Characterisation of the HDRF (as a proxy for BRDF) of snow surfaces at Dome C, Antarctica, for the inter-calibration and inter-comparison of satellite optical data

Measurements of the Hemispherical Directional Reflectance Factor (HDRF) of snow surfaces were performed at Dome C, Antarctica, during the Australis Summer 2011–2012 to support the inter-comparison and inter-calibration of satellite optical sensors. HDRF data were collected with the Gonio Radiometric Spectrometer System (GRASS) which performs hyper-spectral measurements of radiance from the same target surface with independent collectors at a number of viewing and azimuth angles in the 0–60° and 0–360° angular ranges, respectively. The radiance collectors, installed on a hemispheric frame and connected to a spectrometer through fibre optics, have an 8° full cone of acceptance and a viewing footprint varying from 0.049m2 at nadir to 0.142m2 at viewing angles of 60°. These relatively small footprints allow for the characterization of small-scale heterogeneities in the HDRF of observed surfaces. HDRF measurements representative of the Dome C snow surfaces were made at eight different sites along a transect approximately 100 m long. All the sites exhibit similar HDRF distributions with inter-site differences explained by small-scale inhomogeneities of the surface. The measured HDRF display marked forward scattering with anisotropy increasing with wavelength in the 400–1600 nm spectral region. These data complement those from previous measurements performed in the same area with a different technique. Agreement between the two data sets is shown by differences generally lower than 4% between HDRF distributions derived from a previous study and the spatially averaged HDRF from the various sites along a transect presented in this work.JRC.H.1-Water Resource

Toward an assessment of the fitness-for-purpose of Copernicus ocean colour data

The Copernicus Program has been established through the Regulation EU No377/2014 with the objective to ensure long-term and sustained provision of accurate and reliable data on environment and security through dedicated services. Among these, the Copernicus Marine Environment Monitoring Service and the marine component of the Climate Change Service, both rely on satellite ocean colour observations delivering data on water quality and climate relevant quantities such as chlorophyll-a concentration used as a proxy for phytoplankton biomass. This Report, building on the long-standing experience of the JRC on ocean colour, summarizes a number of recent investigations essential to assess the fitness-for-purpose of Copernicus ocean colour data products. These investigations embrace: i. The accuracy of radiometry data from the Ocean and Land Colour Instrument (OLCI) on board Sentinel-3a. The assessment is performed relying on geographically distributed in situ reference measurements from autonomous systems and dedicated oceanographic campaigns. ii. Uncertainty analysis of ocean colours radiometry data from a number of international missions. The analysis aims at assessing the potentials for the construction of Climate Data Records (CDRs) from independent missions. iii. The impact of adjacency effects in coastal data limiting the accuracy of ocean colour radiometry products. The study relies on state-of-the-art radiative transfer simulations and aims at quantifying adjacency effects in space data from sensors exhibiting different signal-to-noise ratios. iv. Uncertainties affecting in situ radiometry data as a result of the lack of comprehensive characterizations of field instruments. This is an attempt to illustrate the fundamental importance of comprehensive radiometric calibrations and characterizations for in situ instruments supporting validation activities. v. Reproducibility of the experimental determination of pigments concentrations for the validation of satellite data products. The analysis documents the differences affecting the quantification of pigments concentrations through the applicationJRC.D.2-Water and Marine Resource