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Giuseppe Zibordi, Jean-François Berthon, and Davide D’Alimonte

Abstract

Radiometric products determined from fixed-depth and continuous in-water profile data collected at a coastal site characterized by moderately complex waters were compared to investigate differences and limitations between the two measurement methods. The analysis focused on measurements performed with the same radiometer system sequentially deployed at discrete depths (i.e., 1 and 3 m) and successively used to profile the water column. Within the 412–683-nm spectral interval, comparisons show uncertainties of 2%–4%, 3%–5%, and 2% for the subsurface values of upwelling radiance, L un, upward irradiance, E un, and downward irradiance, E dn, all normalized with respect to the above-water downward irradiance. The related spectral biases vary from −2% to 1% for L un, are in the range of 2%–3% for E un, and are lower than 0.5% for E dn. Derived products like the irradiance reflectance, R, Q factor at nadir, Q, and normalized water leaving radiance, L WN, exhibit spectral uncertainties of 4%–6%, 2%–3%, and 2%–4%. The related spectral biases vary from 1% to 3%, 2% to 3%, and −2% to 1%, respectively. An analysis of these results indicates a general diminishing of uncertainties and biases with a decrease of the diffuse attenuation coefficient, Kd, determined at 490 nm, for most of the quantities investigated. Exceptions are E dn and Kd because an increase of Kd reduces the perturbations due to wave effects on downward irradiance measurements. An evaluation of the perturbing effects due to the presence of optical stratifications, which lead to a nonlinear decrease with depth of log-transformed radiometric measurements, shows an expected increase in uncertainty and bias specifically evident for Ku, E un, Kl, and L un, and derived quantities like R, Q, and L WN. Overall results, supported by a t-test analysis, indicate the possibility of using moorings in moderately complex coastal waters to determine L WN with a slightly higher uncertainty with respect to that achievable with continuous profiling systems.

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Giuseppe Zibordi, Davide D'Alimonte, and Jean-François Berthon

Abstract

Wave perturbations induce uncertainties in subsurface quantities determined from the extrapolation of optical measurements taken at different depths. An analysis of these uncertainties was made using data collected in the northern Adriatic Sea coastal waters over a wide range of environmental conditions with a profiling system having a 6-Hz acquisition rate, ∼0.1 m s−1 deployment speed, radiance sensors with 20° full angle field of view, and irradiance collectors of ∼1-cm diameter. The uncertainties were quantified as a function of the depth resolution of radiance and irradiance profiles through the percent differences between the subsurface values computed from full and reduced resolution profiles (the latter synthetically created by removing data from the former). The applied method made the analysis independent from instrument calibration; from perturbations induced by instrument self-shading, deployment structure, and bottom effects; and from environmental variability caused by seawater and illumination changes during casts. The results displayed a significant increase in uncertainties with decreasing depth resolution. For instance, in the 443–665-nm spectral range with a depth resolution of 12.5 cm, the largest uncertainties were observed for the subsurface downward irradiance, E d(0, λ), and the near-surface diffuse attenuation coefficient, K d(λ), with spectral average uncertainties of 5.5% and 11.7%, respectively. With the same depth resolution, the smallest uncertainties were observed for the subsurface upwelling radiance, L u(0, λ), and upward irradiance, E u(0, λ), showing spectral average values of 1.0% and 0.6%, respectively. The uncertainties in the irradiance reflectance, R(λ); the Q factor, Q n(λ); and the normalized water-leaving radiance, L WN(λ), gave values in keeping with those of the quantities used for their computation. The uncertainties were also analyzed as a function of sea state S s and diffuse attenuation coefficient K d at 490 nm. These values were used to estimate the depth resolution requirements restricting below given thresholds the wave-induced uncertainties in the computed subsurface optical quantities. To satisfy a 2% maximum uncertainty in the 443–665-nm spectral range, for the specific instrumental and environmental conditions characterizing the data used in the analysis, results suggested minimum depth resolutions of 11, 40, 3, and 2 cm, for L u(0, λ), E u(0, λ), E d(0, λ), and K d(λ), respectively.

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Giuseppe Zibordi, Brent N. Holben, Marco Talone, Davide D’Alimonte, Ilya Slutsker, David M. Giles, and Mikhail G. Sorokin

Abstract

The Ocean Color component of the Aerosol Robotic Network (AERONET-OC) supports ocean color related activities such as validation of satellite data products, assessment of atmospheric correction schemes and evaluation of bio-optical models, through globally distributed standardized measurements of water-leaving radiance and aerosol optical depth. In view of duly assisting the AERONET-OC data user community, this work: i. summarizes the latest investigations on a number of scientific issues related to above-water radiometry; ii. emphasizes the network expansion that from 2002 till the end of 2020 integrated 31 effective measurement sites; iii. shows the equivalence of data product accuracy across sites and time for measurements performed with different instrument series; iv. illustrates the variety of water types represented by the network sites ensuring validation activities across a diversity of observation conditions; and v. finally documents the availability of water-leaving radiance data corrected for bidirectional effects applying a method specifically developed for chlorophyll-a dominated waters and an alternative one likely suitable for any water type.

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Giuseppe Zibordi, Frédéric Mélin, Jean-François Berthon, Brent Holben, Ilya Slutsker, David Giles, Davide D’Alimonte, Doug Vandemark, Hui Feng, Gregory Schuster, Bryan E. Fabbri, Seppo Kaitala, and Jukka Seppälä

Abstract

The ocean color component of the Aerosol Robotic Network (AERONET-OC) has been implemented to support long-term satellite ocean color investigations through cross-site consistent and accurate measurements collected by autonomous radiometer systems deployed on offshore fixed platforms. The AERONET-OC data products are the normalized water-leaving radiances determined at various center wavelengths in the visible and near-infrared spectral regions. These data complement atmospheric AERONET aerosol products, such as optical thickness, size distribution, single scattering albedo, and phase function. This work describes in detail this new AERONET component and its specific elements including measurement method, instrument calibration, processing scheme, quality assurance, uncertainties, data archive, and products accessibility. Additionally, the atmospheric and bio-optical features of the sites currently included in AERONET-OC are briefly summarized. After illustrating the application of AERONET-OC data to the validation of primary satellite products over a variety of complex coastal waters, recommendations are then provided for the identification of new deployment sites most suitable to support satellite ocean color missions.

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