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Giuseppe Zibordi

Abstract

The spectral immersion factor of in-water radiance sensors If quantifies the effects of changes in the sensor's response when operated in water versus in air. The values of If are currently computed with a relationship derived from a basic sensor model, which only requires knowledge of the refractive indices of the water and the material constituting the sensor's optical window in contact herewith. Uncertainties in the computation of If are investigated in the 400–700-nm spectral range for a specific class of widely used multispectral radiometers. The analysis is made by comparing If values from the theoretical relationship currently in use with (i) If from a new relationship based on an extended sensor model accounting for the actual solid-angle field of view and the reflectance and transmittance of the external and internal optical components, and (ii) experimental If determined with sample radiometers having diverse optical windows made of materials with different refractive indices. Results highlight that the relationship derived from the basic sensor model introduces a 0.4% negative bias when applied to the considered class of radiometers having a fused silica optical window, a 13° in-air half-angle field of view, and an estimated detector reflectance of 0.15. Reference values of If for the specific class of radiometers, determined with the newly proposed relationship, are presented, and their dependence on seawater temperature and salinity is discussed.

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Kenneth J. Voss and Giuseppe Zibordi

Abstract

Camera systems which measure a complete hemispherical field (“fisheye” lens systems), can be applied to the measurement of the radiance, but accurate radiometric and geometric calibrations are required to obtain absolute radiance data. The calibration procedure applied to an instrument built for spectral and geometrical radiance distribution measurements in the visible wavelength region (450–650 nm), based on an electro-optic fisheye camera system, is described and validated through comparison with a mere standard radiometer.

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Stanford B. Hooker and Giuseppe Zibordi

Abstract

Two new immersion factor methods are evaluated by comparing them with the so-called traditional (or incremental) method. For the first method, the optical measurements taken at discrete water depths are substituted by continuous profiles created by removing the water from the tank used in the experimental procedure at a constant flow rate with a pump. In the second method, the commonly used large tank is replaced by a small water vessel with sidewall baffles, which permits the use of a quality-assured volume of water. The summary of the validation results produced for the different methods shows a significant convergence of the two new methods with the traditional method with differences generally well below 1%. The average repeatabilities for single-sensor characterizations (across seven wavelengths) of the three methods are very similar and approximately 0.5%. The evaluation of the continuous method demonstrates its full applicability in the determination of immersion factors with a significant time savings. The results obtained with the small water vessel demonstrate the possibility of significantly reducing the size of the tank (along with decreasing the execution time) and permitting a completely reproducible methodology (based on the use of pure water). The small tank approach readily permits the isolation and quantification of individual sources of uncertainty, the results of which confirm the following aspects of the general experimental methodology: (a) pure water is preferred over tap water, (b) the water should not be recycled (so it does not age), (c) bubbles should be removed from all wetted surfaces, (d) the water surface should be kept as clean as possible, (e) sidewall reflections can be properly minimized with internal baffles, and (f) a pure water characterization can be easily corrected to produce an appropriate seawater characterization. Within the context of experimental efficiency and reproducibility, this study suggests that the combination of a properly baffled small tank with a constant-flow pump would be an optimal system.

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Giuseppe Zibordi, Marco Talone, and Lukasz Jankowski

Abstract

The response to temperature of sample hyperspectral radiometers commonly used to support the validation of satellite ocean color data was characterized in the 400–800-nm spectral range. Measurements performed in the 10°–40°C interval at 5°C increments showed mean temperature coefficients varying from −0.04 × 10−2 (°C)−1 at 400 nm to +0.33 × 10−2 (°C)−1 at 800 nm, which are largely explained by the temperature coefficient of the photodetector array constituting the core of the sensor. Overall, the results indicate the possibility of applying temperature corrections with an uncertainty of approximately 0.03 × 10−2 (°C)−1 for the class of hyperspectral radiometers investigated in the study.

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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, John Piero Doyle, and Stanford B. Hooker

Abstract

A field campaign was performed to estimate the shading effect induced on in-water irradiance and radiance measurements taken in the immediate vicinity of the Acqua Alta Oceanographic Tower (AAOT), located in the northern Adriatic Sea, which is regularly used to support ocean color validation activities. Sequences of downwelling irradiance and upwelling radiance profiles were collected at varying distances from the tower to evaluate the shading effects during clear-sky conditions as a function of the deployment distance. The experimental data, as well as Monte Carlo simulations, indicate that the shading effect is negligible for both downwelling irradiances and upwelling radiances at deployment distances greater than 15 and 20 m, respectively. At closer distances, for example, at the 7.5-m deployment distance regularly used at the AAOT for the collection of underwater optical measurements, the shading effect is remarkable: both field and simulated data at a depth of 7 m and a wavelength of 443 nm show that, with a relatively low sun zenith angle of 22°, the shading effect is within 3% for downwelling irradiance and within 8% for upwelling radiance. Monte Carlo simulations at 443, 555, and 665 nm, computed at a depth of 0 m and with values of seawater inherent optical properties representative of the AAOT site, are used to extend considerations on shading effects to measurements taken during different illumination conditions at the 7.5-m deployment distance. Simulations for ideal clear-sky conditions (i.e., in the absence of atmospheric aerosols) show that errors induced by AAOT perturbations significantly vary as a function of wavelength and sun zenith angle. The highest values are observed at 443 nm where, with the sun zenith angle ranging from 20° to 70°, errors vary from 2.4% to approximately 6.2% for downwelling irradiance and from a minimum of 3.0% (occurring at 30°) to almost 6.6% for upwelling radiance. Simulations also show that the shading error can be as high as approximately 20% for both irradiance and radiance measurements taken during overcast sky conditions.

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Stanford B. Hooker, Gordana Lazin, Giuseppe Zibordi, and Scott McLean

Abstract

A high-quality dataset collected at an oceanographic tower was used to compare water-leaving radiances derived from simultaneous above- and in-water optical measurements. The former involved two different above-water systems and four different surface glint correction methods, while the latter used three different in-water sampling systems and three different methods (one system made measurements a fixed distance from the tower, 7.5 m; another at variable distances up to 29 m away; and the third was a buoy sited 50 m away). Instruments with a common calibration history were used, and to separate differences in methods from changes in instrument performance, the stability (at the 1% level) and intercalibration of the instruments (at the 2%–3% level) was performed in the field with a second generation Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM-II). The water-leaving radiances estimated from the methods were compared to establish their performance during the field campaign, which included clear and overcast skies, Case-1 and Case-2 conditions, calm and roughened sea surface, etc. Three different analytical approaches, based on unbiased percent differences (UPDs) between the methods, were used to compare the various methods. The first used spectral averages across the 412–555-nm SeaWiFS bands (the part of the spectrum used for ocean color algorithms), the second used the ratio of the 490- and 555-nm bands, and the third used the individual (discrete) wavelengths. There were eight primary conclusions of the comparisons, which were considered within the context of the SeaWiFS 5% radiometric objectives. 1) The 5% radiometric objective was achieved for some in-water methods in Case-1 waters for all analytical approaches. 2) The 5% radiometric objective was achieved for some above-water methods in Case-2 waters for all analytical approaches, and achieved in both water types for band ratios and some discrete wavelengths. 3) The largest uncertainties were in the blue domain (412 and 443 nm). 4) A best-to-worst ranking of the in-water methods based on minimal comparison differences did not depend on the analytical approach, but a similar ranking of the above-water methods did. 5) Above- and in-water methods not specifically designed for Case-2 conditions were capable of results in keeping with those formulated for the Case-2 environment or in keeping with results achieved in Case-1 waters. 6) There was a significant difference between two above-water instruments oriented perpendicular with respect to the sun, but pointed in the same direction (best agreement) versus the opposite direction (worst agreement). 7) The overall intercomparison of all methods across Case-1 and Case-2 conditions was at the 9.1% level for the spectral averages, and at the 3.1% level for the band ratios (uncertainties other than those associated with implementing the individual methods account for 2%–4% and 1%–3% of these values, respectively). 8) A comparison with traditional regression analyses confirms the UPD conclusions.

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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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