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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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Ilaria Cazzaniga
and
Giuseppe Zibordi

Abstract

The Ocean Color Component of the Aerosol Robotic Network (AERONET-OC) aims at supporting the assessment of satellite ocean color radiometric products with in situ reference data derived from automated above-water measurements. This study, applying metrology principles and taking advantage of recent technology and science advances, revisits the uncertainty estimates formerly provided for AERONET-OC normalized water-leaving radiances L WN. The new uncertainty values are quantified for a number of AERONET-OC sites located in marine regions representative of chlorophyll-a-dominated waters (i.e., Case 1) and a variety of optically complex waters. Results show uncertainties typically increasing with the optical complexity of water and wind speed. Relative and absolute uncertainty values are provided for the various sites together with contributions from each source of uncertainty affecting measurements. In view of supporting AERONET-OC data users, the study also suggests practical solutions to quantify uncertainties for L WN from its spectral values. Additionally, results from an evaluation of the temporal variability characterizing L WN at various AERONET-OC sites are presented to address the impact of temporal mismatches between in situ and satellite data in matchup analysis.

Open access
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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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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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
,
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
,
Davide D’Alimonte
, and
Tamito Kajiyama

Abstract

Quality control (QC) practices are a fundamental requirement for any measurement program targeting the delivery of high-quality data. In agreement with such a need, the Ocean Color component of the Aerosol Robotic Network (AERONET-OC) includes a number of QC steps ensuring the delivery of normalized water-leaving radiance LWN spectra at incremental accuracy levels identified as level 1.0, level 1.5, and level 2.0. Currently, the final QC step allowing for rising level 1.5 LWN spectra to level 2.0 implies the execution of an expert-based procedure, which is extremely time consuming and naturally undergoes subjective decisions on dubious cases. These limitations solicited the development of an automated procedure, so-called A–QCLWN, mimicking the steps supporting an expert analyst during the final QC of AERONET-OC LWN spectra. A–QCLWNapplies hierarchical tests to check (i) the relative consistency of level 1.5 LWN spectra (called candidates) with respect to LWN reference spectra (called prototypes) constructed using LWN spectra formerly and independently quality controlled; (ii) the absence of any pronounced spectral feature in portions of each LWN candidate spectrum expected to exhibit a regular shape; and additionally, when applicable, (iii) the temporal consistency of the LWN candidate spectrum with respect to close-in-time spectra as a criterion to further strengthen the quality of data. A–QCLWN performance has been verified using LWN spectra from AERONET-OC measurement sites representative of various water types embracing oligotrophic/mesotrophic waters dominated by chlorophyll-a concentration and coastal waters exhibiting increasing levels of optical complexity. A–QCLWN has shown an acceptance rate of AERONET-OC level 1.5 LWN candidate spectra varying between approximately 89% and 93% with agreement in the range of 88%–93% with respect to the LWN spectra independently quality controlled through the expert-based procedure. The additional capability of A–QCLWN to rank the fully quality-controlled LWN spectra combining weights depending on the various tests, anticipates the possibility to best support applications with diverse accuracy needs. Finally, acceptance rates of A–QCLWN for LWN prototype spectra built using level 1.5 data, an alternative to fully quality-controlled level 2.0, have shown values generally increased by less than 1%. This indicates the possibility to lessen the constraint implying the existence of reference level 2.0 LWN data for the relative-consistency test at the expense of a fairly low reduction in accuracy.

Open access
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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