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Stanford B. Hooker and André Morel

Abstract

A comparison of above- and in-water spectral measurements in Case-1 conditions showed the uncertainty in above-water determinations of water-leaving radiances depended on the pointing angle of the above-water instruments with respect to the side of the ship. Two above-water data processing methods were used to create a diagnostic variable (formulated for Case-1 waters only) to quantify the presence of superstructure reflections that degraded the above-water intracomparisons of water-leaving radiances by as much as 13%–27% (for far-to-near viewing distances, respectively). The primary conclusions of the above- and in-water intercomparison of water-leaving radiances were as follows: (a) the SeaWiFS 5% radiometric objective was achieved with the in-water instruments; (b) the above-water approach produced agreement to within 5%, but reliably for about half the data, and only with well-controlled procedures and severe filtering to remove glint contamination; (c) a decrease in water-leaving radiance values was seen in the presence of swell, although, wave crests were radiometrically brighter than the troughs; and (d) standard band ratios used in ocean color algorithms remained severely affected, because of the relatively low signal at 555 nm and, thus, proportionally significant ship contamination at this wavelength. Suggestions for a more precise above-water measurement protocol are tentatively proposed.

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Stanford B. Hooker and Giuseppe Zibordi

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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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Stanford B. Hooker and James Aiken

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One of the goals of calibration and validation programs supporting ocean color satellites is to produce water-leaving radiances with an uncertainty of 5% in clear-water regions. This objective requires field instruments with a calibration and measurement capability that is on the order of 1%. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project, in collaboration with the National Institute of Standards and Technology, has developed a portable illumination source with three temperature-stabilized internal monitors designed to provide a stable light field for checking the optical stability of radiometers used to measure the in situ optical properties of seawater. This device is called the SeaWiFS Quality Monitor (SQM). A recent field evaluation during an extensive research cruise indicates the SQM has the following capabilities: (a) the SQM can be used to track the stability of field radiometers at less than the 1% level in terms of the radiometric response of the instruments—on average 0.30% (±0.15%) for radiance sensors and 0.58% (±0.20%) for irradiance sensors; (b) the SQM light field is sufficiently stable to allow for a sensitive measure and, thus, modeling of changes in the radiometric detectors;(c) based on the radiometers used during the field evaluation, daily SQM measurements are needed to resolve the temporal changes in the response of the sensors; and (d) SQM performance, in terms of the generated light field and the SQM internal monitors, is very stable and decayed only by approximately 0.6% during the course of the 36-day deployment with most of the decay attributed to a change in the operating voltage of one of the lamps.

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Stanford B. Hooker and Stephane Maritorena

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The primary objective of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project is to produce water-leaving radiances within an uncertainty of 5% in clear-water regions, and chlorophyll a concentrations within 35% over the range of 0.05–50 mg m−3. Any global mission, like SeaWiFS, requires validation data from a wide variety of investigators. This places a significant challenge on quantifying the total uncertainty associated with the in situ measurements, because each investigator follows slightly different practices when it comes to implementing all of the steps associated with collecting field data, even those with a prescribed set of protocols. This study uses data from multiple cruises to quantify the uncertainties associated with implementing data collection procedures while using different in-water optical instruments and deployment methods. A comprehensive approach is undertaken and includes (a) the use of a portable light source and in-water intercomparisons to monitor the stability of the field radiometers, (b) alternative methods for acquiring reference measurements, and (c) different techniques for making in-water profiles. Three optical systems had quadrature sum uncertainties sufficiently small to ensure a combined uncertainty for the spaceborne and in situ measurements within a total 5% vicarious calibration budget. A free-fall profiler using (relatively inexpensive) modular components performed best (2.7% quadrature sum uncertainty), although a more sophisticated (and comparatively expensive) profiler using integral components was very close and only 0.5% higher. A relatively inexpensive system deployed with a winch and crane was also close, but ship shadow contamination increased the quadrature sum uncertainty to approximately 3.4%.

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Stanford B. Hooker and Donald B. Olson

Abstract

The problem of tracking closed mesoscale vortices using center of mass estimation techniques is studied. Three estimators are evaluated using data from a warm core Gulf Stream ring. The comparisons show that a method based on the intersection of perpendicular bisectors and one using a least-squares fit of a conic section perform comparably. The perpendicular bisector algorithm is used in conjunction with a Gaussian ring model and a star-shaped survey pattern to produce an expected error curve as a function of vortex translation, survey speed and vortex size. For typical ring parameters, center estimation is usually possible to within ±5 km. The feasibility of using differing data sets to construct a history of ring motion based on a coordinate system moving with the ring is also investigated. In this way, the validity of using satellite-derived data and drifter trajectories to estimate the center of mass of a mesoscale feature is assessed. The results of the analysis demonstrate that the location of the deeper structure of the ring and the surface expression are sufficiently well correlated to permit dynamically relevant calculations based on surface measurements. It is shown that satellite-derived data can be used to approximate the center of mass trajectory to within the error in the individual center estimates for the period analyzed. The Lagrangian-drifter-derived centers are offset from the center of mass trajectory in a manner consistent with kinematic arguments.

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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

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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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B. Carol Johnson, Ping-Shine Shaw, Stanford B. Hooker, and Don Lynch

Abstract

A portable and stable source of radiant flux, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM), was developed as a field instrument for use in experiments away from the calibration laboratory such as those encountered during oceanographic cruises. The SQM monitors the radiometric stability of radiance and irradiance sensors during these field experiments; a companion paper gives results acquired during the third Atlantic Meridional Transect cruise. In conjunction with laboratory calibration sources, the SQM can be used to transfer the calibration to the field experiment. Two independent lamp assemblies generate three flux levels, and the lamps are operated at constant current using active control. The exit aperture of the SQM is large and homogeneous in radiance. The SQM was designed to approximate a Lambertian radiator. An internal heater provides operational stability and decreased warmup intervals, which minimizes lamp hours. Temperature-controlled silicon photodiodes with colored-glass filters monitor the stability of the SQM, which is better than 1%. These independent monitors, which are integrated with the SQM, provide information on the flux from the SQM and can be used to normalize the output from the field radiometers during the experiment. Three reference devices, or fiducials, which are designed to mimic the optical surfaces of the field radiometers but are not functioning detector units, are used in place of the field radiometers to produce baseline monitor signals. The front surface of the fiducial is protected when not in use and kept clean during the field experiment. The monitor signals acquired using the fiducials provide additional information on the radiometric stability of the SQM. A kinematically designed mounting ring is used on both the field radiometers and the fiducials to ensure the devices being tested view the same part of the exit aperture each time they are used.

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