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  • Author or Editor: Stéphane Maritorena x
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Stanford B. Hooker and Stephane Maritorena

Abstract

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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Leonel Romero, J. Carter Ohlmann, Enric Pallàs-Sanz, Nicholas M. Statom, Paula Pérez-Brunius, and Stéphane Maritorena

Abstract

Coincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O(100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s−1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O(1) m2 s−1. Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day−1.

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