A chlorophyll (Chl) a retrieval algorithm, originally developed for spectral subsurface irradiance reflectance determined from above-water shipboard measurements, was adapted for use with satellite ...imagery to be acquired by the MERIS (Medium Resolution Imaging Spectrometer) instrument. This MERIS algorithm was calibrated for Chl a concentrations in the range 3–185 mg m−3 using spectral reflectance calculated from shipboard measurements on the IJssel Lagoon (The Netherlands). Next, the algorithm was validated for various inland and coastal waters covering this concentration range. Despite the lower spectral resolution of MERIS as compared to the shipboard spectroradiometer, the standard error of estimate is expected to be similar, i.e. ∼9 mg m−3 of Chl a in mesotrophic and eutrophic lakes, rivers, estuaries and coastal waters.
This paper reports the first measurement using the NOvA detectors of nu sub(mu) disappearance in a nu sub(mu) beam. The analysis uses a 14 kton-equivalent exposure of 2.74x10 super(20) ...protons-on-target from the Fermilab NuMI beam. Assuming the normal neutrino mass hierarchy, we measure (ProQuest: Formulae and/or non-USASCII text omitted) and sin super(2)theta sub(23) in the range 0.38-0.65, both at the 68% confidence level, with two statistically degenerate best-fit points at sin super(2)theta sub(23)=0.4 3 and 0.60. Results for the inverted mass hierarchy are also presented.
A chlorophyll-retrieval algorithm for use with imagery from the Medium Resolution Imaging Spectrometer (MERIS) aboard ENVISAT relying on wavebands centred at 665, 705 and 775 nm was described in a ...previous paper (Gons et al. 2002, J. Plankton Res., 24, 947–951). The present study reexamined the performance for the current nominal setting to 708.75 nm of the previously envisaged 705 nm band. Validation of the algorithm with revised coefficients gave the same standard error of estimate for the inland and coastal waters as in the original work. The algorithm has been transcribed for direct application with the MERIS level-2 standard product ‘water-leaving reflectance’. By this correction, chlorophyll estimation will generally improve, especially for high concentrations.
A hand-held spectroradiometer was used for above-water determination of subsurface spectral irradiance reflectance in the Scheldt Estuary (Belgium/The Netherlands), the North Sea off the Belgian ...coast, and the Hudson/Raritan Estuary (New York/New Jersey). On the North Sea the measurement conditions were adverse, and elsewhere broken cloud caused considerable spectral variation. Despite this variation the retrieval of chlorophyll a (Chl-a) from three reflectance spectra at all sampling stations was stable. The algorithm calibrated for the freshwater IJssel Lagoon (The Netherlands) proved to be applicable to these estuarine and coastal waters (N = 30; standard error of estimate = 7 mg m-3 for corrected Chl-a ranging from 1 to 93 mg m-3).
Three turbulence closure schemes, designed for stratified shallow water flows, are presented. They are based upon κ-ε theory and use respectively two, one or zero transport equations for turbulent ...variables. The models are first tested on the evolution of a wind-driven turbulent layer in a stratified fluid. The results are at least qualitatively in agreement with observational and experimental data. A discussion is given about the existence of self-similar solutions. The models are compared next with the observational data of the Rhine outflow area. The periodic variation in the density structure, forced by wind and tides and which is clearly visible in the data, is predicted by the model. A physical interpretation of the model results is given in the absence of wind forcing. The effects of estuarine circulation, tidal straining and mixing on the development or breakdown of stratification are well represented by the model calculations.
Results are presented of a three-dimensional model study of the tidally-averaged salinity field in the Rhine-Meuse plume. In conditions of low mixing (no wind, neap tide) freshwater emerges from the ...river as a jet, turns right under the influence of Coriolis force and attaches to the coast as a buoyancy current. Surface residual currents are essentially geostrophic. Within the plume the surface layer is strongly stratified and overlies a bottom mixed layer.
Results are strongly sensitive to the parameterization of vertical mixing: models ranging from constant diffusion coefficients through simple algebraic Richardson number dependent formulations to turbulence closure with evolution equations for turbulent kinetic energy and length scale were tested. Turbulence closure with a single evolution equation for turbulent kinetic energy and an algebraic length scale formulation was found to provide a suitable balance between physical realism (assessed by theoretical considerations and the practical ability to represent a well-mixed bottom boundary layer and stratified surface layer) and computational efficiency. Simpler models, which vary diffusion coefficients as function of Richardson number, may produce similar results, though require more careful calibration. Constant diffusion coefficients are clearly inadequate for the application considered. Even the preferred “
k” model requires some calibration as a “background” or “ambient” mixing coefficient had to be introduced to avoid unrealistically strong stratification.
The tidally-averaged salinity field was found to be qualitatively similar at neap and spring tide, though cross-shore penetration of the outflow jet was reduced, bottom-mixed layer thickness increased and overall stratification reduced at springs. In contrast, the salinity field was found to be strongly dependent on wind strength and direction, both through wind-induced surface mixing and advection by wind-driven surface currents.
The goal of this paper is to present in situ data for the reflectance and transparency of river plumes and to interpret the observed variability in terms of hydrodynamic processes. The data was ...obtained for river plumes in the Black Sea and in Arctic Ocean regions. Spectral reflectance was measured in the 400- 700nm band by a ship/helicopter-mounted spectroradiometer and was supplemented by shipboard measurements of vertical profiles of the light attenuation coefficient. Remotely-sensed data shows the transition from freshwater to sea water with both sharp boundary fronts and wide mixing zones. Internal fronts and quasi-periodic spatial variability of reflectance were also observed within the river plume. The observations are related to the relevant hydrodynamic processes (fronts, advection and settling of particulate matter, internal waves). A radiance model was developed to match the vertical profile of optical properties to the observed variation of remotely-sensed reflectance.
We investigate whether weak hydrodynamic interaction theory can explain signatures of river outflow fronts, as observed frequently in radar imagery. Following a brief description of frontal ...hydrodynamics based on flow modelling, we develop a new model of the imaging mechanism. The model is two-dimensional in position, in order to include the effect of current shear, and also two-dimensional in wave number, for inclusion of waves longer than the Bragg waves into the scattering mechanism. The model's hydrodynamic part is based on numerical integration of the ray equations. The radar backscatter is calculated using first-order Bragg scattering, or using a model based on the Stratton-Chu integral equation. The model is also simplified to an analytical expression for the hydrodynamic modulation. Convergence always causes positive (bright) modulations, whereas shear may also cause negative (dark) modulations. The effect of radar look angle and wind direction is investigated. The model predictions are compared with synthetic aperture radar (SAR) measurements over the near- and far-field outflow front of the river Rhine. In the near-field, convergence dominates the modulation in the radar cross section. Good agreement is obtained when the radar look direction is more or less perpendicular to the front line. When the radar is looking parallel to the front line, the width, and the magnitude of the observed modulations are underestimated. Wind stress variations and scattering from sharp-crested or breaking waves (effects not included in the models) may explain this discrepancy in part. A local relaxation source term may not be applicable in such a case. In the far-field, shear is important. Though the simulated modulations are somewhat weaker than observed, the agreement between theory and experiment is satisfactory. Due to uncertainties in the details of the flow field and imperfections in the imaging mechanism, it is not possible to draw firm conclusions on the detailed structure of river outflow fronts based on radar images alone. This can only be achieved when the images are combined with in situ measurements.
The use of satellites to monitor the color of the ocean requires effective removal of the atmospheric signal. This can be performed by extrapolating the aerosol optical properties in the visible from ...the near-infrared (NIR) spectral region assuming that the seawater is totally absorbant in this latter part of the spectrum. However, the non-negligible water-leaving radiance in the NIR which is characteristic of turbid waters may lead to an overestimate of the atmospheric radiance in the whole visible spectrum with increasing severity at shorter wavelengths. This may result in significant errors, if not complete failure, of various algorithms for the retrieval of chlorophyll-
a concentration, inherent optical properties and biogeochemical parameters of surface waters.
This paper presents results of an inter-comparison study of three methods that compensate for NIR water-leaving radiances and that are based on very different hypothesis: 1) the standard SeaWiFS algorithm (Stumpf et al., 2003; Bailey et al., 2010) based on a bio-optical model and an iterative process; 2) the algorithm developed by Ruddick et al. (2000) based on the spatial homogeneity of the NIR ratios of the aerosol and water-leaving radiances; and 3) the algorithm of Kuchinke et al. (2009) based on a fully coupled atmosphere–ocean spectral optimization inversion. They are compared using normalized water-leaving radiance nL
w in the visible. The reference source for comparison is ground-based measurements from three AERONET-Ocean Color sites, one in the Adriatic Sea and two in the East Coast of USA.
Based on the matchup exercise, the best overall estimates of the nL
w are obtained with the latest SeaWiFS standard algorithm version with relative error varying from 14.97% to 35.27% for λ
=
490
nm and λ
=
670
nm respectively. The least accurate estimates are given by the algorithm of Ruddick, the relative errors being between 16.36% and 42.92% for λ
=
490
nm and λ
=
412
nm, respectively. The algorithm of Kuchinke appears to be the most accurate algorithm at 412
nm (30.02%), 510 (15.54%) and 670
nm (32.32%) using its default optimization and bio-optical model coefficient settings.
Similar conclusions are obtained for the aerosol optical properties (aerosol optical thickness τ(865) and the Ångström exponent, α(510, 865)). Those parameters are retrieved more accurately with the SeaWiFS standard algorithm (relative error of 33% and 54.15% for τ(865) and α(510, 865)).
A detailed analysis of the hypotheses of the methods is given for explaining the differences between the algorithms. The determination of the aerosol parameters is critical for the algorithm of Ruddick et al. (2000) while the bio-optical model is critical for the algorithm of Stumpf et al. (2003) utilized in the standard SeaWiFS atmospheric correction and both aerosol and bio-optical model for the coupled atmospheric–ocean algorithm of Kuchinke. The Kuchinke algorithm presents model aerosol-size distributions that differ from real aerosol-size distribution pertaining to the measurements. In conclusion, the results show that for the given atmospheric and oceanic conditions of this study, the SeaWiFS atmospheric correction algorithm is most appropriate for estimating the marine and aerosol parameters in the given turbid waters regions.
► In this study we compare three SeaWiFS atmospheric correction in turbid waters. ► Three AERONET-OC sites (East Coast of US and Adriatic Sea) have been selected. ► We compare the normalized water-leaving radiances through a match-up exercise. ► Based on our dataset, the SeaWiFS standard method is the most adequate to estimate nLw.
The dry weight concentration of suspended particulate material, SPM (units: mg L−1), is measured by passing a known volume of seawater through a preweighed filter and reweighing the filter after ...drying. This is apparently a simple procedure, but accuracy and precision of SPM measurements vary widely depending on the measurement protocol and experience and skills of the person filtering. We show that measurements of turbidity, T (units: FNU), which are low cost, simple, and fast, can be used to optimally set the filtration volume, to detect problems with the mixing of the sample during subsampling, and to quality control SPM. A relationship between T and ‘optimal filtration volume’, Vopt, is established where Vopt is the volume at which enough matter is retained by the filter for precise measurement, but not so much that the filter clogs. This relationship is based on an assessment of procedural uncertainties in the SPM measurement protocol, including salt retention, filter preparation, weighing, and handling, and on a value for minimum relative precision for replicates. The effect of filtration volume on the precision of SPM measurement is investigated by filtering volumes of seawater ranging between one fifth and twice Vopt. It is shown that filtrations at Vopt maximize precision and cost effectiveness of SPM. Finally, the 90% prediction bounds of the T versus SPM regression allow the quality control of SPM determinations. In conclusion it is recommended that existing SPM gravimetric measurements be refined to include measurement of turbidity to improve their precision and quality control.
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