Barry Gross

Fluorescence component in the reflectance spectra from coastal waters. Dependence on water composition

Based on HYDROLIGHT simulations of more than 2000 reflectance spectra from datasets typical of coastal waters with highly variable optically active constituents as well as on intercomparisons with field measurements, the magnitude of chlorophyll fluorescence was analyzed and parameterized as a function of phytoplankton, CDOM, and suspended inorganic matter concentrations. Using the parameterizations developed, we show that variations in the fluorescence component of water leaving radiance in coastal waters are due more to the variability of attenuation in the water than to the variability of the fluorescence quantum yield, which we estimate to be relatively stable at around 1%. Finally, the ranges of water conditions where fluorescence plays a significant role in the reflectance NIR peak and where it is effectively undetectable are also determined.

Novel optical techniques for detecting and classifying toxic dinoflagellate Karenia brevis blooms using satellite imagery

Karenia brevis (K. brevis) blooms are of great interest and have been commonly reported throughout the Gulf of Mexico. In this study we propose a detection technique for blooms with low backscatter characteristics, which we name the Red Band Difference (RBD) technique, coupled with a selective K. brevis bloom classification technique, which we name the K. brevis Bloom Index (KBBI). These techniques take advantage of the relatively high solar induced chlorophyll fluorescence and low backscattering of K. brevis blooms. The techniques are applied to the detection and classification of K. brevis blooms from Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color measurements off the Gulf of Mexico. To assess the efficacy of the techniques for detection and classification, simulations, including chlorophyll fluorescence (assuming 0.75% quantum yield) based on K. brevis blooms and non-K. brevis blooms conditions were performed. These show that effective bloom detection from satellite measurements requires a threshold of RBD>0.15W/m(2)/microm/sr, corresponding to about 5mg/m(3) of chlorophyll. Blooms can be detected at lower concentration by lowering the RBD threshold but false positives may increase. The classification technique is found most effective for thresholds of RBD>0.15W/m(2)/microm/sr and KBBI>0.3*RBD. The techniques were applied and shown to be effective for well documented blooms of K. brevis in the Gulf of Mexico and compared to other detection techniques, including FLH approaches. Impacts of different atmospheric corrections on results were also examined.

Fluorescence component in the reflectance spectra from coastal waters. II. Performance of retrieval algorithms.

Retrieval of chlorophyll fluorescence magnitude using Fluorescence Height algorithms in coastal waters is more complicated than in the open ocean because of the strong deviations of elastic reflectance within the fluorescence band from the derived fluorescence baseline. We use results of our recently established relationship between fluorescence magnitude and concentrations of water constituents together with extensive HYDROLIGHT simulations, field and satellite data to analyze the performance and retrieval limitations of MODIS and MERIS FLH algorithms in the variety of coastal waters and to examine improvements for spectral band selection suitable for future sensors.

Improved MODIS Aerosol Retrieval Using Modified VIS/SWIR Surface Albedo Ratio Over Urban Scenes

In this paper, we demonstrate a regional approach to Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) retrieval over urban scenes. By using simultaneous MODIS and AErosol RObotic NETwork (AERONET) sky radiometer data, we show that for urban scenes, the surface reflectance ratios between the visible (VIS) (0.47- and 0.66-?m) and shortwave-infrared (2.12-?m) channels used in the MODIS Collection 5 (C005) retrieval are significantly underestimated. This leads to an underestimate in the VIS ground albedos resulting in a significant overestimate of the AOD in the VIS channels. Since the dependence on scattering geometry is weak, revised surface reflectance ratio maps at spatial resolutions down to 1.5 km can be developed, permitting aerosol retrievals at these resolutions. Applying this procedure results in significant reductions in MODIS-retrieved AOD bias in comparison to AERONET. As an example, for New York City, at the operational 10-km resolution, the MODIS-AERONET regression using C005 is ?MODIS = 1.24 ?AERONET + 0.11, R 2 = 0.85 , while for the regional method, we apply results in ?MODIS = 1.02 ?AERONET + 0.04, R 2 = 0.92 . This improved performance is maintained even for high-resolution retrieval down to 1.5 km with a regression of ?MODIS = 0.96 ?AERONET + 0.02, R 2 = 0.88. In addition, we show that this method can be extended to other urban scenes such as Mexico City with similar results and discuss the implications of C005 AOD overestimates on PM2.5 monitoring. Furthermore, comparisons with distributed PM2.5 sensors in the region confirm that the high estimates of AOD from C005 cannot be explained by excess urban emissions.

Neural network approach to retrieve the inherent optical properties of the ocean from observations of MODIS

Retrieving the inherent optical properties of water from remote sensing multispectral reflectance measurements is difficult due to both the complex nature of the forward modeling and the inherent nonlinearity of the inverse problem. In such cases, neural network (NN) techniques have a long history in inverting complex nonlinear systems. The process we adopt utilizes two NNs in parallel. The first NN is used to relate the remote sensing reflectance at available MODIS-visible wavelengths (except the 678 nm fluorescence channel) to the absorption and backscatter coefficients at 442 nm (peak of chlorophyll absorption). The second NN separates algal and nonalgal absorption components, outputting the ratio of algal-to-nonalgal absorption. The resulting synthetically trained algorithm is tested using both the NASA Bio-Optical Marine Algorithm Data Set (NOMAD), as well as our own field datasets from the Chesapeake Bay and Long Island Sound, New York. Very good agreement is obtained, with R² values of 93.75%, 90.67%, and 86.43% for the total, algal, and nonalgal absorption, respectively, for the NOMAD. For our field data, which cover absorbing waters up to about 6 m⁻¹, R² is 91.87% for the total measured absorption.

Population dynamics of Yb3+, Er3+ co-doped phosphate glass

Excitation dynamics in Er3+–Yb3+ co-doped phosphate glasses were examined as a function of both erbium and ytterbium concentration by monitoring fluorescence on the 1.5 μm 4I13/2→4I15/2 transition of Er3+ following pulsed laser excitation of the system at 977 nm under room temperature. For the range of donor (Yb3+) and acceptor (Er3+) concentration considered, both nonexponential and exponential relaxation dynamics were observed. Nonexponential acceptor fluorescence is characteristic of noninteracting donor (Yb3+) regime, while the exponential response is consistent with interacting donors. For the latter case, the linear acceptor dependence for the Yb3+–Er3+ transfer rate K implies energy super migration among donor ions.

Superradiant amplification in an optically dense gas

Abstract We solve numerically the coupled integro-differential Maxwell-Bloch equations describing the amplification of a pulse in an inverted pressure broadened resonant medium, without assuming the Slowly Varying Envelope Approximation in space. Our algorithm incorporates the effects of the backward wave and that of the Local Field Corrections. We show that the superradiant regime sets on for much smaller values of αL than that predicted by the Forward Wave Approximation, and that it is possible to convert most of the energy stored in the atomic system into transmitted and reflected bursts whose duration are substantially shorter than either the dephasing or relaxation times of the atomic system, for sample lengths that are only few wavelengths long. Furthermore, we show that in the superradiant regime, except for sample lengths that are in the neighborhood of an integer multiple of the half-wavelength, the atomic polarizability possesses definite spatial symmetry with respect to the midpoint of the sample, resulting there in equal energy release for the transmitted and reflected signals. The transition domain between two consecutive opposite spatial symmetry sectors is manifested through unequal intensity for the transmitted and reflected bursts and in different frequency shifts in the spectral distributions of the two output signals.

Comparison of the paraxial-ray approximation and the variational method solutions to the numerical results for a beam propagating in a self-focusing Kerr medium.

The analytic solutions for the amplitude and phase of a cw cylindrically symmetric beam propagating in a self-focusing Kerr medium obtained from the aberrationless paraxial approximation and the variational method are compared with the numerical results of the theory. For beam energies close to the critical focusing value, the variational method expression for the longitudinal phase is shown, except for an extremely small longitudinal distance of propagation, to give better qualitative and quantitative agreement with the numerical solutions. In particular, we confirm the variational method prediction that the regularized phase does not change sign.

Estimating particle composition and size distribution from polarized water-leaving radiance

The sensitivity of the polarization of water-leaving radiance to the microphysical parameters of oceanic hydrosols, specifically to the real part of the bulk refractive index (nbulk) and to the hyperbolic slope of the Junge-type particle size distribution (PSD, ξ) is analyzed using in situ measurements of the underwater polarized light, in both Case I and Case II waters, and multiple scattering computations. Based on comparisons of experimental and simulated data, estimations of the real part of the refractive index and of the slope of the PSD are given. The study yielded results that generally agreed with expectations and that have accuracies comparable to previously published techniques. The analysis also demonstrates that the inclusion of polarization in addition to traditional radiance measurements can be expected to provide complementary information on the nature of particle populations in the ocean.

Retrieval of chlorophyll fluorescence from reflectance spectra through polarization discrimination: modeling and experiments.

The polarization discrimination technique we recently developed, shows that it is possible to separate the elastic scattering and the chlorophyll fluorescence signal from the water-leaving radiance by making use of the fact that the elastically scattered components are partially polarized, while the fluorescence signal is unpolarized. The technique has been shown to be applicable to a wide range of water conditions. We present an extension of experimental and analytical results, which serve to define the scope of this technique and its range of applicability. A new analysis, based on vector radiative transfer computations, and on laboratory and field measurements on eastern Long Island and in the Chesapeake Bay, shows that the technique is generally effective for both open ocean and coastal waters, but that it is limited if the ocean bottom albedo and/or multiple scattering due to very high mineral particle concentrations result in depolarizing the water-leaving radiance. In addition, we show that in contrast with the polarization-based retrieval, the traditional method of extracting fluorescence height using the baseline method can give significant errors, particularly for coastal waters where it strongly overestimates the fluorescence values.