Sam Ahmed

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.

Effect of multiple light scattering and self-absorption on the fluorescence and excitation spectra of dyes in random media

The absorption, fluorescence, and excitation spectra of a dye in a highly scattering random medium were studied experimentally. The intrinsic absorption spectrum of the dye does not change in the presence of scatterers, but the presence of scatterers in the media will change the observed fluorescence spectra. The observation is accounted for by the change in the photon trajectory path length for the fluorescence emission.

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.

Polarization impacts on the water-leaving radiance retrieval from above-water radiometric measurements

Above-water measurements of water-leaving radiance are widely used for water-quality monitoring and ocean-color satellite data validation. Reflected skylight in above-water radiometry needs to be accurately estimated prior to derivation of water-leaving radiance. Up-to-date methods to estimate reflection of diffuse skylight on rough sea surfaces are based on radiative transfer simulations and sky radiance measurements. But these methods neglect the polarization state of the incident skylight, which is generally highly polarized. In this paper, the effects of polarization on the sea surface reflectance and the subsequent water-leaving radiance estimation are investigated. We show that knowledge of the polarization field of the diffuse skylight significantly improves above-water radiometry estimates, in particular in the blue part of the spectrum where the reflected skylight is dominant. A newly developed algorithm based on radiative transfer simulations including polarization is described. Its application to the standard Aerosol Robotic Network-Ocean Color and hyperspectral radiometric measurements of the 1.5-year dataset acquired at the Long Island Sound site demonstrates the noticeable importance of considering polarization for water-leaving radiance estimation. In particular it is shown, based on time series of collocated data acquired in coastal waters, that the azimuth range of measurements leading to good-quality data is significantly increased, and that these estimates are improved by more than 12% at 413 nm. Full consideration of polarization effects is expected to significantly improve the quality of the field data utilized for satellite data validation or potential vicarious calibration purposes.

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.

Low and optically thin cloud measurements using a Raman-Mie lidar

We analyze the potential of measuring low-altitude optically thin clouds with a Raman-elastic lidar in the daytime. Optical depths of low clouds are derived by two separate methods from nitrogen Raman and elastic-scattering returns. By correcting for aerosol influences with the combined Raman-elastic returns, Mie retrievals of low-cloud optical depth can be dramatically improved and show good agreement with the direct Raman retrievals. Furthermore, a lidar ratio profile is mapped out and shown to be consistent with realistic water phase cloud models. The variability of lidar ratios allows us to explore the distribution of small droplets near the cloud perimeter.

Retrieving quantum yield of sun-induced chlorophyll fluorescence near surface from hyperspectral in-situ measurement in productive water.

Magnitude and quantum yield (eta) of sun induced chlorophyll fluorescence are determined in widely varying productive waters with chlorophyll concentrations from 2- 200 mg/m(3). Fluorescence was estimated using linear fitting of in-situ measured surface reflectance with elastic and inelastic reflectance spectra. Elastic reflectance spectra were obtained from Hydrolight simulations with measured absorption and attenuation spectra as inputs. Eta is then computed based on a depth integrated fluorescence model and compared with Hydrolight calculation results. Despite the large variability of coastal environments examined the ? values are found to vary over a relatively narrow range 0.1%-1% with mean values of 0.33%+/-0.17%.

Improving atmospheric correction for highly productive coastal waters using the short wave infrared retrieval algorithm with water-leaving reflectance constraints at 412 nm.

The recently developed short wave infrared (SWIR) atmospheric correction algorithm for ocean color retrieval uses long wavelength channels to retrieve atmospheric parameters to avoid bright pixel contamination. However, this retrieval is highly sensitive to errors in the aerosol model, which is magnified by the higher variability of aerosols observed over urban coastal areas. While adding extra regional aerosol models into the retrieval lookup tables would tend to increase retrieval error since these models are hard to distinguish in the IR, we explore the possibility that for highly productive waters with high colored dissolved organic matter, an estimate of the 412 nm channel water-leaving reflectance can be used to constrain the aerosol model retrieval and improve the water-leaving reflectance retrieval. Simulations show that this constraint is particularly useful where aerosol diversity is significant. To assess this algorithm we compare our retrievals with the operational SeaWiFS Data Analysis System (SeaDAS) SWIR and near infrared retrievals using in situ validation data in the Chesapeake Bay and show that, especially for absorbing aerosols, significant improvement is obtained. Further insight is also obtained by the intercomparison of retrieved remote sensing reflectance images at 443 and 551 nm, which demonstrates the removal of anomalous artifacts in the operational SeaDAS retrieval.

Potential and limitations of energy-transfer processes in pulsed and cw dye laser mixtures: comparison of theory and experiments.

A comprehensive energy-transfer dye laser (ETDL) simulation model and the premise on which it is based are examined and reviewed. The model permits theoretical predictions of the total transfer efficiency and hence identification of the appropriate mechanisms that are responsible for energy transfer as well as predicting fluorescence spectra of dye mixtures. Using concentration regimes that are generally applicable to laser action, we also show that the model is capable of predicting gain line shapes (and hence tunability) for pulsed ETDLs. An extension of the model also predicts the gain spectrum of cw laser-pumped ETDLs as a function of donor and acceptor concentrations, their spectral data, energytransfer parameters, pump power and wavelength, and dimensions of the active region. Model simulation results are checked against experimental results for pulsed and cw ETDLs with laser outputs in the visible and near-IR spectral regions. The results lead to a better and more detailed understanding of ETDL mec hanisms and the role that the triplet states play in these mechanisms.

Radiometric calibration of ocean color satellite sensors using AERONET-OC data

Radiometric vicarious calibration of ocean color (OC) satellite sensors is carried out through the full sunlight path radiative transfer (RT) simulations of the coupled ocean-atmosphere system based on the aerosol and water-leaving radiance data from AERONET-OC sites for the visible and near-infrared (NIR) bands. Quantitative evaluation of the potential of such approach for achieving the radiometric accuracies of OC satellite sensors is made by means of direct comparisons between simulated and satellite measured top of atmosphere (TOA) radiances. Very high correlations (R ≥ 0.96 for all visible channels) are achieved for the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor when this approach is applied with the data from the LISCO and WaveCIS AERONET-OC sites. Vicarious calibration gain factors derived with this approach are highly consistent, with comparisons between the two sites exhibiting around 0.5% discrepancy in the blue and green parts of the spectrum, while their average temporal variability is also within 0.28% - 1.23% permitting the approach to be used, at this stage, for verification of sensor calibration performance.