Soe Hlaing

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.

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.

Long Island Sound Coastal Observatory: Assessment of above-water radiometric measurement uncertainties using collocated multi and hyperspectral systems

The Long Island Sound Coastal Observational platform (LISCO) near Northport, New York, has been recently established to support validation of ocean color radiometry (OCR) satellite data. LISCO is equipped with collocated multispectral, SeaPRISM, and hyperspectral, HyperSAS, above-water systems for OCR measurements. This combination offers the potential for improving validation activities of current and future OCR satellite missions, as well as for satellite intercomparisons and spectral characterization of coastal waters. Results of measurements made by both the multi and hyperspectral instruments, in operation since October 2009, are presented, evaluated and their associated uncertainties quantified based on observations for a period of over a year. Multi- and hyperspectral data processing as well as the data quality analysis are described and their uncertainties evaluated. The quantified intrinsic uncertainties of HyperSAS data exhibit satisfactory values, less than 5% over a large spectral range, from 340 to 740 nm, and over a large range of diurnal daylight conditions, depending on the maximum sun elevation at the solar noon. Intercomparisons between HyperSAS and SeaPRISM data revealed that an overcorrection of the sun glint effect in the current SeaPRISM processing induces errors, which are amplified through the whole data processing, especially at the shorter wavelengths. The spectral-averaged uncertainties can be decomposed as follows: (i) sun glint removal generates 2% uncertainty, (ii) sky glint removal generates strong uncertainties of the order of 15% mainly induced by sun glint overcorrection, (iii) viewing angle dependence corrections improve the data intercomparison by reducing the dispersion by 2%, (iv) normalization of atmospheric effects generates approximately 4% uncertainty. Based on this study, improvements of the sun glint correction are expected to significantly reduce the uncertainty associated with the data processing down to the level of 1%. On the other hand, strong correlations between both datasets (R(2)>0.96) demonstrate the efficacy of the above-water retrieval concept and confirm that the collocated instrumentation constitutes an important aid to above-water data quality analysis, which makes LISCO a key element of the AERONET-OC network.

Assessment of a bidirectional reflectance distribution correction of above-water and satellite water-leaving radiance in coastal waters

Water-leaving radiances, retrieved from in situ or satellite measurements, need to be corrected for the bidirectional properties of the measured light in order to standardize the data and make them comparable with each other. The current operational algorithm for the correction of bidirectional effects from the satellite ocean color data is optimized for typical oceanic waters. However, versions of bidirectional reflectance correction algorithms specifically tuned for typical coastal waters and other case 2 conditions are particularly needed to improve the overall quality of those data. In order to analyze the bidirectional reflectance distribution function (BRDF) of case 2 waters, a dataset of typical remote sensing reflectances was generated through radiative transfer simulations for a large range of viewing and illumination geometries. Based on this simulated dataset, a case 2 water focused remote sensing reflectance model is proposed to correct above-water and satellite water-leaving radiance data for bidirectional effects. The proposed model is first validated with a one year time series of in situ above-water measurements acquired by collocated multispectral and hyperspectral radiometers, which have different viewing geometries installed at the Long Island Sound Coastal Observatory (LISCO). Match-ups and intercomparisons performed on these concurrent measurements show that the proposed algorithm outperforms the algorithm currently in use at all wavelengths, with average improvement of 2.4% over the spectral range. LISCOs time series data have also been used to evaluate improvements in match-up comparisons of Moderate Resolution Imaging Spectroradiometer satellite data when the proposed BRDF correction is used in lieu of the current algorithm. It is shown that the discrepancies between coincident in-situ sea-based and satellite data decreased by 3.15% with the use of the proposed algorithm. This confirms the advantages of the proposed model over the current one, demonstrating the need for a specific case 2 water BRDF correction algorithm as well as the feasibility of enhancing performance of current and future satellite ocean color remote sensing missions for monitoring of typical coastal waters.

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%.

Sensitivity of the above water polarized reflectance to the water composition

Estimating the Stokes vector components of the polarized water radiance from above water measurements is a challenging task, mainly because of their small magnitude and the strong contamination by the sky light reflected on the sea surface. Consequently, in most applications the Stokes vector components are considered equal to zero except of I, the total reflectance. In this study, both below and above water measurements are used to assess the feasibility of such retrievals and their use to determine the water composition. In-water inherent optical properties (IOPs) were measured with commercially available instrumentation. In addition, in-water polarization characteristics were measured by our multi-angular hyperspectral sensor which provided the Stokes components for a scattering angles range of the 0-180° and a full spectral range between 400 and 750 nm. Second, a customized HyperSAS (Satlantic) instrument is used from the coastal platform in Long Island Sound, NY (LISCO) acquiring above water measurements. That instrumentation includes, in the standard configuration, two hyperspectral radiance sensors for measuring upwelling and sky radiances and one irradiance sensor for measuring downwelling irradiance. In our installation, HyperSAS capabilities were augmented by adding two radiance sensors having two polarizers oriented at 0 and 45°, with respect to a reference axis (HyperSAS-POL). An ad hoc procedure, which included measurements and radiative transfer computations, has been developed enabling to estimate the contribution of the sky glint and subtract it from the signal directly measured by HyperSAS-POL. As a result, the retrieved spectral shape of the underwater degree of polarization is consistent with what obtained from in situ underwater measurements and depends on the IOPs of the ocean itself. In addition, the demonstrated correctness of this polarized measurements from LISCO site enable us to provide continuous time series from the beginning of June 2010.

Validation of ocean color satellite sensors using coastal observational platform in Long Island Sound

The Long Island Sound Coastal Observational platform (LISCO) near Northport, New York, has been recently established to support satellite data validation. LISCO is equipped with both multispectral SeaPRISM and hyperspectral HyperSAS radiometers for ocean color measurements. LISCO substantially expands observational capabilities for the continuous monitoring and assessment of ocean color satellite data quality. This offers the potential for improving the calibration and validation activities of current and future Ocean Color satellite missions, as well as for satellite intercomparisons and spectral characterization of coastal waters. Results of measurements made by both the multi and hyperspectral instruments, in operation since October 2009, are presented, evaluated and compared with ocean color satellite data. The comparisons with the normalized water-leaving radiance derived from SeaPRISM with that from MERIS, MODIS and SeaWiFS showed satisfactory correlations (r > 0.9 at 550nm) and consistencies (APD < 15% at 550nm). Similar and equivalent results are obtained when the hyperspectral HYPERSAS data are compared with the same satellite datasets. The results confirm that the LISCO site is appropriate for use in calibration/validation of the ocean color satellites in coastal waters and as a key element of the AERONET-OC network. This makes it possible to foresee a wider use of the LISCO site to monitor current and future ocean color multispectral (NPOESS, Sentinel) and hyperspectral (HICO) satellite missions.

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.

Evaluation of atmospheric correction procedures for ocean color data processing using hyper- and multi-spectral radiometric measurements from the Long Island Sound Coastal Observatory

In Ocean Color (OC) data processing one of the most critical steps is the atmospheric correction procedure used to separate the water leaving radiance, which contains information on water constituents, from the total radiance measured by space borne sensors, which contains atmospheric contributions. To ensure reliability of retrieved water leaving radiance values, and OC information derived from them, the quality of the atmospheric correction procedures applied needs to be assessed and validated. In this regard, the Long Island Sound Coastal Observatory (LISCO), jointly established by the City College of New York and the Naval Research Laboratory is becoming one of the key elements for OC sensors validation efforts, in part because of its capabilities for co-located hyper and multi-spectral measurements using HyperSAS and SeaPRISM radiometers respectively, with the latter being part of the NASA AERONET - OC network. Accordingly, the impact of the procedures used for atmospheric correction on the retrieval of remote sensing reflectance (Rrs) data can then be evaluated based on satellite OC data acquired from the LISCO site over the last two years. From this, the qualities of atmospheric correction procedures are assessed by performing matchup comparisons between the satellites retrieved atmospheric data and that of LISCO.

Fluorescence contribution to reflectance spectra for a variety of coastal waters

Improved remote sensing retrievals of the chlorophyll fluorescence component in coastal water reflectance can significantly help environmental impact assessments. While retrieval of chlorophyll fluorescence from satellite observations of open ocean reflectance using Fluorescence Line Height (FLH) algorithms is now routine, it is much more complicated in coastal waters where the fluorescence overlaps with a NIR elastic scattering peak arising from the combination of photosynthetic pigment and particulate scattering and absorption, and rapidly increasing water absorption. To examine retrieval accuracies attainable in coastal waters by MODIS and other FLH algorithms, we compared the results of extensive numerical simulations with those of our field measurements in the Chesapeake Bay. The relationship between the contribution of fluorescence in the reflectance spectra and [Chl] and other water constituents was analyzed by simulations of more than 1000 reflectances using the HYDROLIGHT radiative transfer program. For these, IOP were related to parameterized microphysical models, following the same procedures used to generate the IOCCG dataset, but with higher (1 nm) spectral resolution, and wider range of parameters including chlorophyll specific absorption more typical of coastal waters. Results of simulations and field measurements show that the variability of retrieved fluorescence can be attributed largely to its attenuation in the water by algae, CDOM and mineral particles, and much less to the variation of the fluorescence quantum yield. Our systematic parametric study of fluorescence as a function of the other water components is then used to define the range of water parameters where fluorescence contributes significantly to the NIR peak reflectance, and where it is almost undetectable.