Index of Ocean and Coastal Waters Research Areas
The purpose of the Ocean and Coastal ORS Lab is to monitor and investigate the optical properties of complex coastal areas as well as clear open ocean waters. This is accomplished with remotely-sensed data, received from operational and research satellites, observing platforms and in situ data.
Multiangular Hyperspectral Polarimetry in Case I and Case II Waters
Since Waterman’s pioneering observations of the underwater polarized light field (Waterman 1954), scientists have made progress in developing radiative transfer models to predict and accurately measure the spectral and angular distribution of the underwater light field. Yet, a major but vastly understudied component of the underwater light field is the polarized light field. At the ORS lab we are investing the underwater polarization combining both experimental data (acquired through custom-built sensors) and vector radiative transfer calculations.
Dependence of the polarized water-leaving radiance on the particles microphysics
The polarization of light in the atmosphere has been used as a tool for gaining information on aerosol optical properties that could not have been obtained by studying the scalar radiance alone (see, for example, Waquet et al. 2009 and references therein). In the atmosphere, polarization mainly comes from single scattering so that angular features of the phase function are mapped directly onto the polarized radiance. Features in the single scattering can be readily identified in the angular distribution of the degree of (linear) polarization (DOP). In the ocean, the features tend to be washed out due to the presence of multiple scattering by hydrosols. In the open ocean (Case I waters), most particles are organic particles (both living and nonliving), covarying with chlorophyll concentration. These suspended particles have a weak effect on the underwater DOP because of usually low concentrations and low refractive indices (Harmel et al. 2008). The underwater polarization is, therefore, mainly driven by Rayleigh scattering by water molecules resulting in a relatively simple pattern, i.e. maximal DOP between 0.6-0.8 (depending on the wavelength) occurring around 90° scattering angle. However, in Case II waters, inorganic particles, having a relative refractive index much higher than chlorophyllic particles, can significantly change the DOP of the water-leaving radiance.
Our aim is to use polarization to systematically retrieve additional and complementary information on the suspended particles (specifically, refractive index and size distribution) that can not be obtained with methods that only analyze the scalar intensity.
Development of biological response to the dynamic spectral-polarized underwater light field (2009 MURI Topic 7)
The Multidisciplinary University Research Initiative program (MURI) is a program designed to address large multidisciplinary topic areas representing exceptional opportunities for future DoD (Department of Defense) applications and technology options.
One of the winners of the MURI 2009 grants is the research group lead by Dr. Molly E. Cummings at University of Texas. For information click here. Several research groups, including the ORS Lab, are involved in this project, specifically:
Many marine animals make use of the underwater polarization to achieve complete camouflage. Effective camouflage relies on matching the background perfectly. The difference between perfect and imperfect camouflage can mean the difference between life and death for many marine animals. For military applications, consequences resulting from complete versus incomplete camouflage are similarly grave and the success of a mission may critically depend upon perfect camouflage. The underwater polarized light field depend both on the IOPs of the water medium and on the time of the day. The biological world has responded to this dynamically changing portion of the electromagnetic spectrum by developing (a) polarization visual sensitivity, and (b) physiological responses to vary skin reflectance properties to mimic or contrast with the background. Having polarized vision, aids target detection underwater due to enhanced target contrast. Consequently, ignoring the polarized component in a camouflage design may increase its detection by unintended viewers with polarized sensitivities.
The aim of this project is to identify the mechanistic pathways that have evolved in the biological realm to send signals or remain concealed against the underwater polarized light field using a comparative approach. Both vertebrates and invertebrates systems are under investigation as well as molecular and hydrosol scattering water bodies, to address these specific objectives:
Field research is conducted in both in open ocean and coastal waters. Biological responses are studied in the lab on invertebrates and vertebrates selecting species that occupy oligotrophic and eutrophic environments. Physiological experiments are also conducted in the lab to determine the regulatory control of polarization camouflage in both vertebrate and invertebrate systems. By combining polarization modeling, field measurements, and physiological measurements of animal response in the lab in a comparative fashion, it might be possible to identify alternative solutions to the problem of camouflage in a polarized environment.
Observing Systems (LISCO)
Advances in oceanic bio-optical processes are expected to be more heavily focused on improving satellite retrieval products of inherent optical properties (IOPs) of coastal waters, which, because of their complexity, offer more challenges than open ocean waters, where satellite observations and retrieval algorithms are already reasonably effective. Thus, the validation of the current and future Ocean Color satellite data is important for characterizing the optical environment connected with coastal waters, which are of importance because of population concentrations along them and their susceptibility to anthropogenic impacts.
To address these concerns and support present and future multi- and hyper-spectral calibration and validation activities, as well as the development of new measurement and retrieval techniques and algorithms for coastal waters, the ORS Lab along with the Naval Research Laboratory at Stennis Space Center, Mississippi, has established a new, scientifically comprehensive, off-shore platform, the Long Island Sound Coastal Observatory (LISCO). This site has been designed to serve as a venue and framework for combining multi- and hyperspectral radiometer measurements with satellite and in situ measurements and radiative transfer simulations of coastal waters, helping to provide more effective closure for the whole measurement validation and simulation loop. Measurements are utilized for multi-spectral calibration and validation of current Ocean Color satellites (MERIS, MODIS, SeaWIFS) in coastal waters, and for evaluating future satellites missions (NPOESS, OCM2, Sentinel-2) with extension to hyperspectral calibration and validations of the hyperspectral sensors (HICO), as well as for improvements in coastal IOP retrieval and atmospheric correction algorithms.
The platform combines an AERONET SeaPRISM radiometer (CIMEL Electronique) as a part of AERONET Ocean Color Network (AERONET-OC), with a co-located HyperSAS set of radiometers capable of hyperspectral measurements of water-leaving radiance, sky radiance and downwelling irradiance. Both instruments were installed on the Long Island Sound Coastal Observatory (LISCO) in October 2009 and since then have been providing data. SeaPRISM data are transferred by the satellite link to NASA. Raw SeaPRISM data are also collected at the CCNY-ORSL server. HyperSAS data are transmitted via a broadband cellular service as emails to the CCNY-ORSL Sky server. The instruments are positioned on a retractable tower (Floatograph).
In June 2010 the HyperSAS system was upgraded to its polarization version, i.e. HyperSAS POL, which allows the detection of the Stokes components I, Q and U of the upwelling water-leaving radiance.
Additional in-water measurements:
Field measurements are regularly taken near LISCO for the matchups with the instruments as well to determine variability of water parameters and its impact on the validation of the ocean color satellite data. The instruments currently being deployed are:
Retrieval of Chlorophyll Fluorescence
The polarization discrimination technique
The NIR peak in the NIR spectrum of the reflectance can be significantly affected by chlorophyll fluorescence depending on the water composition. The accuracy of [Chl] retrieval depends, therefore, on the ability to separate contributions of elastic scattering from fluorescence spectra.
The polarization discrimination technique we 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.
The fluorescence component in the reflectance spectra
We conducted detailed analysis (through extensive simulations using HYDROLIGHT of the remote sensing reflectance spectra to reassess existing fluorescence algorithms in order to identify sources of errors. In particular, it is important to understand their limits in coastal waters in the context of optically active constituents, including variations in specific absorption of chlorophyll and accessory pigments, CDOM absorption, and scattering and absorption by nonalgal particles (NAP). We also used 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.
Quantum yield of chlorophyll fluorescence
The detection of solar induced chlorophyll fluorescence (SICF) obtained by processing ocean color spectra from satellite sensors (e.g. MODIS and MERIS) has been a very powerful tool for monitoring marine phytoplankton on synoptic scales. As a signal specific to phytoplankton, SICF provides an alternative means to assess the biomass and primary productivity. Our approach for the retrieval of SICF takes advantage of hyperspectral field measurements of absorption and attenuation, which are combined with remote sensing reflectance and used to determine SICF and its quantum yield in highly productive coastal waters.