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Slade W.H.,Sequoia Scientific, Inc. | Boss E.,University of Maine, United States
Applied Optics | Year: 2015

Measurements of the particulate beam attenuation coefficient at multiple wavelengths in the ocean typically exhibit a power law dependence on wavelength, and the slope of that power law has been related to the slope of the particle size distribution (PSD), when assumed to be a power law function of particle size. Recently, spectral backscattering coefficient measurements have been made using sensors deployed at moored observatories, on autonomous underwater vehicles, and even retrieved from space-based measurements of remote sensing reflectance. It has been suggested that these backscattering measurements may also be used to obtain information about the shape of the PSD. In this work, we directly compared field-measured PSD with multispectral beam attenuation and backscattering coefficients in a coastal bottom boundary later. The results of this comparison demonstrated that (1) the beam attenuation spectral slope correlates with the average particle size as suggested by theory for idealized particles and PSD; and (2) measurements of spectral backscattering also contain information reflective of the average particle size in spite of large deviations of the PSD from a spectral power law shape. © 2015 Optical Society of America.


Mobley C.D.,Sequoia Scientific, Inc. | Boss E.S.,University of Maine, United States
Applied Optics | Year: 2012

Accurate calculation of underwater light is fundamental to predictions of upper-ocean heating, primary production, and photo-oxidation. However, most ocean models simulating these processes do not yet incorporate radiative transfer modules for their light calculations. Such models are often driven by abovesurface, broadband, daily averaged irradiance or photosynthetically available radiation (PAR) values obtained from climatology or satellite observations, sometimes without correction for sea-surface reflectance, even though surface reflectance can reduce in-water values by more than 20%. We present factors computed by a radiative transfer code that can be used to convert above-surface values in either energy or quantum units to in-water net irradiance, as needed for calculations of water heating, and to inwater PAR, as needed for calculations of photosynthesis and photo-oxidation. © 2012 Optical Society of America.


Lee Z.,Mississippi State University | Ahn Y.-H.,Korea Ocean Research and Development Institute | Mobley C.,Sequoia Scientific, Inc. | Arnone R.,Naval Research Laboratory Stennis Space Center
Optics Express | Year: 2010

Using hyperspectral measurements made in the field, we show that the effective sea-surface reflectance ρ (defined as the ratio of the surface-reflected radiance at the specular direction corresponding to the downwelling sky radiance from one direction) varies not only for different measurement scans, but also can differ by a factor of 8 between 400 nm and 800 nm for the same scan. This means that the derived water-leaving radiance (or remote-sensing reflectance) can be highly inaccurate if a spectrally constant ρ value is applied (although errors can be reduced by carefully filtering measured raw data). To remove surface-reflected light in field measurements of remote sensing reflectance, a spectral optimization approach was applied, with results compared with those from remote-sensing models and from direct measurements. The agreement from different determinations suggests that reasonable results for remote sensing reflectance of clear blue water to turbid brown water are obtainable from above-surface measurements, even under conditions of high waves. © 2010 Optical Society of America.


Pejrup M.,Copenhagen University | Mikkelsen O.A.,Sequoia Scientific, Inc.
Estuarine, Coastal and Shelf Science | Year: 2010

It has long been recognized that the suspended sediment concentration (SSC) is one of the major determinants for the flocculation of cohesive particles into sediment flocs in estuaries. It is furthermore well known that the turbulent shear of the water significantly influences the flocculation process and the equilibrium settling velocity of flocculated sediment in a turbulent flow. A vast number of authors have reported algorithms relating the median settling velocity (W50) to suspended sediment concentration. However, only a few studies have dealt with the impact of the turbulent shear (in this paper expressed as the root mean square [rms] velocity gradient, [G]) in the water on the W50 in situ. There is a strong need to establish algorithms based on in situ measurements describing the dual impact of both SSC and G on the flocculation process, and hence, W50. The present paper addresses this topic. Field settling velocities of suspended cohesive sediment have been measured in micro-, meso-, and macro-tidal estuaries. Regression analyses between the W50, SSC and G are presented. It is shown that by including both G and SSC in the regression analyses, a significant increase in the correlation of the description of W50 and the controlling parameters from each area can be obtained. A generic algorithm describing the data from all the investigated areas is suggested. It works well within specific tidal areas but fails to give a generic description of the field settling velocity. © 2009 Elsevier Ltd. All rights reserved.


Mobley C.D.,Sequoia Scientific, Inc.
Optics Express | Year: 2011

Ocean physical-biological-optical ecosystem models can require light calculations at thousands of grid points and time steps. Implicit inverse models that recover ocean absorption and scattering properties from measured light variables can require thousands of solutions of the radiative transfer equation. An extremely fast radiative transfer code, EcoLight-S(ubroutine), has been developed to address these needs. EcoLight-S requires less than one second on a moderately fast computer to compute spectral irradiances over near-ultraviolet to near-infrared wavelengths with errors in the photosyntheically available radiation (PAR) of no more than ten percent throughout the euphotic zone. It is thus possible to replace simple and often inaccurate analytical PAR or spectral irradiance models with more accurate radiative transfer calculations, with very little computational penalty. EcoLight-S is applicable to Case 2 and optically shallow waters for which no analytical light models exist. EcoLight-S also computes upwelling and downwelling plane irradiances, nadir and zenith radiances, and the remote-sensing reflectance. These quantities allow ecosystem predictions to be validated with optical measurements obtained from in-water instruments or remotely sensed imagery. © 2011 Optical Society of America.


Mobley C.D.,Sequoia Scientific, Inc.
Applied Optics | Year: 2015

Generation of random sea surfaces using wave variance spectra and Fourier transforms is formulated in a way that guarantees conservation of wave energy and fully resolves wave height and slope variances. Monte Carlo polarized ray tracing, which accounts for multiple scattering between light rays and wave facets, is used to compute effective Mueller matrices for reflection and transmission of air- or water-incident polarized radiance. Irradiance reflectances computed using a Rayleigh sky radiance distribution, sea surfaces generated with Cox-Munk statistics, and unpolarized ray tracing differ by 10%-18% compared with values computed using elevation- and slope-resolving surfaces and polarized ray tracing. Radiance reflectance factors, as used to estimate water-leaving radiance from measured upwelling and sky radiances, are shown to depend on sky polarization, and improved values are given. © 2015 Optical Society of America.


Grant
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 120.50K | Year: 2016

The remote sensing reflectance signal measured by an ocean color satellite is to first order proportional to the ratio of backscattered to absorbed light. Therefore in situ measurements of absorption and backscattering, as functions of wavelength, along with in situ and satellite radiometery, are key to refinement and calibration of legacy ocean color algorithms, as well as development of next generation ocean color products such as phytoplankton functional type. Currently, commercial instruments exist for in situ measurement of the hyperspectral absorption coefficient, but no instrument exists for measurement of the hyperspectral backscattering coefficient. We propose to develop an active sensor for in situ measurement of the hyperspectral backscattering coefficient. We are considering a design based on a broadband white source (chopped), monochromator for varying the source beam wavelength, and hyperspectral detectors (receiving at 2-3 angles) using spectrometers that track the monochromator. There are several configurations of the source, monochromator, spectrometer, and detector that can be considered the aim of the proposed work is to simulate a number of instrument configurations and assess technical and commercial feasibility of such hyperspectral backscattering instrument. The proposed instrument addresses a critical gap in the field of currently available systems for measuring hyperspectral IOPs in situ, in support of hyperspectral ocean color missions.


Grant
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 124.45K | Year: 2013

We propose to develop an active sensor for in situ measurement of the inherent optical properties (IOPs) absorption and backscattering at multiple wavelengths. Multi- or hyper-spectral absorption of particles and dissolved materials is routinely measured in the laboratory and in situ in order to characterize, for example, the quantities and types of phytoplankton based on concentrations of specific absorbing pigments. Similarly, backscattering is employed to estimate the concentration of suspended material. Measurements of absorption and backscattering concurrently, and at multiple wavelengths, are useful as proxies for biogeochemical measurements such as particle composition, concentration of particulate organic carbon, and particle size distribution, as well as for remote sensing calibration and validation.The current state of the art for phytoplankton observation using optical sensors on autonomous platforms relies on linking biomass with optical backscattering and chlorophyll. The ability to quantify phytoplankton using absorption not only overcomes limitations of backscattering and fluorescence-based approaches, but multi-spectral (visible wavelength) measurements of absorption also provide the means to discern the presence of accessory pigments and pigment packaging, ultimately leading to not only improvements in phytoplankton biomass estimates, but also the potential for resolving phytoplankton functional types.Briefly, the proposed sensor emits a collimated beam of light into the water and measures the backscattered light as a radial function from the beam location. An inversion algorithm is then used to convert this backscattered intensity as a function of distance from the beam to the inherent optical properties absorption and backscattering. Multiple source wavelengths are used and the sensor is packaged in a compact, flat-faced geometry easing integration into autonomous platforms.


Grant
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase II | Award Amount: 737.99K | Year: 2016

Particle size distribution (PSD) is a fundamental environmental measurement, with diverse biogeochemical applications including carbon cycle science, ecosystem and fisheries modeling, and harmful algal bloom (HAB) detection/prediction. There is optimism that estimates of PSD will be available from ocean color measurements (such as NASA's upcoming PACE mission), and will be able to help constrain global-scale ecosystem/carbon models and estimates of primary production. However, natural PSD variability is not well understood due to the challenges of routine measurement, and there exists little field data over large space and time scales. We propose to bridge this gap by developing an instrument for ship-based flow-through application that uses laser scattering from multiple wavelengths for estimation of the PSD across a wide range of particle sizes from 0.1 to 500 micron, covering a range from the smallest oceanic pico-plankton to larger meso-plankton.


Grant
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 124.50K | Year: 2015

Particle size distribution (PSD) is a fundamental environmental measurement, with diverse biogeochemical applications including carbon cycle science, ecosystem and fisheries modeling, and harmful algal bloom (HAB) detection/prediction. There is optimism that estimates of PSD will be available from ocean color measurements (such as NASA's upcoming PACE mission), and will be able to help constrain global-scale ecosystem/carbon models and estimates of primary production. However, natural PSD variability is not well understood due to the challenges of routine measurement, and there exists little field data over large space and time scales. We propose to bridge this gap by investigating an instrument for ship-based flow-through application that uses laser scattering from multiple wavelengths and polarizations for estimation of the PSD across a wide range of particle sizes from submicron to >200 micron, covering a range from the smallest oceanic pico-plankton to larger meso-plankton.

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