Robert H. Stavn

Comparison of Numerical Models for Computing Underwater Light Fields

Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

Optical scattering and backscattering by organic and inorganic particulates in U.S. coastal waters

We present the results of a study of optical scattering and backscattering of particulates for three coastal sites that represent a wide range of optical properties that are found in U.S. near-shore waters. The 6000 scattering and backscattering spectra collected for this study can be well approximated by a power-law function of wavelength. The power-law exponent for particulate scattering changes dramatically from site to site (and within each site) compared with particulate backscattering where all the spectra, except possibly the very clearest waters, cluster around a single wavelength power-law exponent of -0.94. The particulate backscattering-to-scattering ratio (the backscattering ratio) displays a wide range in wavelength dependence. This result is not consistent with scattering models that describe the bulk composition of water as a uniform mix of homogeneous spherical particles with a Junge-like power-law distribution over all particle sizes. Simultaneous particulate organic matter (POM) and particulate inorganic matter (PIM) measurements are available for some of our optical measurements, and site-averaged POM and PIM mass-specific cross sections for scattering and backscattering can be derived. Cross sections for organic and inorganic material differ at each site, and the relative contribution of organic and inorganic material to scattering and backscattering depends differently at each site on the relative amount of material that is present.

Optical modeling of clear ocean light fields: Raman scattering effects.

A Monte Carlo simulation (the NORDA optical model) and the Three-Parameter Model of the submarine light field are used to analyze the effect of water Raman emission at 520 nm in clear ocean waters. Reported optical anomalies for clear ocean waters at longer wavelengths (520 nm +) are explained by the effects of water Raman emission, and the simulation results are confirmed by Biowatt-NORDA observations made in the Sargasso Sea. A new optical parametrization for clear ocean water is proposed.

Biogeo-optics: particle optical properties and the partitioning of the spectral scattering coefficient of ocean waters

We propose a direct method of partitioning the particulate spectral scattering coefficient of the marine hydrosol based on the concurrent determination of the concentrations of particulate mineral and organic matter (the total mass of optically active scattering material exclusive of water) with the particulate spectral scattering coefficient. For this we derive a Model II multiple linear regression model. The multiple linear regression of the particulate spectral scattering coefficient against the independent variables, the concentrations of particulate inorganic matter and particulate organic matter, yields their mass-specific spectral scattering cross sections. The mass-specific spectral scattering cross section is simply the particle scattering cross section normalized to the particle mass, a fundamental optical efficiency parameter for the attenuation of electromagnetic radiation [Absorption and Scattering of Light by Small Particles, (Wiley-Interscience, 1983), pp. 80-81, 289]. It is possible to infer the optical properties of the suspended matter from the mass-specific spectral scattering cross sections. From these cross sections we partition the particulate spectral scattering coefficient into its major components.

Effects of Raman scattering across the visible spectrum in clear ocean water: a Monte Carlo study.

Raman-scattering activity in clear ocean waters is documented for the visible spectrum from Monte Carlo simulations. The Raman-scattering activity has a significant effect on the upwelling irradiance value in air and on the submarine light field at the water surface across the visible spectrum. A reduction in Raman-scattering activity at 440 nm that is due to Fraunhofer lines at the Raman source wavelengths is also demonstrated. At wavelengths greater than 500 nm, Raman scattering makes a significant contribution to the in-water light field at depth.

Raman scattering in ocean optics: quantitative assessment of internal radiant emission.

Raman-scattering activity in clear ocean waters is further documented from Monte Carlo simulations and optical data that are collected in the Sargasso Sea. A method is proposed, based on the anomalous absorption coefficient for a nonconservative irradiance field, to assess the percentile composition of internal radiant emission for the irradiance field at any depth.

Suspended minerogenic particle distributions in high‐energy coastal environments: Optical implications

[1] This paper examines suspended minerogenic particle distributions in the near-coastal ocean, Oceanside, California. The environment is dominated by resuspension of particles from a well-sorted sandy sediment. We obtain information on the suspended mineral matter from the relocatable numerical sedimentation model, TRANS98. Inputs for the model include bottom particle size distribution, wind speed, wave period and height, and near-bottom current speed. Model output is a vertical profile of mineral particle concentrations and size distributions at selected depths. The mineral particle size distribution allows the calculation of the total scattering coefficient of suspended minerogenic matter. The particle size distribution for the bed at Oceanside is approximately Gaussian lognormal in character. The predicted size distribution near bottom is a skewed Gaussian lognormal; then the distribution shifts toward the increasing importance of smaller size classes as we approach the water surface. Deeper stations maintain a lognormal character up into the surface layers while the shallower stations switch to a particle size distribution of the smallest size class being the modal class in the surface layers. Hyperbolic size distributions have been proposed for the Oceanside site from optical observations. These distributions would have a slope of 2.1 for coastal water which would decrease to as low as 1.3 for shallow nearshore stations and near the bottom. None of the proposed hyperbolic slopes provides an adequate approximation of the particle distributions predicted by TRANS98. The larger particle size classes are significantly overestimated by hyperbolic models, which causes the total scattering coefficient of the minerogenic matter to be overestimated. These results indicate that hyperbolic slope models cannot be used to retrieve particle size distributions by inversions involving the total scattering coefficient nor are they accurate estimators of the minerogenic scattering coefficient. INDEX TERMS: 4552 Oceanography: Physical: Ocean optics; 4546 Oceanography: Physical: Nearshore processes; 4863 Oceanography: Biological and Chemical: Sedimentation; 1640 Global Change: Remote sensing; KEYWORDS: particle size distributions, optical scattering, sediment resuspension

Mass-specific scattering cross sections of suspended sediments and aggregates: theoretical limits and applications

The spectral mass-specific scattering cross section σ[PIM](λ) is most important for the remote sensing inversion of the concentration of suspended mineral matter in the coastal ocean. This optical parameter is also important in optical theory and therefore the theoretical limits of this parameter are important. There are differing reports in the literature on the magnitude of σ[PIM](λ) and its spectral slope in different coastal ocean systems. To account for and predict these differences, I have applied a model of the size distribution of primary suspended mineral particles and aggregates of these particles to theoretical calculations of σ[PIM](λ). I utilized a model of mineral particle aggregates by Khelifa and Hill [Khelifa, A. and P.S. Hill, J. Hydraul. Res. 44, 390 (2006)] and Latimers optical model of aggregates [Latimer, P., Appl. Opt. 24, 3231, (1985)]. I have been able to account for the variations in magnitude and spectral slope of σ[PIM](λ). This analysis will apply to not only inverting the concentration of suspended mineral matter but also provides the basis for inverting the processes of coagulation and aggregation of primary mineral particles in determining sedimentation rates, budgets, etc.

Hydrodynamics and Marine Optics during Cold Fronts at Santa Rosa Island, Florida

Abstract Keen, T.R. and Stavn, R.H., 2012. Hydrodynamics and marine optics during cold fronts at Santa Rosa Island, Florida. Observations of optical and hydrodynamic processes were made on the open beach on Santa Rosa Island, Florida, in March 1995. This study focuses on the passage of two cold fronts. The observations have been supplemented by a bio-optical model; a suite of hydrodynamic models to simulate coastal flows forced by waves, tides, local wind, and coastal sea level; and a geo-optical model that predicts scattering by mineral particles resuspended by wave action. These models have been used to examine the interaction of atmospheric forcing and hydrodynamics with respect to the observed marine hydrosol. The optical and hydrodynamic measurements, and the model results, have been used to conceive a cold-front regime model of the hydrosol for open beaches in the Gulf of Mexico. The optical environment during the cold front was determined by three hydrosol phases: (1) a prefrontal steady-state hydrosol consisting of fine resuspended mineral particles, phytoplankton cells, organic detritus, and colored dissolved organic matter; (2) a frontal phase dominated by resuspended mineral particles; and (3) a postfrontal hydrosol containing large phytoplankton, detritus, and fine mineral particles. This concept is useful for identifying the physical processes responsible for observed optical properties. It should be applicable to other regions and types of events.

Determining Functional Relations in Multivariate Oceanographic Systems: Model II Multiple Linear Regression

AbstractA method for estimating multivariate functional relationships between sets of measured oceanographic, meteorological, and other field data is presented. Model II regression is well known for describing functional relationships between two variables. However, there is little accessible guidance for the researcher wishing to apply model II methods to a multivariate system consisting of three or more variables. This paper describes a straightforward method to extend model II regression to the case of three or more variables.The multiple model II procedure is applied to an analysis of the optical spectral scattering coefficient measured in the coastal ocean. The spectral scattering coefficient is regressed against both suspended mineral particle concentration and suspended organic particle concentration. The regression coefficients from this analysis provide adjusted estimates of the mineral particle scattering cross section and the organic particle scattering cross section. Greater accuracy and effic...