Andrew H. Barnard

A model for estimating bulk refractive index from the optical backscattering ratio and the implications for understanding particle composition in case I and case II waters

A model based on Mie theory is described that estimates bulk participate refractive index n¯p from in situ optical measurements alone. Bulk refractive index is described in terms of the backscattering ratio and the hyperbolic slope of the particle size distribution (PSD). The PSD slope ξ is estimated from the hyperbolic slope of the particulate attenuation spectrum γ according to the relationship γ ≈ ξ − 3, verified with Mie theory. Thus the required in situ measurements are the particulate backscattering coefficient, the total particulate scattering coefficient, and the particulate attenuation coefficient. These parameters can be measured with commercially available instrumentation with rapid sampling rates and real-time data return. Application of the model to data from the Gulf of California yielded results that agreed with expectations, e.g., predicted n¯p was 1.04–1.05 in the chlorophyll maximum and 1.14–1.18 near sediments. Below the chlorophyll maximum in case I type waters, predicted n¯p values were between 1.10 and 1.12, suggesting the presence of a significant inorganic mineral component in the background or detrital organic particles with low water content.

Hyperspectral temperature and salt dependencies of absorption by water and heavy water in the 400-750 nm spectral range

The temperature and salt dependencies of absorption by liquid water (H2O) and heavy water (D2O) were determined using a hyperspectral absorption and attenuation meter (WET Labs, AC-S). Sodium chloride (NaCl) was used as a proxy for seawater salts. There was no significant temperature (PsiT) or salt (PsiS) dependency of absorption at wavelengths <550 nm. At wavelengths >550 nm, PsiT exhibited peaks at approximately 604, 662, and 740 nm. A small negative trough in PsiS occurred at approximately 590 nm, followed by a small positive peak approximately 620 nm, a larger negative trough at approximately 720 nm, and a strong positive peak at approximately 755 nm. The salt dependency of absorption by heavy water, Psis(H), exhibited a negative power-law shape with very low Psis(H), at wavelengths >550 nm. Our experiments with NaCl, clean open ocean seawater, and artificial seawater support the hypothesis that salts modify the absorption spectra of seawater by modifying the molecular matrix and vibrations of pure water.

Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf

Spectral attenuation and absorption coefficients of particulate matter and colocated hydrographic measurements were obtained in the Mid-Atlantic Bight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixing and Optics experiment. Within the bottom boundary layer (BBL) the magnitude of the beam attenuation decreased and its spectral shape became steeper with distance from the bottom. Concurrently, the slope of the particulate size distribution (PSD) was found to increase with distance from the bottom. Changes in the PSD shape and the magnitude of the beam attenuation as functions of distance from the bottom in the BBL are consistent with particle resuspension and settling in the BBL, two processes that are dependent on particle size and density. For particles of similar density, resuspension and settling would result in a flattening of the PSD and an increase in the beam attenuation toward the bottom. In both fall and spring the magnitude of the particle attenuation coefficient correlates with its spectral shape, with a flatter shape associated with higher values of the attenuation. This observation is consistent with idealized optical theory for polydispersed nonabsorbing spheres. According to this theory, changes in the steepness of the particle size distribution (particle concentration as a function of size) will be associated with changes in the steepness of the attenuation spectra as a function of wavelength; a flatter particle size distribution will be associated with a flatter attenuation spectrum. In addition, the observed ranges of the beam attenuation spectral slope and the PSD exponent are found to be consistent with this theory.

Spatial and temporal variability of absorption by dissolved material at a continental shelf

Optical properties of dissolved (colored dissolved organic material (CDOM)) and particulate matter and hydrographic measurements were obtained at the Mid-Atlantic Bight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixing and Optics experiment. To assess the temporal and spatial variability, time series were obtained at one location and cross-shelf transects were carried out. On short timescales, variability in the vertical distribution of the dissolved fraction was mostly due to high- frequency internal waves. This variability was conservative, resulting in no changes on isopycnals. Over longer periods and episodically, CDOM variability was dominated by storms. The storms were associated with sediment resuspension events and were accompanied by an increase in the absorption by the dissolved materials. Data from spatial transects show that near the bottom, over the shelf, and in both spring and fall, increased particulate absorption and increased CDOM absorption co-occur. These data support the hypothesis that bottom sediments can act as a source of dissolved organic carbon during sediment resuspension events.

Influence of surface waves on measured and modeled irradiance profiles

Classical radiative transfer programs are based on the plane-parallel assumption. We show that the Gershun equation is valid if the irradiance is averaged over a sufficiently large area. We show that the equation is invalid for horizontal areas of the order of tens of meters in which horizontal gradients of irradiance in the presence of waves are much larger than vertical gradients. We calculate the distribution of irradiance beneath modeled two-dimensional surface waves. We show that many of the features typically observed in irradiance profiles can be explained by use of such models. We derive a method for determination of the diffuse attenuation coefficient that is based on the upward integration of the irradiance field beneath waves, starting at a depth at which the irradiance profile is affected only weakly by waves.

In situ determination of the remotely sensed reflectance and the absorption coefficient: closure and inversion

We tested closure between in situ radiometric and absorption coefficient measurements by using a nearly backscattering-independent remote-sensing reflectance model that employs the remote-sensing reflectance at three wavelengths. We show that only a small error is introduced into the closure model when the proper functional relationships of f/Q and the backscattering is taken to be a constant when using the sea-viewing wide field-of-view sensor wavelengths 443, 490, and 555 nm. A method of inverting the model to obtain the absorption coefficient by use of simple linear spectral relationships of the absorption coefficient is provided. The results of the model show that the independent measurements of reflectance and absorption obtain closure with a high degree of accuracy.

In-Water Instrumentation and Platforms forOcean Color Remote Sensing Applications

Remote sensing of reflected sunlight from the upper ocean is a tremendous tool for studying biological, chemical, geological, and physical processes over a broad range of time and space scales. Global biogeochemical phenomena spanning seasonal (e.g., spring bloom), multi-year (e.g., the El Nino Southern Oscillation), to decadal (e.g., climatic variability) time scales can be resolved by an orbiting satellite imager. Reflected light in the visible domain (wavelengths of ~400 to 700 nm) is particularly useful in the study of upper ocean processes, as many important biogeochemical components of seawater absorb and scatter light effectively in this spectral range (the term “ocean color” specifically relates to the spectral character of this water-leaving visible light). These dissolved and particulate seawater components play key roles in the cycling of carbon in the ocean and serve as indicators of ecosystem health. Down-looking, passive remote sensors in air and space measure sunlight that is reflected upward into the sensor; in addition to the atmospherically scattered photons, a portion of the measured radiance results from photons that have exited the ocean and passed back through the atmosphere to the sensor in orbit. This portion is termed spectral upwelled water-leaving radiance, Lu (W m nm sr) and primarily consists of light scattered in the backward direction off the particles and molecules of seawater (for a complete discussion, refer to Chapter 1). Sunlight incident at the ocean surface is represented as spectral downwelling surface irradiance, Ed (W m nm), and the socalled remote sensing reflectance, Rrs, is derived from Lu/Ed, with Lu strictly defined in the nadir direction (normal to the plane of the ocean surface). Although Lu consists of primarily backscattered light, equally important in terms of its information content is the component of incident sunlight missing in the upwelled light. This is light that has been absorbed (or filtered) by the constituents of seawater in the upper ocean. The dependence of Rrs on these optical processes of backscattering and absorption just below the ocean surface (represented as 0) can be simply written (Morel and Prieur 1977):

In situ optical variability and relationships in the Santa Barbara Channel: implications for remote sensing.

Relationships and variability of bio-optical properties in coastal waters are investigated. Optical proxies indicate that these coastal waters are optically complex and highly variable and are categorized as follows: (1) relatively clear and dominated by high index of refraction, biogenic particles, (2) more turbid, consisting of mostly inorganic particles and little phytoplankton, (3) extremely turbid with high concentrations of inorganic particles, and (4) more turbid and dominated by biogenic particles. We present a method, alternative to traditional remote-sensing algorithms, of classifying coastal waters [the Spectral Angle Mapper (SAM)] and utilize the SAM to successfully isolate plume conditions in time series of downwelling irradiance and total absorption coefficient. We conclude with a discussion of the use of the SAM for coastal management operations.

Optical closure in a complex coastal environment: particle effects

An optical dataset was collected on a mooring in the Santa Barbara Channel. Radiative transfer modeling and statistical analyses were employed to investigate sources of variability of in situ remote sensing reflectance [r(rs)(lambda,4 m)] and the f/Q ratio. It was found that the variability of inherent optical properties and the slope of the particle size distribution (xi) were strongly related to the variability of r(rs)(lambda,4 m). The variability of f/Q was strongly affected by particle type characteristics. A semianalytical radiative transfer model was applied and effects of variable particle characteristics on optical closure were evaluated. Closure was best achieved in waters composed of a mixture of biogenic and minerogenic particles.

Aquatic laser fluorescence analyzer: field evaluation in the northern Gulf of Mexico.

The new Aquatic Laser Fluorescence Analyzer (ALFA) provides spectral and temporal measurements of laser-stimulated emission (LSE) for assessment of phytoplankton pigments, community structure, photochemical efficiency (PY), and chromophoric dissolved organic matter (CDOM). The instrument was deployed in the Northern Gulf of Mexico to evaluate the ALFA analytical capabilities across the estuarine-marine gradient. The robust relationships between the pigment fluorescence and independent pigment measurements were used to validate the ALFA analytical algorithms and calibrate the instrument. The maximal PY magnitudes, PYm = PY(1-1.35·10⁻⁴PAR⁻¹, were estimated using the underway measurements of PY and photosynthetically active radiation (PAR). The chlorophyll (Chl) spatial patterns were calculated using the ratio of Chl fluorescence to PY to eliminate the effect of non-photochemical quenching on the underway Chl assessments. These measurements have provided rich information about spatial distributions of Chl, PYm, CDOM, and phytoplankton community structure, and demonstrated the utility of the ALFA instrument for oceanographic studies and bio-environmental surveys. The data suggest that the fluorescence measurements with 514 nm excitation can provide informative data for characterization of the CDOM-rich fresh, estuarine, and coastal aquatic environments.