Grace Chang

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.

Initial results from the Bermuda Testbed Mooring program

The Bermuda Testbed Mooring (BTM) has been deployed since June 1994 and provides the oceanographic community with a deep-water platform for testing and intercomparing new instruments. The mooring is located about 80 km southeast of Bermuda. Surface instruments collect meteorological and spectral radiometric measurements from a buoy tower. Measurements at depth include: currents, temperature, conductivity, optical properties, and nitrate and trace element concentrations. Data have been sent to shore and to a nearby ship using a new inductive-link telemetry system. The high temporal resolution, long-term data collected from the mooring provide important information concerning episodic and periodic processes ranging in scale from minutes to years. For example, short nitrate pulses and associated biological events have been observed in the mooring data sets, which were not seen in the periodic ship-collected time-series data. Evaluation of undersampling and aliasing effects characteristic of infrequent sampling are also enabled with these data sets. The primary purposes of this report are to describe new systems and to illustrate early data resulting from the BTM program.

Toward closure of upwelling radiance in coastal waters

We present three methods for deriving water-leaving radiance L(w)(lambda) and remote-sensing reflectance using a hyperspectral tethered spectral radiometer buoy (HyperTSRB), profiled spectroradiometers, and Hydrolight simulations. Average agreement for 53 comparisons between HyperTSRB and spectroradiometric determinations of L(w)(lambda) was 26%, 13%, and 17% at blue, green, and red wavelengths, respectively. Comparisons of HyperTSRB (and spectroradiometric) L(w)(lambda) with Hydrolight simulations yielded percent differences of 17% (18%), 17% (18%), and 13% (20%) for blue, green, and red wavelengths, respectively. The differences can be accounted for by uncertainties in model assumptions and model input data (chlorophyll fluorescence quantum efficiency and the spectral chlorophyll-specific absorption coefficient for the red wavelengths, and scattering corrections for input ac-9 absorption data and volume scattering function measurements for blue wavelengths) as well as radiance measurement inaccuracies [largely differences in the depth of the L(u)(lambda, z) sensor on the HyperTSRB].

Sediment resuspension over a continental shelf during Hurricanes Edouard and Hortense

The present comprehensive physical and optical measurements have captured sediment resuspension associated with two hurricanes, Edouard and Hortense, that passed over the Coastal Mixing and Optics study site in the fall of 1996. Sediment resuspension associated with Hurricane Edouard was forced by combined current and wave processes. Combined current-wave bottom shear stresses exceeded 3.5 dyn cm−2, well above the shear stress for the observed resuspension, which was determined to be 0.8 dyn cm−2. Hortense sediment resuspension, however, was caused primarily by waves with little or no contribution from subtidal plus tidal currents; combined current-wave bottom shear stress reached 2 dyn cm−2. Beam attenuation data reveal that bottom sediments were resuspended from ∼70 m depth to more than 30 m up into the water column during both hurricanes. The relaxation of sediments to pre-Edouard conditions occurred at about the same time at all depths, indicating advection and transport by subtidal currents. Analyses of optical data reveal that the resuspended sediments consisted mainly of detritus and relict pigments from the ocean bottom.

Sediment resuspension in the wakes of Hurricanes Edouard and Hortense

A unique set of physical and optical observations of sediment resuspension was obtained during the passage of two hurricanes, Edouard and Hortense. The eyes of these hurricanes passed within approximately 110 and 350 km, respectively, of our study site on the continental shelf (100 km south of Cape Cod, Massachusetts) within a two-week period in the fall of 1996. Sediments were resuspended to more than 30 m above the ocean bottom during both hurricanes. The sediment resuspension processes associated with the two hurricanes are shown to differ primarily because of the separation distances between the eyes of the hurricanes and the observational site (e.g., local versus remote forcing). Observed particle-size distributions were shifted toward smaller scales during Hurricane Edouard because of flocculate disaggregation caused by high levels of localized shear and turbulence.

Variability of the downwelling diffuse attenuation coefficient with consideration of inelastic scattering

In situ time-series measurements of spectral diffuse downwelling irradiance from the Bermuda Testbed Mooring are presented. Averaged diffuse attenuation coefficients of downwelling irradiance, Kd,and their elastic and inelastic components are investigated at seven wavelengths. At shorter wavelengths (<510 nm), Kd is weakly dependent on the solar zenith angle owing to the prevailing scattering effect and therefore can be considered a quasi-inherent optical property. At longer wavelengths (>510 nm), Kd shows a strong dependence on the solar zenith angle. As depth increases, inelastic scattering plays a greater role for the underwater light field at red wavelengths.

Comparisons of optical properties of the coastal ocean derived from satellite ocean color and in situ measurements

Satellite-derived optical properties are compared to in situ mooring and ship-based measurements at a coastal site. Comparisons include remote sensing reflectance (R(rs)), chlorophyll concentration (Chl) using two different Chl algorithms, and spectral absorption [a(pg)(lambda)] and backscattering coefficients [b(b)(555)] using three different bio-optical algorithms. For mooring/shipboard comparisons, we observed mean relative errors of 70.5%/-3.8% (SeaWiFS OC4v4), -21.4%/-49.3% (SeaWiFS Stumpf), 109.5%/13.4% (MODIS OC3m) and 0.5%/-48.9% (MODIS Stumpf) for Chl. For satellite-derived and mooring comparisons of a(pg)(412), we found mean relative errors of -69.4% (-67.1%), -52.6% (- 48.9%), and -62.7% (-65.4%) for the Arnone, GSM, and QAA algorithms for SeaWiFS (MODIS), respectively. Mean relative errors of 21.3%, 19.9%, and 16.5% were found between SeaWiFS-derived (Arnone, GSM, and QAA algorithms, respectively) and moored b(b)(555) measurements. Discrepancies in Rrs at blue wavelengths are attributed to the satellite atmospheric correction and sea surface variations of the moored radiometers. High spatial and temporal variability of bio-optical properties coupled with differences in measurement techniques (pixel versus point) contribute to inconsistencies between remotely sensed and in situ biooptical properties.

Partitioning in situ total spectral absorption by use of moored spectral absorption–attenuation meters

High-temporal-resolution spectral absorption data were acquired by use of one bottom-mounted (approximately 68-m) and three moored spectral absorption and attenuation meters (ac-9 meters at 14, 37, and 52 m) on the Middle Atlantic Bight continental shelf during the fall 1996 period of the Coastal Mixing and Optics experiment. We employed a previously published spectral absorption model with the data to partition total absorption into absorption by water, phytoplankton, detritus, and gelbstoff (dissolved matter). We validated the model by comparing its results against coincident in vivo absorption coefficients derived from discrete bottle samples. Correlations between modeled and in vivo spectra range from 0.873 to 0.998. We optimized these correlations to determine the model parameters. These parameters could not be determined solely from the moored ac-9 results. Therefore a separate set of absorption measurements (from discrete bottle samples) was necessary to permit values for the model parameters to be determined. Model results allow us to separate particulate and dissolved components of absorption and to examine the temporal variability and the vertical distributions and concentrations of each component, given the total absorption in the water column.

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.