Scott McLean

An Evaluation of Above- and In-Water Methods for Determining Water-Leaving Radiances

Abstract A high-quality dataset collected at an oceanographic tower was used to compare water-leaving radiances derived from simultaneous above- and in-water optical measurements. The former involved two different above-water systems and four different surface glint correction methods, while the latter used three different in-water sampling systems and three different methods (one system made measurements a fixed distance from the tower, 7.5 m; another at variable distances up to 29 m away; and the third was a buoy sited 50 m away). Instruments with a common calibration history were used, and to separate differences in methods from changes in instrument performance, the stability (at the 1% level) and intercalibration of the instruments (at the 2%–3% level) was performed in the field with a second generation Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM-II). The water-leaving radiances estimated from the methods were compared to establish their performance during the field campaign,...

Spectra of Particulate Backscattering in Natural Waters

Hyperspectral profiles of downwelling irradiance and upwelling radiance in natural waters (oligotrophic and mesotrophic) are combined with inverse radiative transfer to obtain high resolution spectra of the absorption coefficient (a) and the backscattering coefficient (b(b)) of the water and its constituents. The absorption coefficient at the mesotrophic station clearly shows spectral absorption features attributable to several phytoplankton pigments (Chlorophyll a, b, c, and Carotenoids). The backscattering shows only weak spectral features and can be well represented by a power-law variation with wavelength (lambda): b(b) approximately lambda(-n), where n is a constant between 0.4 and 1.0. However, the weak spectral features in b(b)b suggest that it is depressed in spectral regions of strong particle absorption. The applicability of the present inverse radiative transfer algorithm, which omits the influence of Raman scattering, is limited to lambda < 490 nm in oligotrophic waters and lambda < 575 nm in mesotrophic waters.

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.

An Example Crossover Experiment for Testing New Vicarious Calibration Techniques for Satellite Ocean Color Radiometry

Abstract Vicarious calibration of ocean color satellites involves the use of accurate surface measurements of water-leaving radiance to update and improve the system calibration of ocean color satellite sensors. An experiment was performed to compare a free-fall technique with the established Marine Optical Buoy (MOBY) measurement. It was found in the laboratory that the radiance and irradiance instruments compared well within their estimated uncertainties for various spectral sources. The spectrally averaged differences between the National Institute of Standards and Technology (NIST) values for the sources and the instruments were <2.5% for the radiance sensors and <1.5% for the irradiance sensors. In the field, the sensors measuring the above-surface downwelling irradiance performed nearly as well as they had in the laboratory, with an average difference of <2%. While the water-leaving radiance Lw calculated from each instrument agreed in almost all cases within the combined instrument uncertainties (a...

Comparison of spectral radiance calibrations at oceanographic and atmospheric research laboratories

This report describes the first Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Radiometric Intercomparison (SIMRIC-1). The purpose of the SIMRIC-1 is to ensure a common radiometric scale among the calibration facilities that are engaged in calibrating in situ radiometers used for ocean colour-related research and to document the calibration procedures and protocols. SIMBIOS staff visited the seven participating laboratories for at least two days each. The SeaWiFS Transfer Radiometer (SXR-II) measured the calibration radiances produced in the laboratories. The measured radiances were compared with the radiances expected by the laboratories. Typically, the measured radiances were higher than the expected radiances by 0 to 2%. This level of agreement is satisfactory. Several issues were identified where the calibration protocols need to be improved, especially the reflectance calibration of the reference plaques and the distance correction when using the irradiance standards at distances greater than 50?cm. The responsivity of the SXR-II changed from 0.3% (channel 6) to 1.6% (channel 2) from December 2000 to December 2001. Monitoring the SXR-II with a portable light source showed a linear drift of the calibration, except for channel 1, where a 2% drop occurred in summer.

A new instrument for measuring the high dynamic range radiance distribution in near-surface sea water

A new instrument for measuring the full radiance distribution in the ocean interior is introduced. The system is based on CMOS technology to achieve intra-scene dynamic range of 6 decades and system dynamic range of more than 9 decades. The spatial resolution is nominally 0.5 degrees with a temporal frame rate between 1 and 15 frames per second. The general instrumentation, detailed calibration, and a characterization of the system are described. Validity of the camera systems is demonstrated by comparison of the radiance measurements with other classical oceanographic radiometers.

Monitoring the Spring Bloom in an Ice Covered Fjord with the Land/Ocean Biogeochemical Observatory (LOBO)

The Land/Ocean Biogeochemical Observatory (LOBO) system was initially developed by MBARI under the NSF Biocomplexity program to monitor the land/ocean interface for coastal zone management. The system utilizes highly robust and accurate sensors to provide sustained monitoring of critical watershed habitats in challenging environmental conditions in real time. With the incorporation of novel anti-fouling technology developed by WET Labs, the system has an unprecedented six week maintenance cycle, greatly reducing operational costs while providing high quality data sets. LOBO measurements include nitrate, dissolved oxygen, conductivity, temperature, chlorophyll fluorescence, turbidity and CDOM. A LOBO was deployed in the Northwest Arm of Halifax Harbour in January of 2007 and has been monitoring the biogeochemical parameters continuously since (see http://lobo.satlantic.com ). Despite heavy icing at -20degC, heavy winds, and sea ice the system reported data every hour where the detailed nature of the annual spring phytoplankton bloom was recorded at high resolution. The coincident and continuous record of nutrients, phytoplankton and other physical and chemical parameters is unique, and provides a robust means to base predictive coastal ecosystem models in sensitive marine areas.

Field evaluation of antibiofouling compounds on optical instrumentation

Biofouling has been a serious question in the stability of optical measurements in the ocean, particularly in moored and drifting buoy applications. Many investigators coat optical surfaces with various compounds to reduce the amount of fouling; to our knowledge, however, there are no objective, in-situ comparative testing of these compounds to evaluate their effectiveness with respect to optical stability relative to untreated controls. We have tested a wide range of compounds at in-situ locations in Halifax Harbor and in the Adriatic Sea on passive optical sensors. Compounds tested include a variety of TBT formulations, antifungal agents. and low-friction silicone-based compounds; time-scales of up to four months were evaluated. The results of these experiments are discussed.

Closure and uncertainty assessment for ocean color reflectance using measured volume scattering functions and reflective tube absorption coefficients with novel correction for scattering

Optical closure is assessed between measured and simulated remote-sensing reflectance (Rrs) using Hydrolight radiative transfer code for five data sets that included a broad range of both Case I and Case II water types. Model-input inherent optical properties (IOPs) were the absorption coefficient determined with a WET Labs ac9 and the volume scattering function (VSF) determined with a custom in situ device called MASCOT. Optimal matchups were observed using measured phase functions and reflective tube absorption measurements corrected using a scattering error independently derived from VSF measurements. Absolute bias (δ) for simulations compared to measured Rrs was 20% for the entire data set, and 17% if a relatively shallow station with optical patchiness was removed from the analysis. Approximately half of this δ is estimated to come from uncertainty in radiometric measurements of Rrs, with the other half arising from combined uncertainties in IOPs, radiative transfer modeling, and related assumptions. For exercises where such δ can be tolerated, IOPs have the potential to aid in ocean color validation. Overall, δ was roughly consistent with the sum of uncertainties derived from associated measurements, although larger deviations were observed in several cases. Applying Fournier–Forand phase functions derived from particulate backscattering ratios according to Mobley et al. [Appl. Opt.41, 1035 (2002)APOPAI0003-693510.1364/AO.41.001035] resulted in overall δ that was almost as good (23%) as simulations using measured phase functions. Possibilities for improving closure assessments in future studies are discussed.

The Marine Environmental Prediction System (MEPS) - a new generation of moored ocean observing systems

Dalhousie Universitys Marine Environmental Prediction System (MEPS), designed in 2001, has been operated for three years in support of real-time observation, prediction and visualization of ocean properties and processes. The observatory consists of a network of three moored buoys connected to an unmanned shore station using 802.11b wireless LAN technology. The system is controlled, configured and maintained remotely, over the Internet, using the novel DACNet ocean observatory operating system. Each mooring has a full suite of meteorological, oceanographic, acoustical and optical sensors, which collect and provide data in near-to-real-time to an access and visualization system (www.cmep.ca/bav). Currently 20 different sensors are on each platform, with the ability to add virtually any type of oceanographic sensor on spare guest ports. An industry standard PC104 computer is used as the primary controller on each buoy, allowing both Ethernet and a scalable number of high-speed serial inputs, creating a highly flexible acquisition node. The system configuration was designed to meet diverse science team requirements including sensors on the main buoy platform, sensor chains, bottom-mounted sensors and a remote surface tracking optical package. Communications and power are provided from the main buoy, with the option to provide shore-based cabled power. A novel serial network protocol called SatNet was deployed on each mooring to allow groups of standard oceanographic sensors to be connected on the same power and communications bus, simplifying wiring and consolidating sensors on a single interface port. SatNet Interface Modules (SIMs) allow devices without built in SatNet protocols and metadata to communicate on the network. This paper describes the system design, configuration and deployment of the MEPS