AERONET-OC “Helsinki Lighthouse” site in the Gulf of Finland. The site is characterized by optically complex waters dominated by colored dissolved organic matter. AERONET-OC is the Ocean Color component of the Aerosol Robotic Network (AERONET, Holben et al. 1998), a federated instrument network and data archive, managed by the Goddard Space Flight Center (GSFC) of the U.S. National Aeronautics and Space Administration (NASA) and specifically conceived to support aerosol investigations through standardized instruments and methods (Holben et al. 2001). Similar to AERONET, AERONET-OC (Zibordi et al. 2009) relies on NASA’s commitment for field instruments calibration, data processing and archiving. These activities are complemented by independent actions focused on establishing and maintaining CE-318 modified sun-photometers at coastal sites of interest for individual investigators or research institutions. These modified sun-photometers, called SeaWiFS Photometer Revision for Incident Surface Measurements (SeaPRISM), have the capability of performing autonomous above-water radiometric measurements in addition to usual atmospheric measurements (Zibordi et al. 2004). Key features of AERONET-OC are: i. near-real time data collection and processing (i.e., within a few hours); ii. use of standardized instruments, calibration procedure and data processing; iii. open access to measurements and products through a specified data policy. The primary data product of AERONET-OC is Lwn at wavelengths suitable for satellite ocean color applications. An additional product is the aerosol optical thickness, complemented by phase function, particle size distribution and single scattering albedo of aerosols, all having potential importance to assess the performance of the atmospheric correction process applied to satellite data. AERONET-OC sites are located in coastal regions by taking advantage of available and accessible offshore grounded structures. Deployment requirements for the collection of measurements suitable for ocean color validation activities are summarized as follows: i. fixed deployment platforms allowing for measurement of the direct sun irradiance through accurate sun-tracking; ii. superstructures with height and shape minimizing contamination of the measuring system by sea-spray; and iii. deployment positions allowing unobstructed sea observations at the maximum possible distance from the superstructure at the time of satellite overpass. Recalling that the minimization of superstructure perturbations in above-water radiometric measurements requires observations of the sea surface at distances at least equal to the superstructure height, the measurement systems are generally deployed from dedicated extensions of the main structure.
- Giuseppe Zibordi
How to cite
Zibordi, Giuseppe (2014): Normalized Water-Leaving Radiance determined from above-water radiometry - Helsinki_Lighthouse (59N,24E). European Commission, Joint Research Centre (JRC) [Dataset] PID: http://data.europa.eu/89h/36369279-dc2a-42a4-94e1-f23f30eb3652
The Aerosol Robotic Network (AERONET), developed by the National Aeronautics and Space Administration (NASA) to sustain atmospheric studies at various scales with measurements from worldwide distributed autonomous sun-photometers has been extended to support marine applications. This new network component called AERONET – Ocean Color (AERONET-OC), provides the additional capability of measuring the radiance emerging from the sea (i.e., water-leaving radiance) with modified sun-photometers installed on offshore platforms like lighthouses, oceanographic and oil towers. AERONET-OC is instrumental in satellite ocean color validation activities through standardized measurements a) performed at different sites with a single measuring system and protocol, b) calibrated with an identical reference source and method, and c) processed with the same code.
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System description: Recent developments in above water radiometry (Mobley 1999, Hooker et al. 2002, Zibordi et al. 2002) led to the development of a fully autonomous above-water radiometer system (Zibordi et al. 2004). This is based on the extended capability of CIMEL (Paris, France) CE-318 automated sun-photometers to perform marine radiometric measurements for determining the spectral Normalized Water-Leaving Radiance, Lwn(λ), in addition to the regular measurements for retrieving aerosol optical properties. This CIMEL-based system, called SeaWiFS Photometer Revision for Incident Surface Measurements (SeaPRISM), performs multiple sky- and sea-radiance measurements at programmable viewing and azimuth angles at eight (nine in the most recent instrument release) center-wavelengths in the 412-1020 nm spectral range.The most recent SeaPRISM system configuration performs ocean color measurements at the 412, 443, 488, 531, 551 and 667 nm center-wavelengths. Additional measurements are performed at 870 and 1020 nm for quality checks, turbid water flagging, and for the application of alternative above-water methods (Zibordi et al. 2002). These center-wavelengths and additionally that at 940 nm were selected to guarantee basic AERONET atmospheric aerosol and water vapor monitoring capabilities and to support essential validation activities for current satellite ocean color systems. Quality Levels: Normalized water-leaving radiances (Lwn) are provided in three levels: Level 1.0, 1.5, and 2.0. Level 1.0 data include Lwn determined from sequences of sea measurements taken with viewing geometries minimizing the platform perturbations. Level 1.5 data include screened Lwn corresponding to Level 1.0 products from measurement sequences not affected by a) cloud perturbations as determined from direct sun irradiance measurements, b) high variability in sea observations indicating elevated wave perturbations, or c) high Lwn values in the near infrared suggesting the presence of obstacles in the optical path between the instrument and the water surface. Level 2.0 data refer to fully quality-assured Lwn, corresponding to a) Level 1.5 products originated from SeaPRISMs exhibiting differences smaller than 5% between the calibration coefficients determined before and after typical one-year deployment periods, b) coincident aerosol optical depth of Level 2.0, and c) spectral consistency of Lwn data with respect to reference data.