Davide D'Alimonte

An autonomous above-water system for the validation of ocean color radiance data

An operational system for autonomous above-water radiance measurements, called the SeaWiFS Photometer Revision for Incident Surface Measurements (SeaPRISM), was deployed at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea and used for the validation of remote sensing radiometric products in coastal waters. The SeaPRISM data were compared with simultaneous data collected from an independent in-water system for a wide variety of sun elevations along with different atmospheric, seawater, and sea state conditions. The average absolute differences between the above- and in-water determinations of water-leaving radiances (computed linearly) were less than 4.5% in the 412-555-nm spectral interval. A similar comparison for normalized water-leaving radiances showed average absolute differences less than 5.1%. The comparison between normalized water-leaving radiances computed from remote sensing and SeaPRISM matchup data, showed absolute spectral average (linear) differences of 17.0%, 22.1%, and 20.8% for SeaWiFS, MODIS, and MERIS, respectively. The results, in keeping with those produced by independent in-water systems, suggest the feasibility of operational coastal networks of autonomous above-water radiometers deployed on fixed platforms (towers, lighthouses, navigation aids, etc.) to support ocean color validation activities.

An Evaluation of Depth Resolution Requirements for Optical Profiling in Coastal Waters

Abstract Wave perturbations induce uncertainties in subsurface quantities determined from the extrapolation of optical measurements taken at different depths. An analysis of these uncertainties was made using data collected in the northern Adriatic Sea coastal waters over a wide range of environmental conditions with a profiling system having a 6-Hz acquisition rate, ∼0.1 m s−1 deployment speed, radiance sensors with 20° full angle field of view, and irradiance collectors of ∼1-cm diameter. The uncertainties were quantified as a function of the depth resolution of radiance and irradiance profiles through the percent differences between the subsurface values computed from full and reduced resolution profiles (the latter synthetically created by removing data from the former). The applied method made the analysis independent from instrument calibration; from perturbations induced by instrument self-shading, deployment structure, and bottom effects; and from environmental variability caused by seawater and i...

Autonomous Above-Water Radiance Measurements from an Offshore Platform: A Field Assessment Experiment

An autonomous system for making above-water radiance measurements has been produced by adding a new measurement scenario to a CIMEL CE-318 sun photometer. The new system, called the Sea-viewing Wide Fieldof-view Sensor (SeaWiFS) Photometer Revision for Incident Surface Measurement (SeaPRISM), combines the normal CE-318 capability for measuring direct sun irradiance and sky radiance, with a new capability for measuring above-water radiance for the retrieval of water-leaving radiance. The system has been extensively tested during several measurement periods over a 1-yr time frame from August 1999 to July 2000 under various sun elevations along with different atmospheric, seawater, and sea-state conditions. The field assessment of the new instrument was conducted at an oceanographic tower located in the northern Adriatic Sea within the framework of measurement campaigns aimed at supporting ocean color calibration and validation activities. Sample data at 440, 500, 670, 870, and 1020 nm were collected at azimuth and zenith angles satisfying the SeaWiFS Ocean Optics Protocols (and successive revisions) for above-water radiance measurements. Specifically, data were collected with azimuth angles of 908 with respect to the sun plane, and with nadir viewing angles of 308 ,4 08, and 458 for above-water measurements and of 1508, 1408, and 1358 for sky radiance measurements, respectively (the latter are needed for glint correction of the data). The intercomparison between water-leaving radiances computed from SeaPRISM measurements and those obtained from in-water optical profiles taken with the Wire-Stabilized Profiling Environmental Radiometer (WiSPER) system were performed using 113 coincident sets of measurements collected during clear-sky conditions. The SeaPRISM measurements taken at 408 and corrected for glint effects using different methods show the best agreement with WiSPER data. The intercomparisons exhibit average absolute unbiased percent differences, generally lower than 10% at 440 and 500 nm, and lower than 26% at 670 nm. The intercomparison of the water-leaving radiance ratio LW(440)/LW(500) from SeaPRISM data taken at 408 and WiSPER data exhibits average absolute unbiased percent differences lower than 5.6%.

Determination of CDOM and NPPM absorption coefficient spectra from coastal water remote sensing reflectance

Multilayer perceptron (MLP) neural network algorithms were developed to retrieve the absorption coefficient spectra of the colored dissolved organic matter (CDOM) and nonpigmented particulate matter (NPPM) from the remote sensing reflectance R/sub rs/ of optically complex waters. The two MLP algorithms, consisting of one hidden layer with ten neurons and requiring R/sub rs/ at 412, 490, and 665 nm as inputs, were trained with a comprehensive experimental dataset of the Northern Adriatic Sea coastal waters. The products of the proposed regional MLP algorithms showed higher accuracies than regional band-ratio algorithms, and exhibited average uncertainties of 20% and 25% in the determination of CDOM and NPPM absorption coefficients at 412 nm, respectively.

Regional Algorithms for European Seas: A Case Study Based on MERIS Data

Advances in satellite ocean color technologies and methodologies are expected to lead to the generation of coastal water bio-optical products with accuracies close to those targeted for oceanic regions. In view of contributing to such a progress, multilayer perceptron neural networks complying with standard Medium Resolution Imaging Spectrometer (MERIS) pigment indices were developed relying on regional highly accurate in situ data. This work illustrates and discusses the application to sample MERIS imagery of those neural networks trained to produce pigment indices in seas characterized by increased levels of bio-optical complexity: the Baltic, the Northern Adriatic, and the Western Black Seas.

Match-Up Analysis of MERIS Radiometric Data in the Northern Adriatic Sea

This letter discusses the normalized water-leaving reflectance obtained from two atmospheric corrections built in the MEGS 8.0 processor applied to the Medium Resolution Imaging Spectrometer (MERIS) marine data. In situ reference data for intercomparisons with satellite-derived reflectance products are from the Ocean Color component of the Aerosol Robotic Network (AERONET-OC). Case of study is the northern Adriatic Sea AERONET-OC site characterized by both Case-1 and Case-2 waters. The accuracy of MERIS pigment indices is also discussed in relation to uncertainties and biases affecting atmospherically corrected data.

Ocean colour validation activities at the Acqua Alta oceanographic Tower in the northern Adriatic Sea

The development and validation of ocean colour products require the availability of in situ atmospheric and marine data. Since 1995, the Coastal Atmosphere and Sea Time Series (CoASTS) Program ensures the collection of an in situ comprehensive set of measurements taken from an oceanographic tower in the northern Adriatic Sea. An example of a validation exercise is presented through the comparison of in situ and satellite-derived aerosol optical thickness at various wavelengths in the 443–865 nm spectral interval.

Statistical assessment of radiometric measurements from autonomous systems

In situ autonomous systems are commonly used for the collection of measurements for the vicarious calibration of satellite data and the successive validation of derived products. However, the use of autonomous systems creates the need of assessing the quality of the large volume of collected data. Within the framework of ocean color activities, this work investigates the consistency of normalized water leaving radiances spectra produced from measurements taken with an above-water autonomous system installed on an oceanographic tower. The study has shown the need of addressing the problem under two different levels of inference. The first level, so-called self-consistency, has demonstrated the capability of identifying spectra with a low statististical representativeness within the dataset itself. The second level, so-called relative-consistency, has provided the possibility of evaluating whether a spectrum is relatively consistent to a reference set of quality-assured data.