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Merging Water-Leaving Radiances: An Optically-Based Technique for the Mediterranean Sea

This study presents an innovative optically-based technique for merging water-leaving radiances (LWN) from various independent platforms to create a consistent time series with optimal coverage in the Mediterranean Basin. Validation is achieved through inter-comparison of sensor-specific products, ensuring enhanced reliability and accuracy. The approach combines spectral data and employs a two-step procedure for optimal output. The benefits of merging LWN from different sensors are highlighted, showing improved spatial coverage and quality in radiometric data, vital for marine and environmental monitoring.

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Merging Water-Leaving Radiances: An Optically-Based Technique for the Mediterranean Sea

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  1. An Optically-Based Technique for Producing Merged Water-Leaving Radiances; Validation and Application for the Mediterranean Basin Frédéric Mélin , Giuseppe Zibordi Global Environment Monitoring Unit, E.C. - Joint Research Centre Motivation:Combining the data records of LWN from independent platforms to create a single consistent time series with optimal coverage Pre-requisite:Inter-comparison of sensor-specific products (Djavidnia et al., MERSEA report, 2006) Principle of the Technique & Field Data Validation of Radiometric Products (LWN) & Merger Validation Test with a 3rd Sensor (MERIS) Benefits of Merging & Time Series for the Mediterranean http://marine.jrc.ec.europa.eu

  2. Principle of the Technique [1] 1. model inversion MERGER Chla bbp(550) ads+dt(440) LWN,1(λi1) LWN,2(λi2) … LWN,m(λ) arbitrary wavelength bio-optical model parameters selection a*ph , S, η 2. forward mode Schematic of the merger: A two-step procedure - combination of all available spectral information - selection of output wavelengths - the merged product is a primary radiometric quantity Mélin & Zibordi, submitted IOCCG Report #6, Chap.4

  3. Principle of the Technique [2] Bio-optical model: (inversion by non-linear LM inversion technique) variables of the model Back-scattering: parameters of the model Absorption: Sensitivity Analysis: Dependence of the merged outputs on the bio-optical parameter set: < 4% (tested for various combinations of η, S, a*ph) Mélin & Zibordi, submitted

  4. CE-318 (sea-viewing) CE-318(sky-viewing) Optical Field Measurements: Acqua Alta Oceanographic Tower Above-Water Radiometry AERONET-OC (May 2002) SeaPRISM Lwn at 412, 440, 500, 555, 675 nm AERONET-OC Zibordi et al., EOS, 2006 AAOT AERONET (Jul. 1996 - now) Mélin et al., IEEE, 2003 Zibordi et al., IJRS, 2004 Mélin & Zibordi, GRL, 2005 Mélin et al., JGR, 2006 Mélin et al., RSE, accepted Operational validation of OC radiometric products [SeaWiFS, MODIS-T, -A, MERIS, GLI] Zibordi et al., IEEE, 2004, 2006 Zibordi et al., GRL 2006, EOS 2006, ECSS 2006

  5. Validation of SeaWiFS Radiometric Products SeaWiFS vs. SeaPRISM match-ups SeaWiFS SeaPRISM Zibordi, Mélin, Berthon, GRL, 2006

  6. Validation of MODIS Radiometric Products MODIS vs. SeaPRISM match-ups MODIS-A SeaPRISM significant match-up dataset for both sensors 173 of these SeaWiFS and MODIS match-ups are on the same day, and can be the basis for validating the merged product Zibordi, Mélin, Berthon, GRL, 2006

  7. Merger Validation MERGED vs. SeaPRISM match-ups SeaPRISM MERGED SeaWiFS only MODIS only SeaWiFS + MODIS (N=173) Mélin & Zibordi, submitted

  8. Test with 3rd Sensor: MERIS[1] SeaPRISM MERIS Overestimate of LWN in the blue ≠ SeaWiFS & MODIS Mélin & Zibordi, submitted

  9. Test with 3rd Sensor: MERIS[2] SeaPRISM MERGED MERIS+MODIS+SeaWiFS NB: Compensation of Overestimate and Underestimate of LWN in the blue

  10. Benefits of Merging [1] Time Series at AAOT site N=473 over ~ 4 years SeaWiFS only (110) SeaWiFS (363) SeaWiFS + MODIS (213) MODIS (323) MODIS only (150) Mélin & Zibordi, submitted

  11. Benefits of Merging [2] Daily spatial coverage - 19th Jul. 2003 MODIS - 551 nm MODIS - 412 nm SeaWiFS - 412 nm SeaWiFS - 555 nm MERGED - 555 nm MERGED - 412 nm

  12. Benefits of Merging [3] Daily coverage over the Mediterranean Sea for 2003 (2-km gridded products) 36% SeaWiFS 22% MODIS 42% Merged Nb. Days – 2003 Mélin, Zibordi, Djavidnia, submitted

  13. Time Series of Differences 2|SWF-MOD|/(SWF+MOD) 2(SWF-MOD)/(SWF+MOD) RMSD(SWF-MOD) Mélin, Zibordi, Djavidnia, submitted

  14. The Merged Series Mélin, Zibordi, Djavidnia, submitted

  15. CONCLUSIONS Presentation and validation of a merging technique for the production of a LWN multi-sensor record Performance at least as good as for the sensor-specific products Merges in a consistent way the various sensor-specific LWN spectra, taking full advantage of the available spectral bands Allows the subsequent applications of any bio-optical algorithm Application on the Mediterranean Sea with time series of differences and merged products Still a lot to learn from the inter-comparison of the sensor-specific products

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