Fabrizio D’Ortenzio, David Antoine, Guislain Bécu

1. A Multi Sensor (MERIS, SeaWiFS, MODIS-A) Ocean Color Satellite Matchup Analysis in the Mediterranean Sea BOUSSOLE Project Fabrizio D’Ortenzio, David Antoine, Guislain Bécu Laboratoire d’Océanographie de Villefranche, CNRS et Université Pierre et Marie Curie Stanford B. Hooker NASA/GSFC, Greenbelt, MD 20771 USA

2. Overview of the presentation 1. Some elements about the BOUSSOLE project 2. Matchup Procedure and evaluation of the results 3. Conclusions

3. Description of the BOUSSOLE Project Motivations: Establish a time series of inherent and apparent optical properties (IOPs and AOPs). Strategy: • Combination of 3 elements : • A deep-sea mooring, collecting data on a “continuous” basis • Monthly cruises for collecting data that are not accessible to the mooring (vertical profiles, water sampling) • A coastal AERONET station, providing the necessary information about the aerosol properties

4. Characteristics of the buoy site: physical dynamics BOUSSOLE Located in the middle of the Cyclonic Circulation of the NW Med The physical dynamic is prevalently vertical i.e. 1-dimensional Surface currents are generally weak Millot, 1999 Mixed-layer depth varies from ~10 to ~200 meters Marty et al. 2002

5. Characteristics of the buoy site: biological dynamics Biomass concentration is generally low. Intense spring bloom is however observed. Chlorophyll concentration varies from 0.05 to 5 mg/m3 SeaWiFS Chlorophyll Concentration BOUSSOLE 13 April 1999 28 May1998

6. Why a new mooring type? Irradiance profilers provide highly noisy data near the surface because of the effect of capillary and gravity waves  the horizontal gradient in E or L is often an order-of-magnitude larger than the vertical gradient (see, e.g., Zaneveld et al., Appl. Opt., 40(9), 2001). Depth Irradiance 490 The data near the surface are often unusable whereas they are the ones that should be used when satellite validation is concerned.

7. Why a new mooring type? The solution we have adopted: Integration over time with a sensor maintained at a fixed position and depth • The new type of buoy ensures: • the verticality of the instruments, • the stability of the instruments, • the minimisation of shading of the instruments by the buoy structure. See Poster #204 of Bécu et al. for a complete description of the BOUSSOLE programme.

8. Measurement suite, instrumentation +4 meters At 9 meters At 4 and 9 meters Wetlabs C-star 2 ls, Hobilabs’ Hydroscat Chelsea MiniTracka SeaBird’ SBE37 7 ls Satlantic’ OCR-OCI 200 Series Surface irradiance (Es) Downwelling irradiance (Ed) Upwelling irradiance (Eu) Upwelling radiance at nadir (Lu) Attenuation coefficient Backscattering coefficient Chlorophyll fluorescence Temperature, Pressure Salinity Buoy tilt Fully normalized reflectances w (defined as p nLw/Es ) Water leaving radiances Lwn -20 meters

9. nLw Time Series nLw670 All data collected every 15 minutes during daylight nLw560 One day of data collection (15-min. resolution) nLw490 nLw490 nLw443

10. nLw Time Series nLw670 Data further filtered for : 1. Excessive buoy tilt or depth 2. Sun zenith angle > 70° nLw560 nLw490 nLw443

11. Satellite Data SeaWiFS (2003-2004) level-2 MLAC data from reprocessing 5 Obtained at : http://oceancolor.gsfc.nasa.gov/cgi/browse.pl?sen=am AMODIS (2003-2005) level-2 data from reprocessing 1 Obtained at : http://oceancolor.gsfc.nasa.gov/cgi/browse.pl?sen=am MERIS (2003-2005) level-2 1-Km reduced resolution data processed by the MERIS prototype version 7.4.1 (called “MEGS7.4.1”) Obtained at: http://merci-srv.eo.esa.int/merci/welcome.do

12. Filtering of buoy data sun zenith angles < 70°, buoy tilts < 15°, water leaving radiances > 0 Selection of satellite data a 5 pixel by 5 pixel box, centered on the closest pixel to the buoy location 62 Points for MERIS 50 Points for MODIS Filtering / averaging of satellite data all 25 pixels must be valid, based on Level-2 processing flags 72 Points for SeaWiFS Selection of matchups time lag between the satellite pass and a valid buoy measurement +/-1 hour The Matchup procedure The matchup process follows four steps (Bailey and Werdell, 2006, with additional considerations related to the specificity of the buoy-derived observations):

13. Results: MERIS  Satellite Data • , • , • , •Buoy Data w560 w490 w443 w443/ w555 Chl

14. Results: MODIS  Satellite Data • , • , • , •Buoy Data nLw551 nLw448 nLw443 nLw443/ nLw555

15. Results: SeaWiFS  Satellite Data • , • , • , •Buoy Data nLw551 nLw448 nLw443 nLw443/ nLw555

16. Meris MODIS SeaWiFS Results: All sensors, all bands Satellite In Situ

17. Results: Statistics

18. Results: comparison with other studies The overall agreement between the satellite-derived and the in situ reflectances or radiances is similar for the three sensors, and it is close to what has been already reported elsewhere:

19. Conclusions BOUSSOLE provides a suitable set of data to validate Level-2 satellite Ocean Color products. An overall agreement between the in situ and the SeaWiFS, MODIS and MERIS reflectances and radiances is obtained.

20. Conclusions Some specific points: 1) the MERIS data products in the “red part” of the spectrum (i.e., l > 600 nm) are overestimated by a factor of two to four; 2) the MERIS data products for the blue wavelengths appear to be overestimated; 3) the SeaWiFS and AMODIS data products exhibit a slight tendency of underestimation in the blue domain; 4) AMODIS data for July and August 2005 appear to be underestimated but the cause of the error is at the moment unidentified.

21. Thank you for your attention