Assessment of uncertainty in the ocean reflectance determined by three satellite ocean color sensors (MERIS, SeaWiFS and MODIS‐A) at an offshore site in the Mediterranean Sea (BOUSSOLE project)

Abstract

The match‐up of satellite‐derived reflectances with in situ observations is crucial to evaluate their quality and temporal stability. To contribute to this effort, a project has been set up to collect a data set of in situ radiometric and bio‐optical quantities, in support to satellite ocean color calibration and validation. The project has been named “BOUSSOLE”, and one of its key elements is a deep‐sea optics mooring collecting data on a near‐continuous basis since September 2003. This buoy is deployed in the deep clear waters of the northwestern Mediterranean Sea, and is visited on a monthly basis for servicing and acquisition of complementary data. The characteristics of the work area establish the site as a satisfactory location for validating satellite ocean color observations. A description of the data processing protocols is provided, followed by an analysis of the uncertainty of the buoy radiometry measurements. The results of a match‐up analysis of the marine reflectances, diffuse attenuation coefficients, and chlorophyll concentrations for three major missions, i.e., MERIS, SeaWiFS, and MODIS‐A, are then analyzed. They show poor performances for the bluest band (412 nm) of the three sensors, and performances within requirements at 443 and 490 nm for SeaWiFS and MODIS‐A. These results suggest that a vicarious calibration should be introduced for the MERIS sensor. This analysis also demonstrates that a major effort is still required to improve atmospheric correction procedures whatever the mission.