Status on the Implementation of the Bpac

The Bright Pixel Atmospheric Correction (BPAC) is the MERIS operational algorithm for precorrecting the residual marine signal in the near infrared prior to the Clear water atmospheric correction. It is an essential step in the mission for computing Level 2 products on Case 2 waters. It is also always applied on Case 1 waters. The method is based on an iterative coupling between a simplified atmospheric correction and a bio-optical model of backscattering of suspended matter at 709, 779 and 865 nm. We present the status on several practical issues recently raised on the implementation and validation of the algorithm: a) Does the algorithm always converge to a solution ? b) How can we explain the very high value of residual marine signal over clear waters ? c) What is the coherence between the BPAC output and the atmospheric correction, in particular would a full coupling between the Clear water algorithm and the bio-optical model improve the final water-leaving reflectance in the visible? Analyse in conducted theoretically, on MERIS scenes and on the MERMAID matchup database. The analysis brings also to light a problem of calibration in the NIR bands. We propose to change the aerosol reflectance extrapolation, as well as an adjustment at 865 nm, which should be further studied in coherence with the current development of MERIS vicarious adjustment.