The Assessment of Atmospheric Correction Processors for MERIS Based on In-Situ Measurements-Updates in OC-CCI Round Robin

The Ocean Colour Climate Change Initiative (OC- CCI) provides a long-term time series of ocean colour data and investigates the detectable climate impact. A reliable and stable atmospheric correction (AC) procedure is the basis for ocean colour products of the necessary high quality.The selection of atmospheric correction processors is repeated regularly based on a round robin exercise, at the latest when a revised production and release of the OC-CCI merged product is scheduled. Most of the AC processors are under constant development and changes are implemented to improve the quality of satellite-derived retrievals of remote sensing reflectances. The changes between versions of the inter-comparison are not restricted to the implementation of AC processors. There are activities to improve the quality flagging for some processors, and the system vicarious calibration for AC algorithms in their sensor specific behaviour are widely studied. Each inter-comparison starts with an updated in-situ database, as more spectra are included in order to broaden the temporal and spatial range of satellite match-ups. While the OC-CCI's focus has laid on case-1 waters in the past, it has expanded to the retrieval of case-2 products now. In light of this goal, new bidirectional correction procedures (normalisation) for the remote sensing spectra have been introduced. As in-situ measurements are not always available at the satellite sensor specific central wave- lengths, a band-shift algorithm has to be applied to the dataset.In order to guarantee an objective selection from a set of four atmospheric correction processors, the common validation strategy of comparisons between in-situ and satellite-derived water leaving reflectance spectra, is aided by a ranking system. In principal, the statistical parameters are transformed into relative scores, which evaluate the relationship of quality dependent on the algorithms under study. The sensitivity of these scores to the selected database has been assessed by a bootstrapping exercise, which allows identification of the uncertainty in the scoring results.A comparison of round robin results for the OC-CCI version 2 and the current version 3 is presented and some major changes are highlighted.