Other algorithm studies
Read our other algorithm studies for current, future and multi-missions.
27 January 2023
04 October 2021
The Clear Water Atmospheric Correction (CWAC) is a key component of Ocean Colour data processors and has been used operationally by agencies, for more than forty years, to provide Ocean Colour Radiometry (i.e. marine reflectance at sea level) and downstream water bio-optical products. In essence, the role of CWAC is to detect the amount and type of aerosols, variable in space and time, with minimum assumptions on the marine optical properties. With this aim, CWAC relies on radiometric measurements in the near infrared (NIR) and possibly short-wave infrared (SWIR), where water absorption is high and the top-of-atmosphere (TOA) signal is assumed to result from the atmosphere only, and not from the water. To deal with geophysical perturbations of the pure atmospheric signal by sea surface, CWAC is preceded by sunglint correction, whitecaps correction and Bright Pixel Correction (BPC), all together forming the so-called Ocean Colour Standard Atmospheric Correction (OC-SAC). OC-SAC differs from alternative atmospheric corrections (OC-AAC), such as spectral matching methods (e.g. EUMETSAT SACSO study) or artificial neural networks whose ocean model over the full spectrum implies further assumptions, and possibly limitations, on the inversion.
The OLCI operational processor CWAC algorithm, up to EUMETSAT Collection 3, dated 2021, comes directly from the MERIS processor developed in the 1990s (Antoine and Morel, 1999). It is known to create spatial noise (uncertainty amplification, aerosol discontinuity), it has limitations in terms of aerosol optical thickness and Angstrom (e.g. Zibordi et al, 2022), air mass or viewing and solar dependence and its outputs need to be flagged data in many situations (e.g. negative reflectance, sensitivity to perturbations, failure for absorbing aerosols). A complete review and upgrade of this algorithm was required for EUMETSAT, in order to deliver Copernicus Ocean Colour products of higher quality in the near future (Collection 4).
The objective of this study was to provide a robust and versatile OC-SAC module, based on state-of-the-art aerosol modelling and inversion, giving advanced possibilities for future Copernicus Ocean Colour data processing for Sentinel-3 OLCI and, potentially, other contributing polar and geostationary missions (FCI, 3MI, MetImage). The module has integrated the Bright Pixel Correction (BPC) recently developed for OLCI Collection 3, with refinements in terms of convergence, and, above all, a totally revised CWAC algorithm with new aerosol modelling, detection and new capabilities regarding aerosol layer height (ALH) retrieval. The module has been integrated into both the OLCI operational processor and the SACSO prototype processor.
Considering the many options of the CWAC components (aerosol modelling, radiative transfer, aerosol detection, uncertainty propagation, etc.), developing a successful CWAC module in the operational processor was a challenging objective.
The overall strategy was:
The project relied on five main tasks:
Task 1: Scientific review and requirements consolidation — A detailed review of existing CWAC algorithms, together with small-case prototyping, supported the design of the CWAC module in terms of science (aerosol modelling, numerical strategy for aerosol selection) and implementation.
More specifically, the development of the CWAC component was guided through an objective criterion on aerosol reflectance (accuracy of 2*10^(-4) in the blue bands) based on an uncertainty analysis to reach the OC requirements over a wide range of oceanic and atmospheric conditions. Key features of the new module are:
Task 2: OC-SAC algorithm development — This comprised the LUT generation and code implementation. The OC-SAC module was developed and consolidated in a new OC-SAC branch of the SACSO prototype. Support and training were given to EUMETSAT in parallel to transfer and use the same module in the operational processor.
The core module, CWAC, comprises schematically four steps, starting from a family of standard aerosol models (Figure 2):
The algorithm stops either after the 1st pass with the standard aerosols, or after the 2nd pass.
Task 3: Product generation — This task first covered the computation of dedicated SVC gains with the EUMETSAT Ocean Colour system vicarious calibration tool (Figure 3) and then the generation of Level-2 products over the Diagnostic Dataset of the SACSO study (see examples on Figure 4), with the OC-SAC-SACSO prototype).
Task 4: Evaluation and validation — This comprised the assessment of the new marine reflectance reflectance in comparison with existing processor (OLCI Collection 3) over individual scenes (Figure 4), global maps, time-series over selected sites and validation and atmospheric by-products against in-situ radiometric data (AERONET-OC, MOBY, Figure 5).
OC-SAC-SACSO prototype results analysed below include a different cloud screening scheme than the existing Collection 3 processor, which may be not as effective. Nevertheless, the main conclusions of this task were that that the new OC-SAC module presents, in comparison to the existing processor:
Task 5: Project outreach — This comprised the promotion of the study to the Sentinel-3 Validation Team, scientific discussions with NASA’s Ocean Biology Processing Group and provision of a detailed roadmap for extended validation and refinements targeting operationalisation of the new OLCI OC-SAC in 2023.