ALBERT

Description: The Ocean Colour Climate Change Initiative intends to provide a long-term time series of ocean colour data and investigate the detectable climate impact. A reliable and stable atmospheric correction procedure is the basis for ocean colour products of the necessary high quality. 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 extended by a ranking system. In principle, the statistical parameters such as root mean square error, bias, etc. and measures of goodness of fit, 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. Although the presented methodology is intended to be used in an algorithm selection process, this paper focusses on the scope of the methodology rather than the properties of the individual processors.

Description: The established procedure to access the quality of atmospheric correction processors and their underlying algorithms is the comparison of satellite data products with related in-situ measurements. Although this approach addresses the accuracy of derived geophysical properties in a straight forward fashion, it is also limited in its ability to catch systematic sensor and processor dependent behaviour of satellite products along the scan-line, which might impair the usefulness of the data in spatial analyses. The Ocean Colour Climate Change Initiative (OC-CCI) aims to create an ocean colour dataset on a global scale to meet the demands of the ecosystem modelling community. The need for products with increasing spatial and temporal resolution that also show as little systematic and random errors as possible, increases. Due to cloud cover, even temporal means can be influenced by along-scanline artefacts if the observations are not balanced and effects cannot be cancelled out mutually. These effects can arise from a multitude of results which are not easily separated, if at all. Among the sources of artefacts, there are some sensor-specific calibration issues which should lead to similar responses in all processors, as well as processor-specific features which correspond with the individual choices in the algorithms. A set of methods is proposed and applied to MERIS data over two regions of interest in the North Atlantic and the South Pacific Gyre. The normalised water leaving reflectance products of four atmospheric correction processors, which have also been evaluated in match-up analysis, is analysed in order to find and interpret systematic effects across track. These results are summed up with a semi-objective ranking and are used as a complement to the match-up analysis in the decision for the best Atmospheric Correction (AC) processor. Although the need for discussion remains concerning the absolutes by which to judge an AC processor, this example demonstrates clearly, that relying on the match-up analysis alone can lead to misjudgement.

Description: A regional data set of water constituent concentrations and inherent optical properties (light absorption and scattering coefficient) for the German Bight and adjacent waters (River Elbe, North Sea, UK waters, and Southern Norwegian Sea) is presented. The data provide high quality results of in situ measurements and laboratory analysis of samples taken at sea, mainly from the mixed layer, during the years 2008 to 2021. Parameters of the water constituents include concentrations of chlorophyll a, particulate organic and dissolved organic carbon (POC, DOC), total suspended matter (TSM), organic suspended matter (OSM) together with water depth, temperature, salinity, and turbidity. Inherent optical properties (IOPs) are given spectrally as light attenuation, scattering and absorption coefficients. This includes coefficients of light attenuation by all non-water matter (cgp) and particulate matter alone (cp), light absorption by all non-water matter (agp), particulate (ap) and dissolved matter (Gelbstoff, ag), non-algal matter (anap) and phytoplankton (aph), and total scattering (bp) and backscattering (bbp) by particulate matter. The combination of concentrations and IOPS is used to determine specific IOPs of German Bight water and in optical modelling of coastal waters to interpret surface reflectance spectra like in satellite remote sensing approaches.