MERIS-based ocean colour classification with the discrete Forel–Ule scale

摘要

Multispectral information from satellite borne ocean colour sensors is atpresent used to characterize natural waters via the retrieval ofconcentrations of the three dominant optical constituents; pigments ofphytoplankton, non-algal particles and coloured dissolved organic matter. Alimitation of this approach is that accurate retrieval of these constituentsrequires detailed local knowledge of the specific absorption and scatteringproperties. In addition, the retrieval algorithms generally use only alimited part of the collected spectral information. In this paper we presentan additional new algorithm that has the merit of using the full spectralinformation in the visible domain to characterize natural waters in a simpleand globally valid way. This Forel–Ule MERIS (FUME) algorithm converts thenormalized multiband reflectance information into a discrete set of numbersusing uniform colourimetric functions. The Forel–Ule (FU) scale is a sea colourcomparator scale that has been developed to cover all possible natural seacolours, ranging from indigo blue (the open ocean) to brownish-green (coastalwater) and even brown (humic-acid dominated) waters. Data using this scalehave been collected since the late nineteenth century, and therefore, thisalgorithm creates the possibility to compare historic ocean colour data withpresent-day satellite ocean colour observations. The FUME algorithm wastested by transforming a number of MERIS satellite images into Forel–Ulecolour index images and comparing in situ observed FU numbers withFU numbers modelled from in situ radiometer measurements. Similarpatterns and FU numbers were observed when comparing MERIS ocean colourdistribution maps with ground truth Forel–Ule observations. The FUnumbers modelled from in situ radiometer measurements showed a goodcorrelation with observed FU numbers (R2 = 0.81 when fullspectra are used and R2 = 0.71 when MERIS bands are used).