An evaluation of apparent color of seawater based on the in-situ and satellite-derived Forel-Ule color scale

Abstract Apparent color of seawater is a key variable that can be used to describe the optical properties of seawater and plays a crucial role in understanding surface marine ecosystems and processes. The apparent color of seawater traditionally determined from a hand-held Forel-Ule color comparator scale can now be derived from satellite ocean color end-product water leaving radiance or remote sensing reflectance. However, this retrieval algorithm has not been verified in China's offshore waters, which are some of the most turbid waters in the world. Thus, the precision of the satellite-derived Forel-Ule index (FUIsatellite) is unknown in this area. In this paper, FUIsatellite values is evaluated using in-situ FUI (FUIinsitu) observations obtained in the first large-scale systematic survey on the marine environment in China, and the results show that there is a generally good agreement (r = 0.75, N = 2390, p = 0.00, MAD = 2.61, MUPD = 35.02%) between them. Uncertainties of satellite derivation along the coast of the East China Sea (ECS) and the Beibu Gulf are identified. Moreover, the probability of uncertainties of FUIsatellite as a function of FUIinsitu can be well fitted by Gaussian function (r2 = 0.99). Our study also demonstrates that the radial basis function, which uses linear function as the basis function, is the optimal method on the spatial interpolation of FUIinsitu. The FUIsatellite is still in good agreement with the interpolated FUIinsitu field (r = 0.77, N = 265302, p = 0.00, MAD = 1.73, MUPD = 36.06%), which reveals that remote sensing techniques could be an integral tool for operational environmental monitoring of water quality in China's offshore waters.