Atmospheric correction of SeaWiFS imagery over turbid coastal waters: A practical method

The current SeaWiFS algorithms frequently yield negative water-leaving radi ance values in turbid Case II waters primarily because the water-column ref lectance interferes with the atmospheric correction based on the 765-nm and 865-nm spectral bands. Here we present a simple, practical method to separ ate the water-column reflectance from the total reflectance at 765 nm and 8 65 nm. Assuming the type of aerosol does not vary much over relatively smal l spatial scales (similar to 50-100 km), we first define the aerosol type o ver less turbid waters. We then transfer it to the turbid area by using a " nearest neighbor" method. While the aerosol type is fixed, the concentratio n can vary. This way, both the aerosol reflectance and the water-column ref lectance at 765 nm and 865 nm may be derived. The default NASA atmospheric correction scheme subsequently is used to obtain the aerosol scattering com ponents at shorter wavelengths. This simple method was tested under various atmospheric conditions over the Gulf of Mexico, and it proved effective in reducing the errors of both the water-leaving radiance and the chlorophyll concentration estimates. In addition, in areas where the default NASA algo rithms created a mask due to atmospheric correction failure, water-leaving radiance and chlorophyll concentrations were recovered. This method, in com parison with field data and other turbid water algorithms, was tested for t he Gulf of Maine and turbid, posthurricane Gulf of Mexico waters. In the Gu lf of Maine it provided more accurate retrievals with fewer failures of the atmospheric correction algorithms. In the Gulf of Mexico it provided far f ewer pixels with algorithms failure than the other methods, did not overest imate chlorophyll as severely, and provided fewer negative water-leaving ra diance values. (C) Elsevier Science Inc., 2000.