Comparison of measured and satellite-derived spectral diffuse attenuation coefficients for the Arabian Sea
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The results of study comparing the spectral diffuse attenuation coefficients Kd(Lambda) measured in the Arabian Sea with those derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) using three algorithms, of which two are empirical-data-driven and one is semi-analytical are presented. The measurements were carried out in all water types and the mean values of the measured spectral Kd(Lambda) are 0.105, 0.092, 0.077, 0.082, 0.110 and 0.490 m sup(-1) for wavelength Lambda at 412, 443, 490, 510, 555 and 670 nm, respectively. This profile corresponds to a chlorophyll value of about 1 mg m sup(−3). The maximum values of the measured Kd correspond to waters with chlorophyll of about 8 mg m sup(−3). Though the satellite-derived Kd(Lambda) are found to be overestimated in all bands, we have observed good correlations between the measured and satellite-derived values in all bands, and excluding the band at 670 nm, the mean absolute percent deviations are observed to be less than 50% in all bands. The performance of the data-driven empirical methods was found to be consistent in all the bands, except at the red band of 670 nm, which is uncorrelated with the measured values and has large errors. The performances of the empirical methods depend on the accuracy of the band ratios of the retrieved remote sensing reflectance. Though the performance of the semi-analytical algorithm is found to be spectrally varying, with large positive bias observed in the blue regions, this algorithm is recommended for hyperspectral applications. The performance of the semi-analytical algorithm could be improved by having a robust algorithm to accurately derive spectral inherent optical properties of absorption and backscattering coefficients from the satellite data.
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