Evaluation of the SeaWiFS and MODIS Chlorophyll a Algorithms Used for the Northern South China Sea during the Summer Season


The present study made evaluations of SeaWiFS-derived and MODIS-derived Chlorophyll a (Chl a) concentrations in the Northern South China Sea (NSCS), using in situ data collected during two research cruises which were conducted during the summer of 2004 (September 18 to October 8) and 2007 (August 10 to 29). The data of ±48 h and 3 × 3 pixels were used for the comparison between satellite and in situ Chl a data, and the results reveal a systematic overestimation of Chl a concentration by National Aeronautics and Space Administration (NASA) global algorithms (OC2v4, OC4v4, and OC3M). The RMSEs of the selected algorithms are larger than 0.35 except OC2_D'Ortenzio (one regional algorithm for the Mediterranean Sea). The overestimation seems to correlate with numerous (≈77%) low Chl a concentration (< 0.1 mg m-3) due to the oligotrophic characteristics of the South China Sea (SCS) in summer, and to correlate with the error in atmosphere correction introduced by aerosols. Therefore, the OC2 and OC4 algorithms for SeaWiFS and OC3M algorithm for MODIS are adapted to NSCS by fitting the satellite data set to in situ Chl a data in NSCS. With the new coefficients based on our field data, the regional version of the three algorithms (TP series) showed good performance with RMSE values of 0.245, 0.245, and 0.288 respectively, which were slightly higher than the algorithm "noise" (0.222 in RMSE). Those TP series algorithms may be considered preliminary due to the relatively small number of available in situ data, and they are suitable in summer season in NSCS.

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