Validation of MERIS Ocean Color Algorithms in the Mediterranean Sea

Details Validation of MERIS Ocean Color Algorithms in the Mediterranean Sea Validation of MERIS Ocean Color Algorithms in the Mediterranean Sea

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004esasp.549e...7m&link_type=abstract

Proceedings of MERIS User Workshop (ESA SP-549). 10-13 November 2003, ESA-ESRIN, Frascati, Italy. Editor: H. Lacoste. Published

Computer Science

Performance

Scientific paper

Satellite ocean color measurements can contribute, better than any other source of data, to quantify the spatial and time variability of ocean productivity and, tanks to the success of several satellite missions starting with CZCS up to SeaWiFS, MODIS and MERIS, it is now possible to start doing the investigation of interannual variations and compare level of production during different decades ([1],[2]). The interannual variability of the ocean productivity at global and regional scale can be correctly measured providing that chlorophyll estimate are based on well calibrated algorithms in order to avoid regional biases and instrumental time shifts. The calibration and validation of Ocean Color data is then one of the most important tasks of several research projects worldwide ([3], [4]). Algorithms developed to retrieve chlorophyll concentration need a specific effort to define the error ranges associated to the estimates. In particular, the empirical algorithms, calculated on regression with in situ data, require independent records to verify the degree of uncertainties associated. In addition several evidences demonstrated that regional algorithms can improve the accuracy of the satellite chlorophyll estimates [5]. In 2002, Santoleri et al. (SIMBIOS) first showed a significant overestimation of the SeaWiFS derived chlorophyll concentration in Mediterranean Sea when the standard global NASA algorithms (OC4v2 and OC4v4) are used. The same authors [6] proposed two preliminary new algorithms for the Mediterranean Sea (L-DORMA and NL-DORMA) on a basis of a bio-optical data set collected in the basin from 1998 to 2000. In 2002 Bricaud et al., [7] analyzing other bio-optical data collected in the Mediterranean, confirmed the overestimation of the chlorophyll concentration in oligotrophic conditions and proposed a new regional algorithm to be used in case of low concentrations. Recently, the number of in situ observations in the basin was increased, permitting a first evaluation of the DORMA algorithms with data not used for the algorithms empirical coefficient retrieval and an extensive validation of the new MERIS algorithm. Up to now two possible explanations of the fact that the oligotrophic waters of the Mediterranean Sea are greener than would result from their phytoplankton content alone have been proposed. Claustre et al. [8] suggested that the observed distortion effect on the blue to green ratio can be due to the presence of Saharan dust in the upper layers that enhance absorption in the blue and backscattering in the green. DOR2002 also proposed a tentative explanation of the observed distortion effect based on the hypothesis that enhanced CaCO3 concentration, due to relative abundance of coccolithophores in the oligotrophic waters of the Mediterranean Sea, can cause similar results. In this note, we show a first analysis of the performances of two regional Mediterranean OC algorithms based on SeaWiFS bands (the NL-DORMA and Bricaud et al. 2002 algorithms), the standard OC4v4 NASA algorithm and the MERIS algorithm currently used to produce chlorophyll maps for case I waters. Finally we revisit the Claustre et al. (2002) and DOR2002 hypothesis in the light of new measurements recently published by Malinverno et al. [9] and the results of the ADIOS project of EC.

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