Assessment of MERIS reflectance data as processed with SeaDAS over the European seas

The uncertainties associated with MERIS remote sensing reflectance (RRS) data derived from the SeaWiFS Data Analysis System (SeaDAS) are assessed with field observations. In agreement with the strategy applied for other sensors, a vicarious calibration is conducted using in situ data from the Marine Optical BuoY offshore Hawaii, and leads to vicarious adjustment factors departing from 1 by 0.2% to 1.6%. The three field data sets used for validation have been collected at fixed stations in the northern Adriatic Sea and the Baltic Sea, and in a variety of European waters in the Baltic, Black, Mediterranean and North Seas. Excluding Baltic waters, the mean absolute relative difference |ψ| between satellite and field data is 10-14% for the spectral interval 490-560 nm, 16-18% at 443 nm, and 24-26% at 413 nm. In the Baltic Sea, the |ψ| values are much higher for the blue bands characterized by low RRS amplitudes, but similar or lower at 560 and 665 nm. For the three validation sets, the root-mean-square differences decrease from approximately 0.0013 sr-1 at 413 nm to 0.0002 sr-1 at 665 nm, and are found similar or lower than those obtained for SeaWiFS or MODIS-Aqua. As derived from SeaDAS, the RRS records associated with these three missions thus provide a multi-mission data stream of consistent accuracy.

Vicarious calibration of satellite ocean color sensors at two coastal sites

Regionally specific vicarious calibration coefficients for ocean color sensors have been computed for the visible bands using the normalized water leaving radiance L(wn) collected at sites in the northern Adriatic Sea and Baltic Proper. These coefficients are consistent with those found in oligotrophic waters for the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) . The extension of the vicarious calibration process to the near-infrared is investigated by taking advantage of the field data of aerosol optical thickness coincident with L(wn). Results further confirm the importance of accurately modeling and identifying the aerosol components in atmospheric correction schemes.