Sea Wifs

Based on the few available samples for comparisons to the AERONET statistics, SeaWifs single channel ocean retrievals, SeaWifs, o67 and SeaWifs, o87, underestimate optical depth. This was expected, as averages are composed from optical depths under minimum blue conditions. The SeaWifs land-retrieval, SeaWifs, l, was applied to cloud-free dark pixels (low reflecting) over land (e.g. green vegetation). Critical for this retrieval are a correct identification of dark pixels (assuming negligible aerosol contributions to satellite sensing at longer wavelengths) and the validity of solar spectral relationships for the surface albedo. Aerosol optical depths from this retrieval are clearly too large. Incorrect pixel identification, including a sub-pixel contamination by clouds and non-black surfaces, are believed to be largely responsible for optical depth overestimates, which at times are significant. Additional uncertainty is introduced by the screening. Dark-pixel informa tion comes from another satellite (AVHRR) with a different local overpass time and cloud-scene removal is only based on visible thresholds.

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