Abstract
An atmospheric correction algorithm for ocean color data in an extremely clear atmospheric condition is proposed. The correction is based on using the data for all eight SeaWiFS channels instead of using just two of the near-infrared channels. The upwelling radiation from a model atmosphere–ocean was calculated for each of the eight wavelengths of the SeaWiFS bands as a function of aerosol optical thickness. Next, a best-fit optical thickness is grossly determined using the generated look-up-table of the upwelling radiation. Then, a ratio of radiance at 443 nm to that at 560 nm taken by SeaWiFS is compared with the ratio derived using the LUT, to derive a suitable chlorophyll density. The new algorithm was applied to SeaWiFS data over the ocean off Cape Howe (−37.8° S, 150.7° E) and Brisbane (−26.6° S, 154.7° E) in East Australia in an extremely clear atmospheric condition. The data were taken in July 20, 2000 and June 8, 1998, respectively. A comparison of SeaDAS with our new algorithm was undertaken with regard to chlorophyll density. The general patterns of chlorophyll density agreed well. However, the new atmospheric correction code provided reasonable chlorophyll densities for some pixels which failed atmospheric correction using the SeaDAS code. A comparison of SeaDAS derived chlorophyll with sea-truth was carried out. The SeaDAS derived chlorophyll densities are higher than those of sea-truth, if the densities are greater than 0.4 mg/m −3. In contrast the new algorithm gave results which agreed well with the sea-truth. Furthermore, a comparison of SeaDAS derived aerosol optical thickness with that based on the present radiative transfer model was carried out. The data were taken off Tasmania, Australia (−48.9° S, 146.5° E) in October 16, 1999. Again, the general pattern of aerosol optical thickness agreed well. More precisely, optical thickness in a tubid atmosphere derived by SeaDAS code is 10% higher, whereas in clear atmospheres it is about a half, when compared to those of the present method.
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