Measurements of upward radiance in channel 1 (1=0.83 µm) of the advanced very high resolution radiometer (AVHRR) onboard NOAA satellites are operationally used at NOAA/NESDIS to derive aerosol optical depth, a1, over the global oceans. The effort is underway to extend the retrievals to AVHRR channel 2 (2=0.63 µm), a2, and to further use a1 and a2 to estimate an effective Angstrom exponent, =-ln(a1/a2)/ln(1/2). Satellite retrievals of a and from NOAA14 are compared against measurements taken by ground-based (AERONET) and aircraft (C-131) sun-photometers during TARFOX in July 1996. The a1 satellite retrievals are in reasonable agreement with AERONET and aircraft data, whereas a2 are substantially overestimated, and, as a result, satellite retrievals of are underestimated. These results are in agreement with the other two validation studies, carried out earlier using AERONET data over two island stations in Cape-Verde and Lanai. The overestimation in a2 at small optical depths by about 0.1 implies that the aerosol model cannot be solely responsible for the disagreement. Three factors which can explain the overestimation -- the calibration in AVHRR channel 2, oceanic reflectance, and water vapor absorption -- are analyzed in detail using sensitivity analysis. Other TARFOX measurements may be used to constrain the uncertainties in all these parameters.
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Last updated Apr-30-1997
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