Sensitivity study of the Angstrom exponent derived from AVHRR over the oceans

Aerosol optical depth, tau(A), has been operationally retrieved from NOAA satellites over global oceans since 1989 using measurements in the Advanced Very High Resolution Radiometer (AVHRR) channel 1 (lambda(1) = 0.63 mu m). Recently, the question has been raised on developing a two-channel procedure, to additionally retrieve the aerosol Angstrom exponent, alpha, using measurements in AVHRR channel 2 (lambda(2) = 0.83 mu m). Here, we evaluate theoretically the expected uncertainty, delta alpha, resulting from atmospheric, oceanic, and radiometric errors. In general, delta alpha is inversely proportional to tau(A), with the proportionality coefficient depending upon spectral channels used, sun-view geometry, and uncertainties in the model retrieval parameters. For the AVHRR, retrieval of alpha under typical oceanic conditions (tau(A) < 0.1) is highly uncertain. Empirical analysis of AVHRR data is underway, and field programs are being conducted to quantify these errors observationally. (C) 1998 COSPAR. Published by Elsevier Science Ltd.