LONG-TERM DAILY MONITORING OF SAHARAN DUST LOAD OVER OCEAN USING METEOSAT ISCCP-B2 DATA .2. ACCURACY OF THE METHOD AND VALIDATION USING SUNPHOTOMETER MEASUREMENTS

The accuracy of a method for long-term monitoring of the desert aeroso l optical thickness over the oceans using Meteosat low-resolution imag es, presented in a companion paper, is assessed. We present Sun photom eter measurements of aerosol optical thickness and Angstrom wavelength exponent obtained in 1986-1994 at different sites and seasons in the tropical Atlantic and northwestern Mediterranean. Results suggest that in the absence of dust outbreak the optical effects of the marine aer osol are dominated by continental anthropogenic sulphates in the Medit erranean Sea and by continental desert dust in the tropical Atlantic. We rely on this data set to constrain the desert aerosol model used in the Meteosat data inversion. We obtain the best agreement between Met eosat- and Sun-photometer-derived aerosol optical thickness with a siz e distribution typical of background desert aerosol and a refractive i ndex of 1.50 - i 0.010. The main theoretical uncertainties on the dese rt aerosol optical thickness estimated from Meteosat are due to the se nsor calibration and to the radiometric sensitivity. Comparison of Met eosat-derived estimates of the desert aerosol optical thickness with i ndependent Sun photometer measurements exhibits a maximum dispersion o f 25%.