Long-term daily monitoring of Saharan dust load over ocean using Meteosat ISCCP-B2 data .1. Methodology and preliminary results for 1983-1994 in the Mediterranean

In this paper we describe a method to perform an accurate long-term monitoring of the optical thickness and mass column density of airborne desert dust over the Atlantic and the Mediterranean using Meteosat wideband solar (visible (VIS) plus near infrared) sensor. The dust load is retrieved using aerosol models and an Earth-atmosphere radiative transfer model. The method focuses on multiyear(from 1983 to 1994) daily retrieval of the atmospheric dust load using Meteosat low-resolution images prepared for the International Satellite Cloud Climatology Project (B2 format). We account for the variable calibrations, radiometric sensitivities, and spectral bands of the successive sensors (Meteosat 2 to Meteosat 5) as well as for the presence of marine background and stratospheric aerosols. We discuss the sensitivity of the method to different factors, and its accuracy is assessed in a companion paper. The results obtained include the daily geographical distribution of the dust load and the temporal variation of the dust load over marine areas. We illustrate and briefly discuss the results for the western Mediterranean and particularly for the Dynamique des Flux Atmospheriques en Mediterranee (DYFAMED) marine station in the Ligurian Sea. The dust transport mainly takes place during summer in this area. More than half a million metric tons of suspended dusts are occasionally observed over the western Mediterranean, and we observed an average of 16 dust events per year. At DYFAMED station the 11.5-year mean dust optical thickness is 0.11, with annual means ranging from 0.055 in 1985 to 0.19 in 1992.