Radiative and microphysical interactions between marine stratocumulus clouds and Saharan dust 1. Remote sensing observations

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This work emphasizes the radiative impact of Saharan dust particles in the presence of clouds, over the tropical North Atlantic Ocean. A first analysis based upon recent European Centre for Medium‐Range Weather Forecasts data confirms the classical observations; that is, in summer, the dust plume lies over the trade wind inversion, that is to say, over the main cloud layer. In winter, however, dust particles generally are transported in the trade wind layer, namely, where the major part of clouds is developing. In a second part, we present a statistical analysis based upon a 6‐year series of daily VIS and IR Meteosat data. Our study reveals, at any season, a decrease in the apparent cloud reflectivity derived from satellite measurements. Minimum values of this albedo are found in geographical areas where dust particles are most often observed and where they have their highest values in optical thickness. This albedo decrease can exceed 20% during spring months, when the dust outbreaks are the most frequent. A final discussion puts forward some physical processes that may explain the observations. In summer, dust presence may result in a change of the reflectance of the system, due to an increase of the absorption of the surface‐cloud‐dust plume system. On the other hand, during the winter season, the cloud albedo decrease may be in part explained by the drying of the atmosphere due to the dry air masses associated with the dust outbreaks coming from the Saharan desert. However, an interesting and complementary explanation is put forward for the winter case: Because of its lower altitude, the dust plume is likely to mix with cloud water. Consequently, in‐cloud processes may lead to a change in droplet spectrum size, which in turn, could affect the cloud reflectivity and therefore the reflectance measured at the top of the atmosphere. Therefore, whatever the physical processes involved, the dust outbreaks seem always to lead to a decrease in the cloud cover albedo over the tropical North Atlantic Ocean. In part 2 of this study, the observations will be completed by numerical modeling dealing with radiation, microphysics, and mesoscale transport, in order to better explain and understand this radiative effect.