Radiative and microphysical interactions between marine stratocumulus clouds and Saharan dust 2. Modeling

Abstract

In the first part of this study, we attempted to highlight some radiative effects of dust particles in the presence of clouds using satellite observations. A statistical study over 6 years of daily Meteosat images revealed a minimum in the apparent cloud albedo (ocean-cloud-atmosphere system) ranging between 10% and 20%, near the West African coast. These decreases were observed at any season, and over the areas where the dust outbreaks are the most frequent. In order to explain these observations, we begin this second part by radiative simulations using the ECMWF radiative code. It appears that the presence of dust over the cloud cover (i.e., summer configuration) leads to a decrease in albedo of the ocean-cloud-atmosphere system reaching about 15% on average, i.e., a value comparable to the one observed by satellite. In winter, however, dust is found in the trade wind layer, at the same level as the cloud cover, but a simple direct effect (superposition effect) cannot explain the observed albedo decrease. According to earlier experimental studies, dust aging is frequently observed in the atmosphere. Complex interactions involving other particles and chemical species enhance the soluble fraction of dust: in-cloud processing, chemical reactions on dust surface, transfer of small sulfate particles toward the large dust particles. Microphysical simulations using an air parcel model reveal that dust presence in a stratiform cloud field may lead to a decrease in the initial cloud condensation nucleus number, an increase of the effective droplet radius, and eventually, a reduction of the cloud albedo. This albedo decrease may reach more than 10% under certain conditions. On average, the albedo of the system is found to be reduced by a value ranging between 3% and 10%. Simulations performed with a mesoscale model reveal the presence at altitude of carbonaceous particles emitted by the African savanna burning in winter. Dust microphysical effects associated with the presence of absorbent carbonaceous particles at altitude would decrease the system albedo by more than 10% and so would explain the satellite observations during the winter season too in the southern part of the area under study. All these computations are in rather good agreement with the satellite observations and confirm the assumption that dust particles play an important climatic role to the west of the African coast. According to our estimates, the radiative forcing associated with the dust in the presence of clouds is (1) globally always positive along the column and (2) located either near the surface (winter case) or at altitude (summer case) according to the season. In the last case (summer), the radiative forcing remains negative near the surface but will be overcompensated by positive values at altitude (dust level). Copyright 2002 by the American Geophysical Union.