Determination of the radiative heating rates of dust over West Africa and their impact on the atmospherical dynamics using satellite observations during the AMMA campaign

Abstract

The work carried out in this study is devoted to a better understanding of radiative properties for dust in the shortwave (SW) and in the longwave (LW), and also the consequence of this radiative forcing on the atmospheric dynamics. Such a characterization is indeed essential in particular over North of Africa, the main source of mineral aerosols. Our study is based on a synergy between different platforms : in situ ground-based or airborne measurements, together with remote sensing observations. This study takes place in the AMMA campaign realized during 2006. The analysis highlights the importance of dust on radiative balance over West Africa on regional scale (radiative impact significant in SW but also in LW), during daytime and the night with heating rate between 1.5 Kday-1 and 4 Kday-1, on average in dust plume. At midtime, heating rate can reach 8 Kday-1, in optically thickest part of the plume. During the nighttime, cooling is observed as the result of the longwave forcing in the dust layer between 0.5 and 1 Kday-1, while warming is observed below the dust layer, in the monsoon layer. The consequence of this heating on the dynamic of this region is also important because of its space and temporal variability. It leads to modifications not only on the temperature, and moisture, but also on wind speed and direction. These modi cations generate significant changes on the monsoon system, as we can show with numerical simulation using a meso-scale model. More precisely, the presence of dust between 1 and 4 km causes radiative forcing inducing vertical and horizontal temperature gradient. The localization of dust observed north of African Easterly Jet (AEJ) leads to an increase of the vertical wind gradient associated to an AEJ intensification. At low altitude, the heat low and associated wind are also intensifi ed. The last part of this work concerns dust detection in infra-red over West Africa (for forecast objectives for instance) using brightness