Impact of elevated Saharan Air layer on shallow marine convection

Mineral dust is one of the major contributors to the global aerosol load with the Sahara being its largest source. Dust particles can be transported over many days and thousands of kilometers. The main transport route spans from Africa over the Atlantic Ocean towards the Caribbean. Most of the time dust-transport takes place in the so-called Saharan Air Layer (SAL).  During its transport the SAL affects the Earth’s atmosphere by scattering and absorption of solar and terrestrial radiation, and by changing cloud evolution and cloud properties. The main season for the transatlantic dust transport is during the boreal summer months. However, dust can be transported towards the Caribbean also during wintertime, although this happens with less frequency.

Airborne lidar measurements with the combined water vapor differential absorption and high spectral resolution lidar system WALES provide simultaneous measurements of the water vapor mixing ratio and of aerosol properties. We use the measurements during the NARVAL-II experiment in August 2016 and during the EUREC4A experiment in January/February 2020 to characterize the long-range transported SAL in summer- and in wintertime, and to investigate its radiative effect and its impact on the subjacent shallow marine trade wind convection. We found, that a small amount of water vapor embedded in the SAL has a strong impact on the radiative heating effect of this layer and consequently also on the atmosphere’s stability. During summertime, when the SAL is well separated from the marine boundary layer, the radiative effect of the SAL dominates. The evolution of shallow marine clouds below the SAL is suppressed. In wintertime, the SAL is transported at lower altitudes and the dust layer is frequently mixed into the marine boundary layer. During this time of the year the effect of the SAL on the evolution and lifetime of marine trade wind convection is much more complex, as the dust particles within the SAL might additionally act as cloud or ice nuclei.

In our presentation we will give an overview of the performed measurements and the radiative transfer calculations. We will present the radiative effects of the separated summertime SAL, and show first results of the impact of the wintertime SAL on the atmosphere’s stability and cloud properties.