Atmospheric turbulence and stability in and around long-range-transported Saharan dust layers as observed by airborne lidar and dropsondes

Each year, during both boreal winter and summer, large amounts of Saharan mineral dust particles get carried westwards over the Atlantic Ocean towards the Caribbean. During their transport, Saharan dust particles can affect the Earth’s radiation budget in different ways. They can either directly scatter, absorb and emit radiation or have an indirect effect by modifying cloud properties through their interactions as cloud condensation nuclei or ice nucleating particles. While during the summer months – the peak season of transatlantic mineral dust transport – the particles are mostly advected in elevated Saharan Air Layers at altitudes of up to 6 km and at latitudes around 15°N, wintertime transport takes place at lower atmospheric levels (<3 km altitude) and lower latitudes. Our recent studies have shown that, during both boreal winter and summer, transported Saharan dust layers are characterized by enhanced concentrations of water vapor compared to the surrounding atmosphere. In this way the dust layers have to potential to modify the radiation budget not only through particle-radiation-interactions, but also through the absorption and emission of radiation by water vapor. This in turn may affect the atmospheric stability and stratification in and around the aerosol layers.

In this study, the turbulent structure as well as the atmospheric stability in and around transported Saharan mineral dust is analyzed and possible differences between summer and wintertime are investigated. Therefore, measurements by both the water vapor and aerosol lidar WALES as well as by dropsondes are studied. They were collected upstream the Caribbean island of Barbados aboard the German research aircraft HALO (High Altitude and Long Range). To identify possible seasonal differences, not only data collected in boreal summer in the framework of the NARVAL-II campaign (August 2016), but also data collected in winter during the EUREC4A research campaign (January & February 2020) are analyzed. During both campaigns several research flights were designed to lead over long-range-transported Saharan mineral dust, thus allowing and in-depth investigation of their properties. The analysis shows that dust layers are highly turbulent and therefore help dust particles to stay airborne for a longer time. Additionally, the dust layers modify the atmospheric stability in a way that the evolution of marine clouds can be affected.

In our presentation, we will give an overview of the performed measurements over long-range-transported Saharan dust layers and present the conducted analyses on atmospheric stability and turbulence from dropsonde measurements and calculated power spectra from lidar data.