Impact of Saharan Dust Aerosols on Radiation and Clouds Over the Tropical East Atlantic Ocean

Aerosols affect the atmospheric processes by interacting with shortwave (SW) and longwave (LW) radiations and also by modifying the cloud microphysics (direct and indirect effects respectively). This study investigates the direct and indirect effects of Saharan dust aerosols, and the subsequent perturbations of the radiation budget and tropical convection by using the Weather Research and Forecasting model coupled with Chemistry module (WRF-Chem). Simulations are conducted for July 1-31, 2010 over two-way nested domains in the tropical East Atlantic Ocean. Results show that the interactions of dust particles with the SW and LW radiations enhance the heating rates in the atmosphere and primarily affect the upwelling SW radiation at the top of the atmosphere, and downwelling SW and LW radiations at the surface. Dust induced lower level heating increases the stability of the lower troposphere. As a result, convection is inhibited below the dust layers and enhanced in the levels inside and above the dust layers. Dust particles acting as the cloud condensation nuclei (CCN) lead to enhanced condensation and produce numerous cloud droplets with reduced droplet sizes. Consequently, slower collision coalescence among the cloud droplets affects the droplet autoconversion rate and reduces the rain production. However, there is no evidence of the suppression of accumulated precipitation at the surface that would otherwise increase the cloud lifetime.
The net radiative forcing of dust is positive inside the atmosphere, and negative at the surface, and top of the atmosphere. Due to the overall positive radiative feedback from the cloud, the magnitudes of the forcing at surface and top of the atmosphere become weaker in the presence of clouds.