Impact of Saharan Dust Aerosols on Convections over the Tropical East Atlantic Ocean

Saharan dust aerosols carried in the Saharan Air Layer (SAL) have been thought to affect convection and tropical cyclones over the Atlantic Ocean. But the sign of the impact is still debatable. This study investigates the direct and indirect effects of Saharan dust aerosols on tropical convection and tropical cyclones over the Atlantic Ocean using the Weather Research and Forecasting model coupled with Chemistry module (WRF-Chem).

Simulations for one summer month show that the interactions of dust particles with the shortwave (SW) and longwave (LW) radiations enhances the heating rates in the atmosphere (ATM) and primarily affects the upwelling SW radiation at the top of the atmosphere (TOA), and downwelling SW and LW radiations at the surface (SUR). Dust induced lower level heating increases the stability of the lower troposphere. As a result, convection is inhibited below dust layer and enhanced in the levels inside and above the dust layer. Availability of dust as 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 SUR that would otherwise increase the cloud lifetime. The net radiative forcing of dust is positive inside the ATM, and negative at the SUR, and TOA. Due to the overall positive radiative feedback from the cloud, the magnitudes of the forcing at SUR and TOA become weaker in the presence of cloud.

Simulations for one tropical storm show dust aerosols increase the cloud mixing ratio, cloud droplet number and affect the rain, ice and snow content. Although dust aerosols were heavily wrapped inside the tropical storm, the intensity and the track of the storm are not sensitive to dust aerosols in this case.