Influence of Saharan Dust on the Microphysical Structure of Deep Tropical Convection

This study investigates the role of dust aerosols on the development, evolution, and structure of deep tropical convective storms through the use of numerical simulations. Cloud resolving model simulations were initialized with a moist, unstable sounding, from 30 Aug 2006 at 1406Z, obtained during the NAMMA field campaign in the tropical east Atlantic. Representative bi-modal vertical profiles of dust and cloud nucleating aerosol were constructed from aerosol observations during NAMMA. Simulations were performed with and without the presence of dust, while the background cloud nucleating aerosols were present in all model runs. Dust particles were initially allowed to have properties of both ice nuclei (IN) and cloud condensation nuclei (CCN). Additional simulations were performed to examine the convective response when dust acts solely as IN and when dust acts solely as CCN.

Analyses suggest that when dust acts as CCN, cloud droplet nucleation and conversion to rain tend to limit the amount of dust that is transported above the freezing level for activation as IN by efficiently removing the dust via nucleation scavenging and precipitation scavenging. In this case, a relatively high concentration of dust acts to suppress the conversion of cloud droplet to rain, leads to production of fewer, but larger rain drops, reduces the magnitude and intensity of rainfall, and produces weaker cold pools. The weaker cold pools allow the leading edge of the outflow boundary to stay closely coupled with the main updraft, thus leading to longer lived storms. When dust behaves solely as IN a large fraction of the dust is transported aloft where it enhances ice nucleation and leads to higher concentrations of smaller cloud ice particles.

These results suggest that dust has the potential to indirectly influence the radiation budget by increasing the longevity of deep convective storms when behaving as CCN or by modifying the cloud ice spectra and cloud top albedo when acting as IN.