Numerical Simulation on Indirect Effects of Saharan Air Layer Dust on Tropical Deep Convection

Observational studies suggest that the Saharan Air Layer (SAL), an elevated layer (850-500hPa) of Saharan air and mineral dust, has strong impacts on the microphysical as well as dynamical properties of tropical deep convection clouds system on its track. In this case study, numerical simulations using a two-dimensional detailed cloud resolving model (DCRM) are performed to investigate the dust-cloud interactions in the tropical deep convection, focusing on the dust role as ice nuclei (IN).

The simulations reproduce many observational features of the dust effects. It shows that mineral dust considerably enhances heterogeneous nucleation and freezing at temperatures warmer than -40°C, especially the former, resulting in more ice hydrometeors number concentration and reduced precipitating size of ice particles. With sufficient supply of water vapor during rigorous convection stage, ice deposition growth is also increased, and consequently the updraft velocity is much stronger due to increased latent heat releasing.

On the other hand, the increased ice deposition consumes more water vapor at middle troposphere, which induces a competition for water vapor between heterogeneous and homogeneous nucleation. As a result, dust suppresses the homogeneous nucleation because of weakening transport of water vapor at lower stratosphere, leading to decreased number concentration of ice cloud particles, and consequently lowers the cloud top height during the stratus precipitating stage.

Dust also influences precipitation in deep convection. It initiates earlier the coalescence because dust-related heterogeneous nucleation and freezing at middle troposphere occur earlier than homogeneous nucleation at higher altitudes. Nevertheless, the convective precipitation is suppressed by the reduced riming efficiency and insufficient fallout related to decreased sizes of precipitating ice hydrometeors. In contrast, dust increases the precipitation in stratiform clouds through deposition growth.