Impacts of Saharan Dust Acting as CCN on the Evolution of an Idealized Tropical Cyclone

The impact of dust in the Saharan Air Layer (SAL) acting as cloud condensation nuclei (CCN) on the evolution of a tropical cyclone (TC) was examined by conducting simulations initialized with an idealized pre-TC mesoscale convective vortex (MCV) using the Regional Atmospheric Modeling System (RAMS). Two groups of simulations initialized with either a strong MCV or a weak MCV were conducted. Within each group, there are five simulations with the maximum CCN concentration varying from 100, 500, 1000, 1500 and 2000 cm-3 in a layer between 1 and 5 km, where the dust in the SAL are typically found.

Within each group of simulations, the MCVs showed similar evolution cycles in terms of the starting time of rapid intensification and occurrence of spiral rainbands. The starting time of rapid storm intensification and intensity divergence among simulations with various CCN concentrations were 24 hours earlier in the strong vortex experiments compared to the weak vortex ones. Through the simulation period, both experiments showed a maximum difference in the azimuthally averaged surface wind speed about 20 m s-1 among simulations. The storm intensities did not show a monotonic trend with increasing or decreasing initial CCN concentration. Latent heating due to important microphysical processes including vapor diffusional growth of hydrometeors, melting of ice particles, activation of CCN, homogeneous and heterogeneous ice nucleation, were analyzed. Differences in latent heating evolution and distribution among simulations with various initial CCN concentrations were examined in an effort to explain the differences in storm evolution among the simulations.