Aerosol Impacts on the Structure, Intensity, and Precipitation of the Landfalling Typhoon Saomai (2006)

Typhoon Saomai (2006) was simulated using the Weather Research and Forecasting Model (WRF) with explicit spectral (bin) microphysics (SBM) to investigate the effects of aerosol from mainland China on the intensity, structure and precipitation of the landfalling storm. MAR (maritime), MIX (semi-continental) and CON (continental) experiments were conducted with different initial aerosol concentrations. Varying aerosol concentrations had little influence on the storm track but resulted in pronounced deviations in intensity and structures. The experiment with a high initial aerosol concentration showed invigorated convection at the periphery of the tropical cyclone (TC), which interfered with the reformation of the eyewall, leading to TC weakening. The minimum pressures in the CON and MIX experiments were increased by more than 30 hPa and 14.6 hPa, and their maximum wind speeds were 20 m s-1 and 13 m s-1 weaker than that in the MAR experiment, respectively. The rain rates in the MIX and CON experiments were 16.6% and 56.2% greater than that in the MAR run, with the differences mainly occurring in the outer rainbands. These results indicate that the aerosol concentration substantially affects the spatial distributions of cloud hydrometeors and rainfall. The increase of rainfall was triggered by an increase in the melting of graupel and cloud droplets collected by raindrops. Similarly, the graupel melting process also enhanced in the outer rainbands with increasing aerosol. Furthermore, a positive microphysics feedback associated with the rainwater in the outer rainbands played an important role in increasing the rain rate in more aerosol scenarios.