Effect Of Aerosol On Cloud And Precipitation: Numerical Simulation

Recent studies showed that, aerosol can serve as cloud condensation nuclei(CCN) and ice nuclei(IN) and thus have a substantial effect on cloud properties and precipitation. In order to make clear the function of aerosol on cloud microphysical processes and precipitation evolution, a heavy precipitation process occurred over Shanxi area on 13 June 2007 has been simulated using Weather Research and Forecasting (WRF) mesoscale model coupled with the Morrision's double-moment bulk microphysical scheme.Firstly, sensitivity tests are conducted and compared for different background conditions to understand the response of cloud microphysical processes and precipitation to changes in concentrations of aerosol particles. The results show that the total rainfall amount is reduced by about 0.8% in the polluted case as compared with the clean case, but a more intensive rainfall rate at the central region of the rained area and no significant change in rainfall area are observed in the former case than in the latter. Snow and graupel crystals are the main sources for rain at the surface. In the polluted case considered here, higher concentration of aerosol particles leads to the decrease of warm rain at the early stage, and consequently more numerous snow particles can grow by coagulations with cloud droplets and rain droplet in the air for longer, intensifying precipitation at the later stage.Secondly, numerical experiments are conducted to investigate the effects of giant cloud condensation nuclei(GCCN) on the development of precipitation and cloud microphysical processes in mixed-phase clouds. The results show that the total rainfall amount is increased by about 0.69% due to the inclusion of giant CCN. At the early stage, the autoconversion rate of cloud water to rain water is enhanced due to the introduction of giant CCN, resulting in an early appearance of large drops and graupel particles. Without giant CCN, the autoconversion rate of cloud water to rain water becomes very slow and large amount of cloud water are sustained in cloud, resulting in an increase of the accretion of ice crystals and snow particles by cloud water, leading to the decrease of rain at the early stage. The melting of snow and graupel particles intensify precipitation at the later stage on account of enhanced and longer growth by coagulations of those hydrometeors with cloud droplets and rain droplet in the cloud.Finally, numerical simulations are conducted and compared for different background conditions to study the effect of dust particles. With lower background concentration of aerosols, the amount of giant CCN in the atmosphere increases with dust particles, leading to more activated droplets, enhanced condensation of water vapor, and slightly increased rainfall rate. When dust particles serve as ice nuclei(IN), the increased initial concentration of ice nuclei results generates the decrease in the effective radius of ice phase particles and the amount of precipitation, which inhibits the effect of giant CCN to a certain extent. Under the condition with higher concentration of background aerosol, the inclusion of giant CCN causes an early development of large drops. The increase in the concentration of IN results in a decrease in the effective radius of ice phase particles and the total amount of precipitation.