Impact of aerosol number concentration on precipitation under different precipitation rates

Mainly by modifying the number concentration and size of cloud droplets, the dynamic and thermodynamic effects of aerosols are important in the formation of precipitation, but so far different and occasionally opposite responses of precipitation to aerosol concentration have been noted. By conducting two numerical experiments with different aerosol concentrations, the impact of aerosol number concentration on precipitation under different precipitation rates is examined. To this end, the Weather Research and Forecasting (WRF) model with a two-moment aerosol-aware bulk microphysical scheme was applied. Results indicate that mainly through redistribution of precipitation both spatially and temporarily, more hygroscopic aerosols in the atmosphere could potentially reduce light precipitation events, but increase heavy precipitation events. In regions with a high precipitation rate (> 1 mm/hr), an ample influx of water vapour exists. Thus, cloud droplets have no need to compete for water vapour in order to grow, which results in an enhancement of precipitation under the polluted atmosphere. On the other hand, less efficient autoconversion processes and/or increased cloud-top evaporation may contribute to the decrease in light (< 0.6 mm/hr) to moderate (0.6-1 mm/hr) precipitation under the polluted atmosphere. A delay in the onset of precipitation is identified in the polluted experiment, which leads to a significant increase in precipitation rate, primarily due to more freezing of cloud droplets and extra release of latent heat of freezing, which invigorates cloud developments, while intensification of secondary clouds by aerosols might have also partly contributed.