Aerosol-induced enhancement of precipitation in mixed phase convective clouds: a modeling study

In general, increase of aerosol concentration has been known to decrease the precipitation efficiency. However, many recent studies showed that precipitation amount also depends crucially on the environmental condition and the cloud type for a given aerosol condition. Especially, precipitation of convective clouds with mixed phases of hydrometeors is often enhanced by enhanced aerosol concentration. Aerosol-induced invigoration of convective clouds is caused by increased release of latent heat in those clouds. However, a clear understanding of its detailed mechanism is not still sufficient. The main purpose of this study is to investigate why precipitation is enhanced due to aerosol increase in mixed phase convective clouds, using a bin resolving cloud model. The three aerosol concentrations, 203, 4980 and 6630 m-3 are chosen, and called maritime, continental and polluted cases, respectively. It is found that the increase of aerosol number concentration leads to an increase of droplet number concentration but a decrease of droplet sizes . However, this reduction of droplet sizes under higher aerosol condition leads to a decrease of ice particle number concentration because small droplets are difficult to freeze. Therefore ice particle concentration of cleaner clouds is higher than that of polluted cloud. Above the freezing level, ice particles grow faster by deposition in higher aerosol cloud since ice particle concentration is lower and more numerous but smaller supercooled droplets evaporate faster. Moreover, riming process in the higher aerosol cloud is more efficient than that of cleaner cloud, because supercooled cloud droplets are more numerous in higher aerosol cloud. Consequently, aerosol increase leads to an enhancement of precipitation particle formation above the freezing level. More detail will be discussed at the conference.