Interactions of mineral dust particles and clouds: Effects on precipitation and cloud optical properties

Abstract

Numerical simulations were performed to investigate the effect of cloud‐processed mineral dust particles on the subsequent development of cloud and precipitation and possible effects on cloud optical properties. A two‐dimensional (2‐D) nonhydrostatic cloud model with detailed microphysics was used. The initial aerosol spectra used in the 2‐D model consisted of both background cloud condensation nuclei and mineral dust particles. These were taken from the results of three successive runs of a parcel model that simulates the interaction of dust and sulfate particles with cloud drops and trace gases and then evaporates the cloud drops. The results show that insoluble mineral dust particles become effective cloud condensation nuclei (CCN) after passing through a convective cloud. Their effectiveness as CCN increases because of a layer of sulfate that is formed on their surface as they are first captured by growing drops or ice crystals and then released as these hydrometeors evaporate. Upon entering subsequent clouds, these particles increase the concentration of the activated drops and widen the drop size distribution. The present work shows that in continental clouds the effect of cloud‐processed dust particles is to accelerate the formation of precipitation particles, although the amount of precipitation depends on the concentration of the large and giant CCN. In maritime clouds the addition of cloud‐processed aerosol and mineral dust particles has a minimal effect on precipitation because the cloud starts with many large particles already. The addition of more CCN to either maritime or continental clouds increases their optical depth, even for those cases in which the precipitation amount is increased.