The effect of aerosol layers on convective cloud microphysics and precipitation

Abstract

The effects of aerosols transported at different altitudes on the microphysical and dynamic processes of clouds formed on different background aerosol concentrations have been investigated using a dynamic cloud model with spectra-resolved microphysics. Two scenarios are conducted to represent the continental and maritime aerosol types. Under the same initial thermodynamic conditions, the continental case generates larger number of liquid drops compared to the maritime case due to the enhancement of background aerosol load, whereas the consumption of water vapor weakens the deposition nucleation and growth of ice particles, leading to reduced number and mass concentration of ice crystals and graupel particles. Aerosols transported either in the boundary layer (0-2. km) or mid-troposphere (2-6. km) can change the characteristics of cloud and precipitation. For continental cases, these transported aerosol particles lead to an increase in the drop number concentration, but decrease in the updraft velocity during cloud development stage, the maximum effective radius of drops, the maximum number concentration of ice crystals and graupel particles, and suppress the ground rainfall. The rainfall shows high sensitivity to changes in microphysics due to enhanced aerosol load. Aerosols transported in the boundary layer have more significant effect on the cloud microphysics and precipitation than that at mid-troposphere. For maritime cases, the transported aerosol particles show similar enhancement effect on number concentration of drops with longer cloud lifetime and hence delayed and suppressed precipitation occurring when aerosol concentration is enhanced in boundary layer, whereas the precipitation increases when aerosols transport in mid-troposphere due to larger maximum effective radius of drops, contributed by melting of larger graupel particles with efficient accretion growth. The results from marine cases and different initial aerosol concentration of continental cases show that the effect of transport of aerosols exhibits more notable effects for lower initial aerosol concentrations. The influence of the environmental wind shear has not been included in the present study. © 2011 Elsevier B.V.