Effects of stratocumulus clouds on aerosols in the maritime boundary layer

Abstract

Evolution of the aerosol size distribution, drop salinity and the rate of aerosol scavenging in drizzling maritime stratocumulus clouds are analyzed using a Lagrangian model of a cloud topped boundary layer in which 1344 adjacent Lagrangian parcels move within a turbulent like flow with statistical parameters derived from observations made during the research flight RF07 in the DYCOMS-II field experiment. The effects of cloud microphysical processes on the evolution of aerosol size distribution are investigated. A significant difference between aerosol particle sizes and droplet salinity in cloud updrafts and downdrafts was found and analyzed. Collisions of drizzle and cloud droplets lead to formation of large aerosol particles with radii up to 3.3 μm, which are 2 μm larger than the maximum aerosol radius in the initial aerosol spectrum. However, the concentration of such large aerosols is very low, so that it is concluded that stratocumulus clouds with comparatively weak drizzle fluxes cannot be a source of giant cloud condensation nuclei. The salinity distribution of droplets and haze particles in the cloud was analyzed. Differences in the salinity distribution were found in ascending and descending parcels. These results were compared to the existing conceptual model of salinity distribution within drops.