Droplet nucleation and growth in orographic clouds in relation to the aerosol population

The formation and development of orographic clouds was studied in a field e xperiment comprising several measurement sites at a mountain ridge. The inf luence of the aerosol population present on the cloud microstructure was st udied in relation to the dynamics in the cloud formation. Droplet nucleatio n scavenging was investigated by the introduction of a non-dimensional part icle diameter related to the process, and it was found that the scavenging rose rapidly in a relatively narrow particle size interval. The size depend ency of the scavenging could partly be explained by external mixture of the aerosol. The large particles in the cloud interstitial aerosol was found t o be of a chemical nature which allows for only a very weak uptake of water , implying that the chemical composition of these particles rather than ent rainment of dry air prevented the droplet nucleation. The aerosol particle number concentration was found to strongly influence the cloud microstructu re. Droplet number concentrations up to approximately 2000 cm(-3) were obse rved together with a substantially reduced effective droplet diameter. The observed effect of elevated particle number concentrations in orographic cl ouds was generalised to the climatologically more important stratiform clou ds by the use of a cloud model. It was found that the microstructure of str atiform clouds was strongly dependent on the aerosol population present as well on the dynamics in the cloud formation. (C) 1999 Elsevier Science B.V. All rights reserved.