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 experiment comprising several measurement sites at a mountain ridge. The influence of the aerosol population present on the cloud microstructure was studied in relation to the dynamics in the cloud formation. Droplet nucleation scavenging was investigated by the introduction of a non-dimensional particle diameter related to the process, and it was found that the scavenging rose rapidly in a relatively narrow particle size interval. The size dependency of the scavenging could partly be explained by external mixture of the aerosol. The large particles in the cloud interstitial aerosol was found to 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 entrainment of dry air prevented the droplet nucleation. The aerosol particle number concentration was found to strongly influence the cloud microstructure. Droplet number concentrations up to approximately 2000 cm(-3) were observed together with a substantially reduced effective droplet diameter. The observed effect of elevated particle number concentrations in orographic clouds was generalised to the climatologically more important stratiform clouds by the use of a cloud model. It was found that the microstructure of stratiform 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.