The influence of hygroscopic flare seeding on drop size distribution over Southeast Queensland

This study presents measurements of the initial cloud-base droplet spectra of seeded and unseeded clouds observed during the Queensland Cloud Seeding Research Program field campaign. The measurements were stratified by the aerosol regime that the cloud formed within, and then analyzed statistically as well as compared to parcel model simulations. Initial droplet spectra were obtained within 300-600 m of cloud base to identify the spectra resulting from nucleation and to avoid effects from collision-coalescence broadening processes. The goal of this study is to assess the impact of hygroscopic flare seeding on cloud-droplet growth as a first step toward understanding the effect of hygroscopic seeding on rainfall. Results indicate that initial droplet spectra are altered by seeding, such that the mean diameter and tail of drops larger than 20 mu m increased in seeded clouds within a given aerosol regime. This effect is most pronounced in the continental-influenced aerosol regime, that generally lacked coarse mode aerosols, but was also observed to occur in the maritime-influenced regime. To further investigate if the observational results were supported by theory, a parcel model was used to improve the process-level understanding of the seeding impact on the initial evolution of the droplet spectra. The simulated results qualitatively agree with the observations that noted the seeding effect is more prominent in the continental-influenced regime.