The influence of hygroscopic flare seeding on droplet spectra in southeast Queensland

In response to a severe drought experienced over the past few years, the Queensland government sponsored the Queensland Cloud Seeding Research Program (QCSRP) in southeast Queensland near Brisbane, Australia. The goal of this paper 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.

This paper presents measurements of the initial cloud base droplet spectra of seeded and unseeded clouds observed during the QCSRP field program. Since there were no tracers released with the flares to explicitly measure spectra in seeded plumes, these spectra are compared by first stratifying them according to the aerosol regime that the cloud formed within and then analyzed statistically. By approaching this analysis in a statistical way we are studying the bulk tendencies of initial droplet spectra of seeded clouds (not necessarily in seeded plumes) compared to the bulk characteristics of the background (unseeded) spectra that have been identified within a given aerosol regime. Initial droplet spectra were obtained within 300-600 m of cloud base, in order to identify the spectra resulting from nucleation and to avoid effects from droplet broadening processes.

Results indicate that initial droplet spectra are altered by seeding, such that the mean diameter and tail of drops larger than 20 µm increased in seeded clouds among a given aerosol regime. This effect is most pronounced in the continental aerosol regime, but was also observed to occur in the maritime regime. This result suggests that the first step in the hygroscopic seeding conceptual model is occurring from hygroscopic flare seeding. Nonetheless further analysis, including numerical simulations, need to be done in order to test if this alteration to the droplet spectra is sufficient to enhance a cloud's precipitation efficiency and rainfall production.