Cloud base aerosol characteristics and implications for cloud microphysics 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 characterize the variety of aerosol regimes in the QCSRP domain and subsequent influences on precipitation formation processes.

On each flight in the QCSRP 2008-2009 field program, a research aircraft took standard measurements of cloud base aerosol and cloud condensation nuclei (CCN), as well as the initial drop size distribution (DSD) in the cloud above cloud base. In addition, some growing clouds were also probed at freezing and mixed-phase temperatures. These basic measurements allowed us to build a climatology of cloud base aerosol conditions and relate them to the initial DSDs and subsequent microphysics in the clouds.

Our observations indicate that the domain of the QCSRP experienced great variations in sub-cloud aerosol conditions, even over the course of a few days, from more continental to more maritime in nature. HYSPLIT back trajectories for each cloud base measurement were run and classified into regimes with similar trajectory characteristics that could influence the aerosol properties (i.e., time spent over land or ocean, proximity to Brisbane, influence of fires, etc). The aerosol measurements in the maritime regime were cleaner and less variable than those in the continental regime. Surprisingly our measurements of initial cloud droplet concentrations were similar between the regimes, however the corresponding cloud DSDs indicate that the continental regime exhibited consistently narrow droplet spectra, while the maritime regime exhibited slightly broader droplet spectra. Environmental characteristics of each regime were also investigated and have shown that each regime had similar cloud base temperatures and instability, although the maritime regime had lower cloud base heights. Such factors are being assessed to explain the similar droplet concentrations.

This paper presents the characteristics of cloud base aerosol and CCN regimes observed during the QCSRP field program and how they relate to the cloud droplet spectra under each observed regime to understand the influence of the cloud base aerosol on the cloud microphysics.