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 study is to assess the variety of aerosol regimes, as well as the frequency of occurrence for each regime, in the QCSRP domain and investigate potential influences on precipitation formation processes.

Aircraft observations collected during the QCSRP 2008-2009 field season included fine through coarse mode aerosol measurements (utilizing DMA, PCASP, and FSSP instrumentation) and aerosol filter sampling to assess the composition and deliquescence of the measured aerosol. Cloud microphysical measurements included a cloud condensation nuclei (CCN) counter, and cloud droplet spectrometers and imaging probes. On each flight in the field program, the aircraft took standard measurements of cloud base aerosol and CCN, as well as the initial drop size distribution (DSD) in the cloud above cloud base. These basic measurements allowed us to build a climatology of cloud base aerosol conditions and relate them to the initial DSDs 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 (i.e., time spent over land or ocean, proximity to Brisbane). The aerosol measurements in the most maritime regime are cleaner and less variable than those in the most continental regime. Furthermore, our preliminary measurements of initial cloud DSDs in each regime suggests that the continental regime exhibits more narrow droplet spectra, while the maritime regime exhibits broader, but more variable, droplet spectra.

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