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June 9, 2011 | Prior to massive flooding early in 2011, long-term drought plagued the Australian state of Queensland. As part of a broad research program on cloud seeding, NCAR researchers have been steadily crunching data from a 2008–09 field project that looked into how to make the clouds drop more rain on the region. The vast data set collected during the campaign, unprecedented for a cloud seeding research project, is yielding insight on how airborne particles might affect seeding success.
It was the severe drought in the southeast corner of Queensland that prompted the state government to explore the potential for cloud seeding. (The government’s focus has since shifted away from drought mitigation in the wake of the recent floods.) The resulting Queensland Cloud Seeding Research Program took a two-pronged approach: collecting data on physical processes from cloud and rain formation to seeding and precipitation, and carrying out a randomized cloud seeding experiment.
During field campaigns conducted near Brisbane, researchers used airborne instruments and weather radars to sample clouds and precipitation. They also undertook trials using hygroscopic seeding, a technique in which hygroscopic (water-attracting) salt particles are injected into clouds, where they function as seeds upon which large droplets can form. The larger droplets fall more quickly, colliding with smaller droplets on the way down and accelerating rainfall development.
In particular, the data is shedding light on how natural variations in Brisbane’s background aerosol affect the way that raindrops form and evolve, according to NCAR scientist Sarah Tessendorf. Aerosol particle concentrations decrease when onshore wind brings cleaner air from the ocean and increase when winds blow from the interior. Radar observations taken during the project show that the way raindrops evolve in size may vary depending on a cloud’s background aerosol regime. In some cases, it’s possible that adding seeding particles would not be helpful if the natural rain formation process is already efficient, Tessendorf says.