THE QUEENSLAND CLOUD SEEDING RESEArcH PrOGrAM BY SARAH A. TE ss ENDORF , ROELOF T. BRUINtJEs, COUR t NEY WEEK s , JAME s W. WIL s ON , CHARLE s A. KNIGH t , RI t A D. ROBER ts , JU st IN R. PE t ER , Sc O tt COLLI s , PE t ER R. BU s E c K , EVELYN FRENEY , MI c HAEL DIXON , MA tt HE w PO c ERNI c H , KYOKO IKEDA , DUN c AN AXI s A , ERI c NEL s ON , PE t ER T. MAY , HARALD RI c H t ER , St UAR t PIKE t H , ROELOF P. BURGER , LOUI s E WIL s ON , St EVEN T. SIEM s , MI c HAEL MAN t ON , ROGER C. St ONE , Ac A c IA PE p LER , DON R. COLLIN s , V. N. BRINGI , M. THURAI , LYNNE TURNER , AND DAVID McRAE An innovative approach to studying the effects of cloud seeding on precipitation is to focus on understanding the natural variability of precipitation and the microphysical responses to aerosol. or much of the last decade, most of eastern and southern Australia was under severe F drought conditions due to mounting rain- fall deficiencies. The Bureau of Meteorology (2011) reported that the lack of late winter and spring rains in 2006 made matters worse; in fact, that period was the third driest August– November period across Australia based on the historical record dating from 1900. Needless to say, the availability of water became a com- munity concern, such that water restrictions were commonplace and conservation pro- grams became a top priority. Plans to build desalination plants in all the major coastal cities of Australia were enacted. In addition, cloud seeding was considered by a number of communities. Australia has a long history of cloud seeding (Kraus and Squires 1947; Smith et al. 1963, 1979; Warburton and Wetzel 1992; Ryan and King 1997; Huggins et al. 2008; Morrison et al. 2009, 2010), although most of these efforts have been focused on glaciogenic seeding techniques (using silver iodide) over the mountains in southeastern Australia and Tasmania. In late 2006 the Queensland government FIG . 1. Map of southeast Queensland region targeted for the decided to establish the Queensland Cloud QCSRP field effort and associated facilities and landmarks. Seeding Research Program (QCSRP) in The 30º beam crossing angle dual-Doppler lobes are overlaid southeastern Queensland (Fig. 1) to determine in black. [Courtesy of Kevin Sampson, NCAR.] AMERICAN METEOROLOGICAL SOCIETY JaNUaRY 2012 | 75 Unauthenticated | Downloaded 02/04/21 12:33 AM UTC the feasibility of cloud seeding as a component been inundated by catastrophic flooding. While this of its long-term water management strategy. The did not affect the purpose and need for the QCSRP at Queensland water management strategy recognizes the time, it is pertinent to point out that the dramatic the need for a broad portfolio of water sources to climate changes this region of the world is undergoing account for the uncertainties and costs associated has an extraordinary impact on the communities in with each type of source. While it was not expected the region, and thus developing methods to mitigate that cloud seeding would restore southeastern these impacts is crucial to the sustainability of these Queensland’s water supply levels to predrought communities. values, it seemed valuable to determine whether certain types of seeding techniques might impact EXperImentaL strateGY. The concep- rainfall and water supplies in the region and whether tual model of hygroscopic seeding is based on the that impact could be quantified. The project was principle that if you can enhance the collision and developed as a collaboration between a number of coalescence process, then it will yield more rainfall institutions from Australia, the United States, and at the ground, thereby improving the precipitation South Africa, and included field measurements over efficiency of a cloud [for more background, see re- the course of two wet seasons. A two-pronged ap- views by Cotton (1982), Bruintjes (1999), and Cotton proach was taken to a) conduct a randomized cloud (2009)]. Thus, clouds are seeded with hygroscopic seeding experiment and b) assemble state-of-the- materials (particles that take on water easily, such as art instrumentation systems to collect data on the salts) in the updraft region of the cloud just below the complete physical process from cloud formation to cloud base. These particles are then carried into the seeding to precipitation. cloud by the updraft where water vapor condenses on Meanwhile, in stark contrast to the drought them to form additional liquid cloud droplets, whose and since the conclusion of the QCSRP, the state of size depends on the size of the hygroscopic particles Queensland has undergone a dramatic climatic shift, introduced into the cloud. Adding hygroscopic par- having had several rainy seasons and recently having ticles of larger sizes should help enhance collision and coalescence processes and convert more of the cloud water to rainfall. AFFILIatIOns: TE ss ENDORF , BRUINtJEs, WEEK s , J. W. WIL s ON , There are many complicating factors, however, KNIGH t , ROBER ts , DIXON , PO c ERNI c H , IKEDA , AXI s A , AND NEL s ON — behind trying to assess the impact of hygroscopic National Center for Atmospheric Research, Boulder, Colorado; cloud seeding, or even pollution aerosols, on clouds PE t ER —Queensland Climate Change Centre of Excellence, Indooroopilly, Queensland, and Monash University, Melbourne, and ultimately on rainfall. In the atmosphere, we Victoria, Australia; COLLI s AND MAY —Centre for Australian do not have control cases as one would create in a Weather and Climate Research, Melbourne, Victoria, Australia; lab setting, and thus such atmospheric experiments RI c H t ER —Bureau of Meteorology Training Centre, Melbourne, always fall short of being able to isolate causality of Victoria, Australia; BU s E c K AND FRENEY —Arizona State University, such aerosol effects. Statistical analysis has often Tempe, Arizona; PIKE t H AND BURGER —University of the been used in cloud seeding assessment studies (e.g., Witwatersrand, Johannesburg, South Africa; L. WIL s ON , SIEM s , AND Mather et al. 1997; Bruintjes 1999; Silverman 2003), MAN t ON —Monash University, Melbourne, Victoria, Australia; St ONE —University of Southern Queensland, Toowoomba, in which clouds are seeded (or not) based on a set Queensland, Australia; PE p LER —Macquarie University, Sydney, of predetermined randomized decision cards. Once New South Wales, Australia; COLLIN s —Texas A&M University, the seeding aircraft finds a cloud that meets a set of College Station, Texas; BRINGI AND THURAI —Colorado State basic criteria for the randomized experiment, a case University, Fort Collins, Colorado; TURNER AND McRAE — is declared, the next sequential randomized decision Queensland Climate Change Centre of Excellence, Dutton Park, card is opened, and the cloud is then either seeded (or Queensland, Australia not) based on the predetermined action. The criteria COrrespOndInG aUtHOr: Sarah A. Tessendorf, Research Applications Laboratory, NCAR, P.O. Box 3000, Boulder, CO used in the QCSRP randomized experiment to select 80307-3000 a cloud was that a cloud needed to have a rain-free E-mail: [email protected] cloud base at least 2 km in diameter, with an updraft that was detectable by the pilot of roughly 2 m s−1 or The abstract for this article can be found in this issue, following the table of contents. greater. This builds up a sample of cases that ideally DOI:10.1175 / BAMS - D -11- 0 0 060.1 are all similar in nature, with roughly half having been seeded, and statistical tests can be performed In final form 19 July 2011 ©2012 American Meteorological Society on various characteristics of the observed cells. These characteristics, such as precipitation flux, rain mass, 76 | JaNUaRY 2012 Unauthenticated | Downloaded 02/04/21 12:33 AM UTC and storm duration, are traditionally derived from single-polarization1 radar echoes of the randomized cases.2 However, this sta- tistical approach is usually limited by the number of randomized cases that can be declared and measured by radar (and/or aircraft) for a given limited time frame experiment as well as by the lack of physical ob- servations to substantiate FIG . 2. Facilities operated during the QCSRP included (a) the CP2 radar and explain any statistical (photo depicts the S-band and X-band antennae), and (b) the SAWS Aero relationships that may be Commander research/seeding aircraft depicted in flight on a research mis- found. Furthermore, in sion. [Photos courtesy Scott Collis, CAWCR.] regions with considerable natural variability, it is even more challenging to in getting a large enough sample of cases in random- obtain a large enough sample to achieve statistical ized seeding statistical experiments. This is especially significance because of the underlying “noise” that true in regions with great natural variability, such as results from that variability. in the coastal region of southeast Queensland. Thus, For the first objective of our two-pronged ap- the rationale for using physical measurements to first proach in the QCSRP, we conducted a traditional understand the natural variability, perhaps even prior randomized cloud seeding experiment (Mather et al. to conducting a randomized seeding experiment, has 1997). Over the course of two seasons, we performed been an important outcome from this program. randomized cloud seeding in 127 clouds and then For the second objective of our two-pronged a statistical analysis was performed using single- approach, we focused on gaining a physical under- polarization, radar-derived quantities to character- standing of the effects of both ambient aerosols ize the rainfall and behavior of each randomized and seeding material on precipitation formation in cloud. Despite collecting that reasonable sample of southeast Queensland clouds.