85th AMS Annual Meeting

Wednesday, 12 January 2005: 4:30 PM
The Southern Plains Experiment in Cloud seeding of Thunderstorms for Rain Augmentation (SPECTRA) Project: The Study and Validation of Rain-Enhancement Strategies for the Mitigation of Drought in the Southern U. S. Great Plains Region
Duncan Axisa, SOAR program, Plains, TX; and G. Bomar and W. L. Woodley
Operational cloud seeding has been conducted on a massive scale in Texas and Oklahoma since the mid 1990s, despite the fact that scientific evidence of its efficacy, as suggested by stringent “proof-of-concept” criteria proferred in recent years, has been lacking. It is generally recognized that a rigorous validation of seeding's efficacy is essential if cloud seeding projects are to avoid the kind of disillusionment and controversy that have brought about the demise of similar projects in years past. Consequently, an intensive series of laboratory studies and field investigations, funds for which were provided by the U. S. Bureau of Reclamation as part of its Weather Damage Modification Program (WDMP), was conducted in 2004 to document the role of artificial nuclei (both glaciogenic and hygroscopic) in promoting the growth of convective towers. One phase of the research conducted in the southern U. S. Great Plains in the late summer of 2004 focused on the collection, and analysis, of cloud condensation nuclei (CCN), to promote understanding of how CCN, of varying consistencies and concentrations, either facilitate or impede the rain-producing mechanisms within growing cumulonimbus clouds. Thirty-four (34) missions using a high-performance twin-engine aircraft with an assortment of atmospheric probes were performed over a 7-week period to quantify, and classify, these CCN concentrations. Among the convective towers sampled were those associated with the residue of Tropical Storm Ivan, as well as towers in a heavily populated metropolitan area impacted by the prevalence of pollutant materials. SPECTRA 2004 also entailed the statistical analysis of surface and meso-network weather data to compare the areal and temporal distribution of rainfall events within cloud seeding target areas with nearby control areas, including regions downwind of areas where seeding has been concentrated for the past six to eight years. Another facet of the research embodied the analysis of Next-Generation Radar (NEXRAD) data in specific instances of intensive seeding of convective complexes in western Texas and Oklahoma to discern differences in cloud evolution that may be attributable to the impact of seeding on the coalescence and precipitation formation processes in those clouds. Finally, one phase of SPECTRA research that had to be suspended due to unforeseen changes in late-season weather patterns along the Texas-New Mexico border focused on the use of hygroscopic materials on highly continental clouds with high concentrations of very small supercooled cloud drops. Nonetheless, one case study of seeding with milled salts was obtained, and measurements of cloud response from hygroscopic seeding were contrasted with observations taken from nearby “control” clouds.

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