P1.36 The impact of glaciogenic cloud seeding on snowfall from winter orographic clouds

Monday, 28 June 2010
Exhibit Hall (DoubleTree by Hilton Portland)
Bart Geerts, University of Wyoming, Laramie, WY; and Q. Miao and Y. Yang

In a 2008 editorial column in Nature, it was argued that “… weather modification is one of those areas in which science can have an immediate and obvious benefit for society”. Cloud seeding probably has been the most widely practiced method of advertent weather modification for the last few decades. It is remarkable that notwithstanding all the data collected and the stronger experimental control than in typical research on cloud and precipitation processes, the effectiveness of cloud seeding in enhancing precipitation remains uncertain. Numerous statistical studies have been conducted to assess changes in surface precipitation, with mixed and questionable results. The level of noise in natural systems compared to the magnitude of the signal makes verification of precipitation enhancement extremely difficult. Survey reports point to the need for field measurements that document the cloud microphysical “chain of events” that lead to an alteration of surface precipitation. This talk uses data from an airborne vertically-pointing mm-wave Doppler radar to study the cloud microphysical effect of glaciogenic seeding of cold-season orographic clouds. Fixed flight tracks were flown downstream of ground-based silver iodide (AgI) generators in the Medicine Bow Mountains of Wyoming. Data from seven flights, each with a no-seeding period followed by a seeding period, indicate that AgI seeding significantly increased radar reflectivity and thus snowfall rate near the ground. Caution is warranted in view of the large natural variability of weather conditions and the small size of the dataset, but a partitioning of the data yields physically meaningful results that corroborate the interpretation.
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