Identification of a Seeding Signature in Texas Using Multi-Spectral Satellite Imagery
AbstractThe method of Rosenfeld and Lensky (1998a) to infer cloud microstructure using multi-spectral, AVHRR, satellite imagery is being used in Texas to determine whether operational cloud seeding is producing recognizable changes in cloud microstructure. The seeding signature is expected to take the form of ice formation at relatively warm temperatures (i.e., > -10°C) as is required by the conceptual model guiding the seeding programs (Rosenfeld and Woodley, 1993). Two cases are presented. The first on 10 August 1999 involved seeding with ejectable silver iodide flares near cloud top and the second on 28 August 1999 involved seeding in updraft regions near cloud base with silver iodide generators and flares affixed to the wings of the seeder aircraft. Both apparently produced seeding signatures. The analysis of the first case suggests anomalous glaciation was achieved in the seeded clouds at -13°C whereas it was achieved at about -19°C to -21°C in the clouds in other areas. These differences are consistent with an effect of seeding in that the seeded clouds glaciated at a warmer temperature than the other clouds. The second case involved operational seeding in the target area of the West Texas Weather Modification Association. Base silver iodide seeding of a short cumulonimbus line began 56 min before the pass of the monitoring satellite when the line was in its formative stages. The maximum updraft at cloud base was estimated at 3,000 ft/min along the line just before the satellite overpass. The analysis revealed a strong apparent seeding signature in the cloud line. The signature took the form of complete glaciation at about -9°C, whereas other similar clouds in the region glaciated between -20°C to -30°C. This appears to constitute a strong glaciation signature resulting from the seeding at cloud base. The detection of apparent seeding signatures in Texas is an important development. It indicates that the top and base AgI seedings are producing ice at temperatures around -10°C. According to the conceptual model guiding the operational seeding programs in Texas, this should be the first step in the process leading to more rainfall. The TEXARC research program has already established that the seeding agents used in Texas produce more ice in individual clouds, but such glaciation had never been documented on a scale that can be seen by satellite sensors until this study. There is now greater reason to take the next step and look for increases in rainfall as a result of the seeding.
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