Use of a Sailplane to Measure Microphysical Effects of Silver Iodide Seeding in Cumulus Clouds

J. E. Dye National Center for Atmospheric Research, Boulder, Colo. 80303

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G. Langer National Center for Atmospheric Research, Boulder, Colo. 80303

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V. Toutenhoofd National Center for Atmospheric Research, Boulder, Colo. 80303

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T. W. Cannon National Center for Atmospheric Research, Boulder, Colo. 80303

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C. A. Knight National Center for Atmospheric Research, Boulder, Colo. 80303

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Abstract

Experiments have been conducted to determine the effects, within cumulus clouds, of seeding with silver iodide; such effects are determined by correlating the presence of ice particles with the presence of silver iodide. This correlation is made possible by the use of a sailplane, which has the capability of remaining in the cloud updraft and the seeded plume for relatively long periods of time. The correlation method provided a very sensitive test for distinguishing seeding effects from natural ice development. Case studies using this technique are discussed.

Measurements in one small cloud having a base at −12°C showed concentrations at −15°C in the plume of up to 400 ice crystals per liter about 4 min after seeding. Aggregation of stellar dendrites occurred for particles as small as 400–500 μm diameter and about half of the available crystals were in aggregates after about 15 min. The output efficiency of the TB-1 pyrotechnic flares used in the experiment is estimated to be at least 1011 ice particles per gram of silver iodide in the temperature range −12 to −17°C which is close to previously reported values. Significant nucleation of ice by silver iodide occurred for at least 20 min after seeding. Linear ice crystal growth rates of stellar dendrites of the order of 0.15 mm min−1 were determined from these observations.

Abstract

Experiments have been conducted to determine the effects, within cumulus clouds, of seeding with silver iodide; such effects are determined by correlating the presence of ice particles with the presence of silver iodide. This correlation is made possible by the use of a sailplane, which has the capability of remaining in the cloud updraft and the seeded plume for relatively long periods of time. The correlation method provided a very sensitive test for distinguishing seeding effects from natural ice development. Case studies using this technique are discussed.

Measurements in one small cloud having a base at −12°C showed concentrations at −15°C in the plume of up to 400 ice crystals per liter about 4 min after seeding. Aggregation of stellar dendrites occurred for particles as small as 400–500 μm diameter and about half of the available crystals were in aggregates after about 15 min. The output efficiency of the TB-1 pyrotechnic flares used in the experiment is estimated to be at least 1011 ice particles per gram of silver iodide in the temperature range −12 to −17°C which is close to previously reported values. Significant nucleation of ice by silver iodide occurred for at least 20 min after seeding. Linear ice crystal growth rates of stellar dendrites of the order of 0.15 mm min−1 were determined from these observations.

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