Experimental Investigations of Ice in Supercooled Clouds. Part 1: System Description and Growth of Ice by Vapor Deposition

Naihui Song Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Dennis Lamb Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Abstract

A continuous flow cloud chamber system was constructed for studies of microphysical and chemical processes in supercooled clouds. An important feature of the cloud chamber was the generation of the components of the supercooled clouds external to the main wind tunnel where crystal growth took place. A population of ice crystals was allowed to grow to relatively large sizes in a steady-state environment with specially imposed flow gradients. Thus, microphysical and chemical processes in supercooled clouds could be simulated under realistic and controlled conditions.

The cloud chamber was utilized here to study ice crystal growth by vapor deposition over a broad range of supercooled cloud conditions. The crystal habit, size, and mass were measured for growth times up to 4 min, temperatures between −6° and −16°C, and liquid water contents from 0.3 to 6 g m−3. The data indicate that the liquid water content enhances the crystal vapor growth rates from less than 2% to almost 20% per unit increase in liquid water content (g m−3), depending on the crystal habit. The growth enhancement that arises from the presence of supercooled liquid water is explained in terms of the transient “vapor flush” effect from the repeated close passage of supercooled water droplets during crystal sedimentation.

Abstract

A continuous flow cloud chamber system was constructed for studies of microphysical and chemical processes in supercooled clouds. An important feature of the cloud chamber was the generation of the components of the supercooled clouds external to the main wind tunnel where crystal growth took place. A population of ice crystals was allowed to grow to relatively large sizes in a steady-state environment with specially imposed flow gradients. Thus, microphysical and chemical processes in supercooled clouds could be simulated under realistic and controlled conditions.

The cloud chamber was utilized here to study ice crystal growth by vapor deposition over a broad range of supercooled cloud conditions. The crystal habit, size, and mass were measured for growth times up to 4 min, temperatures between −6° and −16°C, and liquid water contents from 0.3 to 6 g m−3. The data indicate that the liquid water content enhances the crystal vapor growth rates from less than 2% to almost 20% per unit increase in liquid water content (g m−3), depending on the crystal habit. The growth enhancement that arises from the presence of supercooled liquid water is explained in terms of the transient “vapor flush” effect from the repeated close passage of supercooled water droplets during crystal sedimentation.

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