Ice Crystal Development in a Seeded Cumulus Cloud

Clement J. Todd Atmospheric Research Group and Meteorology Research, Inc., Altadena, Calif.

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Abstract

A cumulus cloud was seeded by an aircraft flying 1780 meters below cloud base. An observation plane spiraled up through the cloud base and made replicas of the cloud particles while recording pressure altitude, temperature, mixing ratio, liquid water, and other factors. In the updraft below cloud base the temperature followed the dry adiabatic potential temperature curve. In the cloud, the temperature showed an increase above the pseudo-adiabatic wet-bulb potential temperature. This increase corresponded to the development of ice crystals and was in the amount expected from the release of heat during freezing. The ice crystals were found slightly above cloud base, −9.5C. The concentration increased in a manner which was consistent with the calibrated temperature activation curve of the AgI generator and the diffusion in the column of air above the seeding aircraft. The crystals were primarily unrimed hexagonal plates. The a axis growth fits the relationship 2a = Kal3/4 where Ka = f(T), which increased five-fold between −9.5C and −13C, a is the length along the axis, T is temperature, and t is time.

Abstract

A cumulus cloud was seeded by an aircraft flying 1780 meters below cloud base. An observation plane spiraled up through the cloud base and made replicas of the cloud particles while recording pressure altitude, temperature, mixing ratio, liquid water, and other factors. In the updraft below cloud base the temperature followed the dry adiabatic potential temperature curve. In the cloud, the temperature showed an increase above the pseudo-adiabatic wet-bulb potential temperature. This increase corresponded to the development of ice crystals and was in the amount expected from the release of heat during freezing. The ice crystals were found slightly above cloud base, −9.5C. The concentration increased in a manner which was consistent with the calibrated temperature activation curve of the AgI generator and the diffusion in the column of air above the seeding aircraft. The crystals were primarily unrimed hexagonal plates. The a axis growth fits the relationship 2a = Kal3/4 where Ka = f(T), which increased five-fold between −9.5C and −13C, a is the length along the axis, T is temperature, and t is time.

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