Applications of Fast Response Continuous SF6 Analyzers to In Situ Cloud Studies

Abstract

The airborne applications of two recently developed analyzers for sulfur hexafluoride (SF6) to investigations of cloud top mixing and cloud seeding are described. The analyzers were developed by AeroVironment (AV) and by Washington State University (WSU). Both analyzers were capable of detecting cumulus-scale plume features. The more elaborate flow control mechanism in the AV analyzer was helpful in reducing the effects of altitude on the instrument response, while the faster response and lower baseline noise level of the WSU analyzer were necessary to detect many plume features. A midcloud injection of SF6 was followed as it mixed through the tops of a small cumulus cloud. The tracer plume was first detected upshear, then mixed through the cloud top region as the cloud top began to collapse. A plume of AgI cloud seeding agent mixed with SF6 was used to investigate the activation and growth of ice particles in a stratocumulus cloud which was overseeded. The SF6 tracer and ice particle plumes remained colocated during the 45 min sampling period, except for one region of ice particles which had begun to separate from the SF6 26 min after the cloud was treated. The growth of ice was limited by water vapor diffusion into the seeding plume. The measured tracer concentrations were used to estimate the fraction of the seeding nuclei which had activated and grown to detectable sizes. A maximum fraction of 54% was observed 17.5 min after seeding. Several other applications for SF6 tracer applications are recommended.