Laboratory Studies of Ice Initiation Mechanisms in Cumulus Clouds

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K. Beard/Abstract 9505298 The effectiveness of evaporation ice nuclei (EIN) will bestudied in laboratory simulations of cloud drop formation, charging, evaporation and reentrainment in cumulus clouds developing through the freezing level. Such nuclei are potentially effective ice nuclei because of organic and inorganic materials in cloud drops. Charge accumulation at cloud top is a source of ice nucleus enhancement by electrofreezing. For the proposed nuclei continuous-flow experiment, cloud drops will be obtained from cloud condensation nuclei (CCN) using laboratory mixing and cloud chambers and the Illinois State Water Survey continuous flow diffusion-chamber system, and from ultrasonic nebulization of aloud water. Cloud drops will be charged using a corona device and evaporated by mixing with dry air. The activation (threshold) temperature of the resultant nuclei will be measured in the presence of condensing, supercooled drops that act as condensation freezing nuclei as a function of temperature. The EIN electrofreezing mode will be evaluated by measurements at several charge levels, and the EIN evaporation-freezing mode will be deduced from measurements using EIN particle generated both warmer and colder than freezing. A separate set of nuclei activity measurements will be made to distinguish contact-freezing from condensation-freezing by using an evaporating cloud in the freezing chamber, so that contact of nuclei is enhanced by phoresis and condensation-freezing is suppressed. A complete set of experiments is planned using nebulized cloud water that has been collected during research flights. Another complete set of experiments will be conducted with cloud-water formed on CCN using the continuous flow diffusion system with corresponding chemical analysis. The nuclei experiment and the diffusion system will be transportable so that measurements can be made at various locations in the future.