An Experimental Study of Ice Crystal Growth and Evaporation

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The physical mechanisms by which ice crystals in the atmosphere grow by vapor deposition and shrink by evaporation are complicated and still uncertain. Yet the exchange of vapor between a crystal and its environment is central for ice-phase evolution. Vapor deposition establishes the initial shapes, masses, and fallspeeds of the particles and hence the time needed for interactions with other cloud particles to become important and for precipitation to form. Evaporation of ice crystals determines the lifetime of cirrus clouds in their decaying stage. This project is an experimental study of the growth and evaporation of individual ice crystals in controlled, laboratory conditions, over a range of temperature, pressure, and humidity appropriate for conditions in the free atmosphere. The experiments employ the method of electrodynamic balance for levitating individual particles in a controlled environment. In addition, there is provision for moving the air through the chamber to study the effects of ventilation on crystal growth. The research will shed new light on vapor exchange at a crystal interface and provide the information needed to improve the parameterization of crystal growth in numerical models of cloud development and the transport of chemical species in cloudy atmospheres.