New model for the vapor growth of hexagonal ice crystals in the atmosphere

We present a new microphysical model for the vapor growth and aspect ratio evolution of faceted, hexagonal ice crystals in the atmosphere. Our model i s based on a novel, efficient numerical method for solving: Laplace's equat ion for steady state diffusion on the surface of a three-dimensional hexago nal prism, and also takes into account the surface kinetic processes of cry stal growth. We do not include ventilation, so our model is limited to stat ionary crystals or falling crystals smaller than 100 mum. We calculate a se lf-consistent solution for the distribution of the supersaturation and the condensation coefficient on each crystal face, for several different assump tions regarding the crystal growth mechanism and ice surface properties. We use this model to predict the aspect ratios expected for faceted ice cryst als over a range of temperatures and supersaturations, as well as to estima te the conditions for which faceted growth becomes unstable and the crystal s become hollowed or dendritic, We compare these predictions to observed fe atures of ice cloud crystals to infer some microphysical characteristics of ice crystals and their temperature dependence. We also compare our predict ed mass growth rates with those of the capacitance model for spheres and el lipsoids to look at the effects of shape and surface kinetics. Finally, we insert the single-particle code into a simple parcel cloud model to investi gate the feedbacks between crystal surface kinetics, shape? and the thermod ynamic properties of clouds.