New model for the vapor growth of hexagonal ice crystals in the atmosphere (original) (raw)

2001, Journal of Geophysical Research

We present a new microphysical model for the vapor growth and aspect ratio evolution of faceted, hexagonal ice crystals in the atmosphere. Our model is based on a novel, efficient numerical method for solving Laplace's equation for steady state diffusion on the surface of a three-dimensional hexagonal prism, and also takes into account the surface kinetic processes of crystal growth. We do not include ventilation, so our model is limited to stationary crystals or falling crystals smaller than 100/•rn. We calculate a self-consistent solution for the distribution of the supersaturation and the condensation coe•cient on each crystal face, for several different assumptions regarding the crystal growth mechanism and ice surface properties. We use this model to predict the aspect ratios expected for faceted ice crystals over a range of temperatures and supersaturations, as well as to estimate the conditions for which faceted growth becomes unstable and the crystals become hollowed or dendritic. We compare these predictions to observed features of ice cloud crystals to infer some microphysical characteristics of ice crystals and their temperature dependence. We also compare our predicted mass growth rates with those of the capacitance model for spheres and ellipsoids to look at the effects of shape and surface kinetics. Finally, we insert the single-particle code into a simple parcel cloud model to investigate the feedbacks between crystal surface kinetics, shape, and the thermodynamic properties of clouds. 1. Introduction' Habit Evolution in Vapor-Grown Ice Crystals The shapes of ice crystals in the atmosphere, and the physical processes that determine them, have long been subjects of great interest, not only because of their importance for radiative transfer [Vogelmann and Ackerrnan, 1995; Baker, 1997] and cloud particle dynamics [Jensen et al., 1994b], but also because of their inherent beauty and complexity [Frank, 19821. The primary growth habit, or shape, of a faceted ice crystal is a hexagonal prism with two basal faces and six prism faces. The aspect ratio is defined as I'-c/a, where 2c is the height, or distance between the basal faces, and 2a is the width, or distance between opposite prism faces.