NEW THEORETICAL FRAMEWORK FOR STUDIES OF VAPOR GROWTH AND SUBLIMATIONOF SMALL ICE CRYSTALS IN THE ATMOSPHERE

We present a new microscopic model of growth and sublimation (g/s) of ice crystals in the atmosphere. This model is based on the assumption that the flux of vapor to the crystal surface is uniform over each fla t crystal face. It thus differs fundamentally from the standard ''capa citance'' model for crystal growth, in which the mixing ratio is assum ed uniform at the surface. In the new model the surface influence on g rowth is calculated self-consistently in terms of local environmental conditions, again differing sharply from the standard models in which this influence is either ignored or assigned a uniform, externally pre scribed value. The new model leads to predictions of the evolution of ice crystal shape as well as mass. We find that predicted g/s rates ar e generally smaller than those predicted by the earlier models. The ge neral trends both in g/s rates and in crystal hollowing predicted by t he model are consistent with field and laboratory observations. The va lues of certain surface parameters needed for application of our model must be found from experiment. We review and compare the relevant lab oratory experiments on ice crystal g/s rates and show their lack of mu tual consistency. Therefore the surface parameters inferred from these experiments are necessarily uncertain. We show that the surface param eter values can be inferred from observations of crystal hollowing, si nce our model allows the prediction of environmental conditions at whi ch hollowing should occur.