Modeling the growth of ice from the vapor: approximations, problems, and possible improvements

 

Authors

Jerry Y. Harrington — Pennsylvania State University
Kara Jo Sulia — University of Albany
Chengzhu Zhang — Lawrence Livermore National Laboratory
Hugh Clifton Morrison — University Corporation for Atmospheric Research

Category

Cloud Properties

Description

Ice crystal vapor growth is a physical process that is critical for the evolution of the vast majority of atmospheric clouds. Despite its importance, numerical cloud models have primarily used a single method for approximating growing ice from the vapor, that being the capacitance analogy. In this analogy, a crystal is assumed to have vapor fields that can be approximated as the electrostatic potential surrounding a capacitor. This approximation is typically combined with mass-size relations derived from in situ cloud data to close the growth equations. These two approximations (capacitance and mass-size relations) are not only used in cloud models but are also routinely used to interpret laboratory measurements. Nevertheless, these approximations can produce large inaccuracies in the modeling of ice crystal growth, particularly at certain temperatures and supersaturations, even for idealized crystal shapes. One of the major shortcomings of this method is that the surface boundary condition requires the vapor density to be constant, which leads to an aspect ratio that does not vary in time. We use a combination of prior laboratory data, theoretical analysis, and detailed ice growth models to expose the temperature and supersaturations where these approximations are appropriate, where they break down and why, and possible methods to correct for the inaccuracies. Our analysis leads to a unified ice growth model that provides improved growth estimates for simple crystal shapes and can be used to interpret laboratory data and for growing ice in cloud models.