Parameterizing ice nucleation rates using contact angle and activation energy derived from laboratory data
The rate of ice nucleation in clouds is not eas-ily determined and large discrepancies exist between model predictions and actual ice crystal concentration measured in clouds. In an effort to improve the parameterization of ice nucleating in cloud models, we investigate the rate of het-erogeneous ice nucleation under specific ambient conditions by knowing the sizes as well as two thermodynamic param-eters of the ice nuclei – contact angle and activation energy. Laboratory data of freezing and deposition nucleation modes were analyzed to derive inversely the two thermodynamic pa-rameters for a variety of ice nuclei, including mineral dusts, bacteria, pollens, and soot particles. The analysis considered the Zeldovich factor for the adjustment of ice germ forma-tion, as well as the solute and curvature effects on surface tension; the latter effects have strong influence on the con-tact angle. Contact angle turns out to be a more important factor than the activation energy in discriminating the nucle-ation capabilities of various ice nuclei species. By extracting these thermodynamic parameters, laboratory results can be converted into a formulation that follows classical nucleation theory, which then has the flexibility of incorporating fac-tors such as the solute effect and curvature effect that were not considered in the experiments. Due to various uncertain-ties, contact angle and activation energy derived in this study should be regarded as " apparent " thermodynamics parame-ters.