Parameterizing ice nucleation rates using contact angle and activation energy derived from laboratory data

[摘要] The rate of ice nucleation in clouds is not easily determined and largediscrepancies exist between model predictions and actual ice crystalconcentration measured in clouds. In an effort to improve theparameterization of ice nucleating in cloud models, we investigate the rateof heterogeneous ice nucleation under specific ambient conditions by knowingthe sizes as well as two thermodynamic parameters of the ice nuclei –contact angle and activation energy. Laboratory data of freezing anddeposition nucleation modes were analyzed to derive inversely the twothermodynamic parameters for a variety of ice nuclei, including mineraldusts, bacteria, pollens, and soot particles. The analysis considered theZeldovich factor for the adjustment of ice germ formation, as well as thesolute and curvature effects on surface tension; the latter effects havestrong influence on the contact angle. Contact angle turns out to be a moreimportant factor than the activation energy in discriminating the nucleationcapabilities of various ice nuclei species. By extracting thesethermodynamic parameters, laboratory results can be converted into aformulation that follows classical nucleation theory, which then has theflexibility of incorporating factors such as the solute effect and curvatureeffect that were not considered in the experiments. Due to variousuncertainties, contact angle and activation energy derived in this studyshould be regarded as "apparent" thermodynamics parameters.

[学科分类] 大气科学