Abstract
Abstract. The rate of ice nucleation in clouds is not easily 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 heterogeneous ice nucleation under specific ambient conditions by knowing the sizes as well as two thermodynamic parameters 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 parameters 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 formation, as well as the solute and curvature effects on surface tension; the latter effects have strong influence on the contact angle. Contact angle turns out to be a more important factor than the activation energy in discriminating the nucleation 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 factors such as the solute effect and curvature effect that were not considered in the experiments. Due to various uncertainties, contact angle and activation energy derived in this study should be regarded as "apparent" thermodynamics parameters.
Highlights
Ice processes in clouds are important to precipitation formation, the hydrological cycle
Marcolli et al (2007) applied the classical nucleation theory to fit their measurement of heterogeneous freezing from Arizona test dust (ATD) in emulsified droplets, and they found that the dependence of the heterogeneous freezing temperatures on ATD concentrations could not be described by assuming a constant contact angle for all ATD particles
Comprehensive representations of heterogeneous ice nucleation rate were achieved by a combination of classical theory and experimental data, from which we derived the activation energy and contact angle for various ice nuclei that are important to the formation of ice in clouds
Summary
Ice processes in clouds are important to precipitation formation, the hydrological cycle. The above threshold concept is adopted popularly in many meteorological cloud models Their formulations present the number of ice particles, generated through deposition or condensation-freezing nucleation, as a function of either temperature or ice supersaturation, or a combination of the two variables (e.g., Fletcher, 1962; Wisner et al, 1972; Cotton et al, 1982; Lin et al, 1983; Myers et al, 1992; Reisner et al, 1998). Based on classical nucleation theory, Khvorostyanov and Curry (2004) parameterized the heterogeneous freezing of deliquescent interstitial aerosols to serve as ice nuclei and used the results to explain empirical observations Their parameterization was purely theoretical and did not utilize the information provided by laboratory measurements of ice nucleation. We obtain basic thermodynamic parameters as described by the classical theory to provide a more general and accurate parameterization of ice nucleation rate for use in cloud microphysical models
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