Quantifying ice nucleation by silver iodide aerosols

Abstract

Laboratory studies of artificial ice nucleating aerosols used for weather modification by cloud seeding have generally been inadequate for describing their complex action in the varied temperature, pressure humidity, and cloud conditions that can be encountered in the atmosphere. This study provides a quantitative framework for predicting ice formation by aerosol particles based on experiments which specifically target currently accepted mechanisms by which ice can form. A physical system for reproducing realistic atmospheric cloud conditions, the Colorado State University dynamic (controlled expansion) cloud chamber, is described. Physical simulations of adiabatic cloud formation and growth are demonstrated. Methodologies were also formulated to use the cloud chamber and other available instrumentation to isolate the action of ice nucleating aerosols by accepted primary ice nucleation modes. These methods were applied to the study of two chemically different silver iodide (AgI) - type aerosols, generated in the exact form in which they have been used for seeding natural clouds . The results were formulated as a function of basic thermodynamic quantities and particle size. An available one dimensional numerical cloud model with microphysical detail was adapted for the equivalent simulation of experiments performed in the cloud chamber. The model was utilized as a diagnostic tool to estimate water supersaturation in association with experiments and it was used for comparison of the predictions of new ice nucleus formulations with observations from generalized seeding simulations conducted in the cloud chamber. The nucleant and mode-specific formulations represent vast improvements compared to available formulations for "pure" AgI. The general implications of these new results were tested by using the model to simulate a few common seeding situations in the atmosphere, and the transferability of results was evaluated by modeling two actual seeding experiment s conducted in summertime cumuli . Within the limitations of the cloud model used, agreement with the atmospheric results was very good. The results of this study should be most useful for designing standard and better methods for the quantitative, study of ice nucleation by artificially generated and natural aerosols, and for evaluating cloud seeding methodologies and potential seeding effects using more complex microphysical - dynamic cloud models.