Abstract
This chapter studies numerical modeling to help define the potential roles of homogeneous freezing of soluble cloud condensation nuclei (CCN) aerosols and heterogeneous freezing by ice nuclei in determining the composition of cirrus clouds. The apparent sensitivity of cirrus cloud properties to changes in the abundance and sizes of heterogeneous IN is much greater than that for soluble CCN. Consideration of existing data on upper tropospheric aerosol characteristics and cirrus cloud microphysics also suggest that the heterogeneous process dominates. It describes laboratory and field studies underway to address uncertainties in understanding the two ice formation processes. It shows that both the magnitude of ice formation and the relative humidity needed for ice formation are typically overestimated by the homogeneous freezing process compared to available observations from cirrus clouds. These discrepancies do not appear explainable by uncertainties in aerosol composition and abundance and/or a poor knowledge of the freezing behavior of small solution droplets in upper tropospheric conditions. However, it was demonstrated that the presence of insoluble aerosol components acting as heterogeneous ice nuclei could explain the concentrations of ice crystals formed in cirrus and the observed relative humidities of their formation. Laboratory and field studies relevant to these issues are in progress.
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