Ice clouds are important in regulating climate, in producing precipitation, and in providing sites for chemical reactions. Ice nucleation occurs in the atmosphere in two primary ways: through homogeneous freezing of liquid solution droplets and through heterogeneous interaction on a particulate nucleus. Homogeneous ice nucleation can be predicted by theory, while heterogeneous ice nucleation is much more complex. There are several mechanisms by which a heterogeneous ice nucleus may initiate ice formation, including deposition, contact, immersion, and condensation. The types and numbers of particles that act as ice nuclei under various conditions in the atmosphere are very poorly characterized. Changes to particulate loadings by anthropogenic activity, whether direct emissions or through land-use and precipitation changes, are likely to influence characteristics of ice and mixed-phase clouds. However, these "aerosol indirect effects" on ice clouds cannot be accurately predicted until we understand the physics of ice formation.
Prior measurements provide a beginning, but much more information is needed. Further comprehension of ice nucleation will require a combination of field experiments, laboratory studies, and modeling. The Principal Investigator will undertake an experimental approach involving direct impaction and evaporation of ice crystals to evaluate the properties of their non-volatile nuclei. This information will be integrated with that from a series of complimentary measurements by other investigators. The primary objective in this work is to better understand ice nucleation in relatively simple clouds in the Ice in Clouds--Layer (ICE-L) experiment. Problems associated with sampling ice from aircraft, and their potential influence on collected data, will be studied using modeling calculations and field measurements. This will add to the intellectual merit of the basic ice nucleation studies.
There are several aspects of this work that are important to the broader community. First, there is the primary importance of understanding how aerosol production due to human activity (whether directly or indirectly through land-use changes) influences precipitation development and the radiative impact of clouds on the earth's climate. Second, there is the positive impact on human resources. A strong educational component is planned for the ICE-L experiment, with new technology allowing real-time data to be integrated into university meteorology classes. Additionally, an undergraduate student and graduate students will work with the PI and colleagues to analyze and interpret the data collected. Additionally, this project will yield improved understanding of ice sampling by aircraft and its implications for past and future measurements in numerous other experiments.
