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This proposal seeks to help improve our understanding of aircraft icing occurrence through better parameterizations of the ice microphysical cloud properties. The goal of this proposal is to create a new Global Climate Model (GCM) parameterization for Arctic ice and mixed-phase clouds, and explore possible relationship between different type size distributions (SDs), and airplane icing. The study will utilize data for different ice crystal size spectra in arctic cold clouds, and data for the corresponding airplane icing occurrences. The PI has already developed and published parameterizations for mid-latitude and tropical ice clouds (Ivanova 2001, Ivanova 2004, Mitchell and Ivanova 2006, Mitchell et al. 2008). The tropical and mid-latitude schemes predict different behavior of the SDs for the same ice water content (IWC) and temperatures. As temperature decreases beyond -35ï‚°C, the concentration of the small crystals is enhanced with the tropical scheme, but the opposite occurs with the mid-latitude scheme. This finding indicates that the microphysics properties of tropical and mid-latitude cold clouds are considerably different for the same IWC. It may also point to the different mechanisms by which convective and non-convective cold clouds are generated. Clearly, there is a need for Arctic and polar ice cloud parameterization, and for a study to explore the possibility of a relationship between the environmental conditions (temperature, IWC, supercooled liquid water content), different predicted size spectra, and aircraft icing. Cold cloud interactions with aircrafts that fly through them require knowledge of cloud microphysics. Aircrafts must be designed to fly into supercooled clouds, or they must avoid those clouds in order to prevent problems associated with airframe and engine icing. De-icing or anti-icing systems must be engineered to withstand reasonable extremes in terms of ice water content (IWC), supercooled liquid water content (LWC), ice particle size distributions (SDs), and temperature. The aircraft design or certification envelopes (FAR 25, Appendix C; Federal Aviation Administration, 1999) were developed before the advent of modern cloud physics instrumentation. In the case of ice and mixed-phase clouds, data from the new arctic field campaigns suggest that cloud temperature is one of the main parameters governing cloud microstructure, the size distributions, and ice water content affecting aircraft icing. Korolev et al. (2001) showed that the cold cloud size distributions may depend on the value of the ice particle size assumed. Parameterizations of ice particle sizes for mid-latitude and tropical ice clouds (Ivanova et al., 2001, Boudala et al., 2002; Ivanova 2004; Mitchell et al., 2008) appear in recent literature, and were implemented in the U. S. Community Climate model 3 (CCM3) Global Climate Model (GCM), and U.K. MetOffice GCM, but little is done to study high latitude cold clouds size distributions and how they may be related to the aircraft icing.