Freezing Drizzle and Supercooled Large Droplet (SLD) Formation in Stably Stratified Layer Clouds: Results from Detailed Microphysical Simulations and Implications for Aircraft Icing

In this paper we evaluate the role of : 1) low Cloud Condensation Nuclei (CCN) conditions, and 2) preferred radiative cooling of large cloud drops as compared to small cloud drops near cloud top, on cloud droplet broadening and subsequent drizzle formation in stably stratified layer clouds. The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical model implemented into the NCAR/Penn State MM5 mesoscale model. The height and width of the two-dimensional mountain was designed to produce an updraft pattern with extent and magnitude similar to observed freezing drizzle cases. In addition, we also evaluate two different methods of ice initiation on freezing drizzle formation, and the role of cloud depth and cloud top temperature on the formation of freezing drizzle.