Sensitivity of freezing drizzle formation in stably stratified clouds to ice processes

This study examines the impact of ice formation and growth processes on freezing drizzle formation in stably stratified clouds. In particular we investigate the reason why freezing drizzle is rarely observed in clouds with top temperatures less than -15 °C. We also investigate the sensitivity of freezing drizzle formation to the Hallett Mossop secondary ice process (Hallet and Mossop, 1974). The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical scheme. The height and width of the two-dimensional mountain were designed to produce an updraft pattern with extent and magnitude similar to documented freezing drizzle cases. The simulations show that: (i) drizzle formation is very sensitive to the ice crystal concentration, with a significant reduction in the area over which drizzle forms and the maximum drizzle water content as the cloud top temperature decreases below -10 °C, and (ii) secondary ice crystal formation has a significant effect on drizzle formation at cloud top temperatures below -10 °C. The above two factors are likely t he main cause for the lack of freezing drizzle at cloud top temperatures less than -15 °C. We also found that neglecting the depletion of ice forming nuclei resulted in considerable overestimation of the ice crystal concentration and suppression of drizzle, even for the -10 °C case.