An investigation of ice production mechanisms in small cumuliform clouds using a 3D model with explicit microphysics. Part I: Model description

A new cloud model that combines a three-dimensional nonhydrostatic dynamica l framework with explicit liquid- and ice-phase microphysics and a detailed treatment of ice nucleation and multiplication processes is presented. The use of 28 size bins to describe each cloud drop and ice particle spectra a llows the authors to account for all major microphysical processes. A derai led scavenging model has been implemented to calculate the collection rare of contact ice nuclei by cloud drops. In addition to contact nucleation, th e model accounts for deposition, condensatisn-freezing, and immersion ice n ucleation mechanisms, as well as for secondary ice production via rime spli ntering. The model performance is illustrated by a simulation of a New Mexican cumul us cloud. Combination of high 100-m spatial resolution with a new initializ ation procedure that promotes development of small eddies results in a clou d with a more realistic distribution of liquid water content compared to si mulations initialized by the standard "bubble" approach.