Collection growth of ice particles is one of the most complex problems in mixed-phase microphysics, since the resulting type of particle may be of a type different from the colliding particles. Usually this process is described by a system of kinetic collection equations of great complexity.

In this work, a stochastic microphysical model for calculating the collection growth in a mixed phase cloud is developed. The stochastic algorithm of Gillespie (1976) for chemical reactions in the formulation proposed by Laurenzi et al. (2002) was used to simulate the kinetic behavior of the particle population. Within this framework, species are defined as ice particles of specific mass and crystal habit. The proposed stochastic algorithm permits the investigation of the effect of ice crystal type on collection growth in mixed-phase clouds and could improve cloud parameterizations for cloud-resolving models.