The growth trajectories of precipitation particles that attain diameters from 0.5 to 2.0 cm are modeled within the wind field of a small, relatively steady-state, southeastern Montana thunderstorm. The trajectories are calculated backwards, from systematic arrays of particles of specified sizes at a level near cloud base. Using a simple set of criteria for rejecting the obviously impossible trajectories, the patterns of accepted trajectory end-points are compared with the radar echo patterns. Good agreement lends credence to the qualitative aspects of the trajectories. For a given size of precipitation particle, the method helps one to assign different trajectory types to specific regions within the horizontal plant on which the calculations were started. The relative importance of the different types of trajectories can thus be estimated. Particle origin mechanisms are discussed in terms of the regions in which the trajectories are found to start. The variety of successful trajectories leading to 1 cm and larger hail in a storm of considerable structural simplicity is noteworthy.
Sensitivity tests indicate that the liquid water content is by far the most important specification in this framework. Ongoing work is directed toward improving this specification and deriving estimates of particle concentration.