Case Study of a Halistorm in Colorado. Part IV: Graupel and Hail Growth Mechanisms Deduced through Particle Trajectory Calculations

The processes of development of graupel and had which fell to the ground from a storm in northeastern Colorado on 22 July 1976 are investigated over a one-hour period. The growth and trajectories of 130 000 particles of different types and sizes are calculated in the measured three-dimensional wind fields. The growth conditions that these graupel and hail (spherical ice particles smaller than and larger than 1 cm, respectively) experience are presented and their trajectories are described. Particles which become hail are those whose terminal velocities are nearly equal to the vertical velocities of the air parcels in which they develop. This enables them to fall into regions of relatively high liquid water content in the main updraft cores. The terming velocities of the particles which become graupel are not as well-matched to the parcel velocities; particles grow with lower liquid water contents. Embryos of the graupel and hail are found to be aggregates (snowflakes) of 0.5–1.5 cm in diameter. Embryo formation takes place in a 1 km wide region of divergence located along the forward portion of the storm where the mid-level air is flowing around the updraft cores Aggregates becoming hail are ingested by feeder cells located adjacent to the main updraft cores and are then carried into newly forming cells which become the main updraft region, wherein they complete their development. Aggregates becoming graupel are ingested directly by the main updrafts. Mechanisms by which particles are selected to become graupel and hail embryos are related to the position at which they develop in the storm, the processes by which they initially develop, and the stage of development of the main updraft core at the time at which they begin to grow in that region.