A Comparative Study of the Rates of Development of Potential Graupel and Hail Embryos in High Plains Storms

Abstract

The rates of development of graupel and hail in High Plains storms are calculated based on mechanisms for the growth of particles of various types. In the first part of this study, planar crystals, aggregates, graupel particles and frozen drops grow in a region of updraft specified by a one-dimensional parcel model. Their rates of development are calculated from a model which considers their growth through accretion and diffusion, ?dry? or ?wet? growth, and melting. At most initial embryo diameters and for liquid water content of 1 g m?3, frozen drops grow most rapidly, followed in order by graupel, aggregates and planar crystals. Particle growth rates over the range of liquid water contents from 0.5 to 2.0 g m?3 can be estimated from those given for 1 g m?3 through the use of a simple parameter. In the second part of the study, the processes of hail production are deduced by calculating the times required for each of the above embryo types to develop into hail from the time of nucleation over the range of typically observed liquid water contents. These times are then compared to the typically observed cell lifetimes to infer the predominant hail growth mechanisms. The calculations suggest that hail is not likely to be produced when particle growth is confined to one region of updraft. A particle which grows in the peripheral regions of a storm through diffusion and then aggregation, and is then introduced into a region of updraft is the most likely candidate to become a hailstone in a High Plains storm. An unrimed aggregate which melts completely to form a drop and then subsequently freezes and develops into hail is the most likely growth sequence that produces hail from frozen drops.