A Study of Hail Growth Utilizing Observed Storm Conditions

Abstract

Hail growth is studied in the Westplains, Colorado, storm for which a variety of observations were collected. Measurements from a triple-Doppler radar network and a penetrating aircraft are used to synthesize fields of liquid water content and three-dimensional, evolving air motion from which detailed hail growth trajectories are computed. Explicit treatment of the growth of hail embryos is sidestepped. Rather, for the purpose of the study a wide range of embryo sizes is assumed to exist over a broad region of the storm and the analysis examines which particles are predicted to grow and what their growth trajectories are. Sensitivity of individual trajectories to many parameter such as initial position, embryo size, and particle drag law, is found. However, the general results, including the following, are insensitive: the storm flow field is organized such that an efficient mechanism exists for transporting particles into the updraft; following growth of the embryo outside the large updraft, a single pass though the updraft is sufficient to account for hail of the observed maximum size (1.5- 2.0 cm); fall-out positions and growth times are consistent with the observations; growth takes place primarily between -10 and -25°C. A wide variety of interweaving trajectories is predicted. In general they conform to the supercell model of Browning and Foote, though several differences are noted. Hail apparently can grow in the storm from a wide range of initial positions, but the favorable initial region for the smaller and more numerous embryos that are expected to dominate the hail production is more restricted. The width of the updraft is found to be an important factor in limiting hail growth in the storm. Melting during fall-out is also significant. The present results are shown to be consistent with a generalized hailstorm model termed an open-cell simple-trajectory model. By this it is meant that (a) the embryo stage of growth occurs outside the large updraft in which (b) the hail itself grows by making a single traverse, as opposed to following several looping "trajectories. Evidence supporting the general importance of this mode of behavior of hailstorms is presented. Alternative theories using them concepts are also discussed.