The Hybrid Multicellular–Supercellular Storm—an Efficient Hail Producer. Part I: An Archetypal Example

Abstract

This is the first of a two part series describing storms termed "hybrid? because their characteristics are intermediate between those classically defined as multicellular and supercellular. They are important because they tend to produce more hail and, in fact, often produce the most severe hailfalls in the central Oklahoma area. Part I discusses the structure and hail growth characteristics of an archetypal example as derived from Doppler radar and surface hail collections. The storm produced maximum diameter hail of 75 mm and a hailswath that was about 300 km long and, at one point, 40 km wide. Although the storm had the large-bounded weak echo region and generally steady appearance associated with supercellular storms, the data suggest that at any time there were three updrafts separated by a distance of 5 to 8 km. Another distinctive flow feature is an intense downdraft in an area of weak reflectivity to the west of the updraft. Hailstone embryos collected over a distance of 100 km and 1.5 h showed a predominance of graupel embryos, rare for Oklahoma storms. Sequential, time-resolved hail samples were obtained nearly simultaneously at locations separated by 7 km perpendicular to the long hailswath axis. Both samples showed similar distributions with modal values of 1.2 cm, but air bubble and crystal structure were remarkably different. In one case, the hailstones grew in a dry regime at cold temperature (≤?20°C), whereas the other sample grew in a wet regime at temperatures warmer than about ?20°C. Trajectory analyses show both samples came from the same updraft, indicating that more than one genus of trajectory may be capable of producing large hail. Part II uses information from Part I and from earlier investigations to give a description of the general characteristics of hybrid storms and the importance of the flow structure of these storms to hail production.