Abstract
Rotating “supercell” thunderstorms are shown to be characterized by high helicity, the vector inner product of velocity and vorticity, which is obtained both from the mean flow in which they are embedded and from buoyancy enrichment. Some unique properties of supercell helical flow are described, including a tendency for trajectory rotation to be reversed from parcel vorticity. A simple helical (Beltrami) flow model resembles gross supercell structure and also provides a prediction of storm motion. Since theory, closure model calculations and numerical simulations indicate that helicity suppresses turbulent dissipation, it is suggested that supercells owe their noted stability and long life to this effect. Enhanced predictability of such storms is then expected and is apparently seen in some results of Wilhelmson and Klemp. It is concluded that rotating storm structure and propagation must involve a compromise between the energetic effects discussed by Lilly in Part I of this study and those considered here, but that the helicity effects seem to be dominant in long-lived storms.
