DYNAMICS OF TORNADIC THUNDERSTORMS

Tornadic thunderstorms are the most intense and most damaging type of convective storm. Whereas ordinary convective cells grow, produce rain, and then decay over a period of 40 rain to an hour, certain thunderstorms may develop into a nearly steady-state structure that persists for several hours, producing heavy rain, large hail, damaging surface winds, and tornadoes. Although tornadoes may arise in a variety of storm conditions, these long-lived storms produce tornadoes most frequently and generate virtually all of the most damaging ones. Prominent features of these tornadic thunderstorms, which are particularly common i the Great Plains and midwestern regions of the United States, are illustrated in an idealized schematic in Figure 1. Our understanding of tornadic storms has evolved gradually over the years as technological advances have been made in observing systems and computer models. The first indications that certain storms exhibit a special behavior came from early studies of data from upper-air soundings and surface stations. While most thunderstorms moved with the mean winds over the lower and middle troposphere (Byers & Braham 1949), certain large storms were found to propagate consistently to the right of the mean winds (Byers 1942, Newton & Katz 1958). Observing the shift in the wind direction as storms passed by surface stations, Byers (1942) and later Brooks (1949) surmised that these intense storms have cyclonic ci rculation (counterclockwise rotation about a vertical axis, viewed from above). Using data from scanning radar and ground observations, case studies of a severe thunderstorm near Wokingham, England (Browning Ludlam 1962), and another near Geary, Oklahoma (Browning & Donaldson 1963), revealed numerous distinctive features that were strikingly

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