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ABSTRACT
The primary objective of this study was to estimate the percentage of U.S. tornadoes that are spawned annually by squall lines and bow echoes, or quasi-linear convective systems (QLCSs). This was achieved by examining radar reflectivity images for every tornado event recorded during 1998-2000 in the contiguous United States. Based on these images, the type of storm associated with each tornado was classified as cell, QLCS, or other.
Of the 3828 tornadoes in the database, 79% were produced by cells, 18% were produced by QLCSs, and the remaining 3% were produced by other storm types, primarily rainbands of landfallen tropical cyclones. Geographically, these percentages as well as those based on tornado days exhibited wide variations. For example, 50% of the tornado days in Indiana were associated with QLCSs.
In an examination of other tornado attributes, statistically more weak (F1) and fewer strong (F2-F3) tornadoes were associated with QLCSs than with cells. QLCS tornadoes were more probable during the winter months than were cells. And finally, QLCS tornadoes displayed a comparatively higher and statistically significant tendency to occur during the late night/early morning hours. Further analysis revealed a disproportional decrease in F0-F1 events during this time of day, which led the authors to propose that many (perhaps as many as 12% of the total) weak QLCSs tornadoes were not reported.
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1. Introduction
The understanding of tornadoes and their formation has grown dramatically over the past several decades, owing to the ever-increasing availability of visual and in situ observations, weather radar and satellite data, and sophisticated computer model simulations of severe convective storms (see the recent reviews by DaviesJones et al. 2001 and Wilhelmson and Wicker 2001). Concomitant with this growth in knowledge has been an expansion of the accepted realm of the types of possible convective storms that produce tornadoes. Based on our current understanding, these range from supercell/mesocyclonic thunderstorms, complete with subtypes such as "low topped" or "mini" (e.g., Kennedy et al. 1993) and "high precipitating" (Moller et al. 1990); nonsupercell/nonmesocyclonic thunderstorms (Wakimoto and Wilson 1989; Brady and Szoke 1989); prefrontal rainbands (Carbone 1983); tropical cyclones (Spratt et al. 1997; McCaul 1987); and squall lines and bow echoes (e.g., Hamilton 1970; Forbes and Wakimoto 1983;...