A climatology of quasi-linear convective systems in the U.S.
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Quasi-linear convective systems, or QLCSs, are a common, organized thunderstorm mode in the U.S. Over the last fifty years, severe weather research has focused on the supercell, but, recently, QLCSs have become an increasingly important area of study. Researchers and operational meteorologists realize that this morphology is difficult to forecast and may be responsible for a large proportion of the severe weather reports in the eastern two-thirds of the U.S. This study seeks to determine the degree to which QLCSs threaten humans and their assets by, first, assessing their climatology and, second, measuring their contribution to the severe report record. Initially, an objective classification scheme was developed and employed to detect and track QLCSs on conterminous U.S. composite radar data spanning the most recent two decades. The objective classification scheme, or computer algorithm, was constructed using machine learning techniques on thousands of subjectively, expert-defined QLCSs from a sample of observed mesoscale convective system events during the observed record. After the scheme was tested and verified, a descriptive climatology of QLCSs was produced. The climatology assessed algorithm-identified QLCS slices (instantaneous footprint of an event) and swaths (the entire footprint of an event) as units of measure, providing the first, long-term spatiotemporal analysis of this morphology. Thereafter, QLCS slices were linked with the severe storm database to determine the proportion of thunderstorm hazard reports produced by this morphology. Results show that, on average, there are 139 QLCSs in any given year. The majority occur in three corridors, depending on the year and season: the eastern High Plains into western Missouri and Arkansas; the Midwest, stretching from Iowa through Wisconsin and northern Illinois to Indiana; and the South, from the central Gulf Coast to Tennessee. QLCSs are found to account for more than one-third of severe wind reports, as well as more than one-fifth of reported tornadoes. The overarching goal of the research was to 1) develop an objective analysis routine that can assist meteorologists in identifying QLCSs in observed and simulated radar reflectivity data, 2) provide the first objectively identified, long-term climatology of QLCSs in the U.S., and 3) increase scientific understanding of the impacts of QLCS hazards on the populace and built environment.