| Severe local storms generated by atmospheric convection and enhanced by wind shear may increase in frequency within the United States, forced by increased surface temperature and other climate changes documented during the 20th Century. However, archives of thunderstorm and tornado events may not accurately represent the climate system, because storm reporting has been influenced by changing detection technologies, enhanced public awareness, and, most importantly, population patterns. Given the possible biases in available storm data and likely underreporting in earlier decades, the climatology of these events over the last ∼50 years was addressed in two ways. First, temporal changes in severe weather potential across the coterminous United States were explored using sounding-derived atmospheric stability indices, by calculating trends in the number of days registering extreme values of each selected index (the Lifted Index, the K-Index, the Severe Weather Threat Index, and Convective Available Potential Energy). This analysis was performed for spring and summer seasons only, when the vast majority of severe weather events occur.{09}Second, spatial and temporal patterns in observed U.S. tornadoes were elucidated, and their relationship to population patterns was assessed with several methods, including: a comparison of trends in population density and tornado density (tornadoes per unit area) for all U.S. counties, the calculation of spatial and temporal correlations between the two variables, and the formal definition of a population bias in the tornado record. Observed county tornado densities were then adjusted for population bias, and the corrected changes in tornado occurrence were mapped. Because population is not the only factor that could produce a spurious rise in the storm record, the average increase in population-corrected tornado patterns was removed, in order to highlight possible climate-driven changes.; These investigations revealed that the number of extremely unstable days, and therefore the potential for severe storm and tornado development, has increased over the eastern two-thirds of the nation. The stability index trends were quite consistent in the U.S. plains and southern regions in summer, while few changes in severe weather frequency were suggested in the west. Increases in observed tornadoes also emerged over most of the country. These changes appear to be partly related to population increases in Colorado, the southwest, the Gulf Coast, Florida, and around major urban centers. (Abstract shortened by UMI.)... |
