Trend Analysis of U. S. Tornado Activity Frequency

As one of the most severe and high-impact weather phenomena, tornadoes and their long-term frequency trends have received lots of attention from both the scientific community and the public. Here, we show that over the last six decades (1954-2018), U.S. (E)F1 tornadoes have a statistically significant upward trend but (E)F2-E)F4 tornadoes have statistically significant downward trends based on both solid trend analyses of three independent methods and robust verifications of reported tornado data using a recently developed approach called sample generation by replacement (SGR). Furthermore, we develop a statistical framework to quantitatively explain two-way interconnections between long-term climate trends and internal variabilities. With the support of quantile-quantile plots, we find that a large positive trend of U.S. (E)F1 tornadoes over the last six decades statistically results from a small internal variability of (E)F1 tornado activities. The long-term trends of U.S. (E)F2-(E)F4 tornadoes are also inversely proportional to their internal variabilities, as anticipated by the two-way interconnection theory developed in this study.