Variability of tornado activity in the United States and its link to weather regimes
Graber, Matthew
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https://hdl.handle.net/2142/125721
Description
Title
Variability of tornado activity in the United States and its link to weather regimes
Author(s)
Graber, Matthew
Issue Date
2024-07-16
Director of Research (if dissertation) or Advisor (if thesis)
Trapp, Robert J
Wang, Zhuo
Committee Member(s)
Sriver, Ryan
Department of Study
Climate Meteorology & Atm Sci
Discipline
Atmospheric Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
M.S.
Degree Level
Thesis
Keyword(s)
Variability
Tornadoes
Weather Regimes
Abstract
Continued efforts to build human resilience to the impacts of tornadoes require updated knowledge of tornado occurrences as well as how their occurrence characteristics may be changing in time and varying by region. We have temporally and geospatially disaggregated annual tornado reports in the United States and revealed that significant, long-term decreases in tornado days from 1960 to 2022 have occurred over the months of June through August, primarily within the Southern Great Plains. In contrast, long-term increases in days of tornado outbreaks have occurred over this period, particularly within the Southeast U.S. and during warm- as well as cool-season months. There are indications that these dichotomous linear trends in tornado days and tornado outbreaks have relaxed over the most recent decade.
The contiguous United States (CONUS) experiences considerable interannual variability in tornado activity. The high impacts of tornadoes motivate the need to better understand the link between seasonal tornado activity and large-scale atmospheric circulations, which may contribute to better seasonal prediction. We employed K-means clustering analysis of 500 hPa geopotential height (500H) anomalies from the ERA-5 reanalysis and identified five warm-season weather regimes (WRs). Certain WRs are shown to strongly affect tornado activity, especially outbreaks, due to their relationship with environmental parameters including convective available potential energy (CAPE) and vertical wind shear (VWS). For example, WR-B, which is characterized by a three-cell wave-like pattern with an anomalous low over the central-CONUS, is associated with enhanced CAPE and VWS in tornado-prone regions and represents a tornado-favorable environment. Persistent WRs, those that last ≥5 consecutive days, are associated with 76% of all tornado outbreaks (days with >10 EF-1+ tornadoes) since 1960, with a persistent WR-B accounting for about 30%. An empirical model using WR frequency and persistence captures the year-to-year variability of warm-season tornado days and outbreaks reasonably well, including some years with high-impact outbreaks. Our study highlights the potential application of WRs for better seasonal tornado prediction.
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