Statistical Examination of Tornado Report and Warning Near-Storm Environments

Open Access
Author:
Anderson-Frey, Alexandra Kate
Graduate Program:
Meteorology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 14, 2017
Committee Members:
  • Yvette Pamela Richardson, Dissertation Advisor
  • Yvette Pamela Richardson, Committee Chair
  • Paul Markowski, Committee Member
  • David Jonathan Stensrud, Committee Member
  • Andrew Mark Carleton, Outside Member
Keywords:
  • tornado
  • quasi-linear convective system
  • supercell
  • convection
  • forecasting
  • self-organizing maps
  • kernel density estimation
  • near-storm environment
Abstract:
This study makes use of a 13-year dataset of 14,814 tornado events and 44,961 tornado warnings in the continental United States, along with near-storm environmental data associated with each of these tornado events and warnings, to build a methodology that can be used to create nuanced climatologies of near-storm environmental data. Two key parameter spaces are identified as being particularly useful in this endeavor: mixed-layer convective available potential energy (MLCAPE) versus 0--6-km vector shear magnitude (SHR6) and mixed-layer lifting condensation level (MLLCL) versus 0--1-km storm-relative helicity (SRH1). In addition, the Significant Tornado Parameter (STP) is identified as a useful composite parameter that can highlight near-storm environments that are particularly favorable for the development of significant tornadoes. Two particular statistical methods for the analysis and characterization of near-storm environments are described and applied: Kernel Density Estimation (KDE), which is applied to bulk (proximity sounding-like) parameter values associated with each event or warning, and Self-Organizing Maps (SOMs), which are applied to fully two-dimensional plots of STP in an area surrounding each event or warning. The KDE approach characterizes and identifies differences in the environments of tornadoes forming in quasi-linear convective systems versus those forming in right-moving supercells; specific environmental traits are also identified for different geographical regions, seasons, and times of day. Tornado warning performance is found to be best in environments with particularly large values of MLCAPE and SHR6. The early evening transition (EET) period is of particular interest: MLCAPE and MLLCL heights are in the process of falling, and SHR6 and SRH1 are in the process of increasing. Accordingly, tornadoes rated 2 or greater on the enhanced Fujita scale (EF2+) are also most common during the EET, probability of detection (POD) is relatively high, and false-alarm ratio (FAR) is relatively low. Overall, when comparing the distribution of environments for events versus those for warnings, there is no "smoking gun" indicating a systematic problem with forecasting that explains the high overall false-alarm ratio, which instead seems to stem from the inability to know which storms in a given environment will be tornadic. The SOM approach establishes nine statistically distinct clusters of spatial distributions of STP values in the 480 km x 480 km region surrounding each tornado event or warning. For tornado events, distinct patterns are associated more with particular times of day, geographical locations, and times of year, and the use of two-dimensional data rather than point proximity sounding information means that these patterns can be identified and characterized with still more detail; for instance, the archetypal springtime dryline environment in the Great Plains emerges readily from the data. Although high values of STP tend to be associated with relatively high POD and relatively low FAR, the majority of tornado events occur within a pattern of low STP, with relatively high FAR and low POD. The two-dimensional plots produced by the SOM approach provide an intuitive way to create distinct climatologies of tornadic near-storm environments. Having established a methodology through the use of KDE and SOM, this research then examines the topic of tornado outbreaks [defined as ten or more (E)F1+ tornadoes that occur with no more than 6 h or 2,000 km between subsequent tornadoes]. Outbreak tornadoes in a given geographical region have greater SRH1 and SHR6 than isolated tornadoes in the same region, and also have considerably higher POD than isolated tornadoes. When SOMs are created for all (E)F1+ tornadoes, the percentage of outbreak tornadoes in a given node is found to depend more strongly on the magnitude of the STP surrounding the tornado than its orientation. For the SOM of outbreak tornadoes, outbreaks occurring in environments with higher magnitudes of STP will generally also have the highest casualty rates, regardless of the specific two-dimensional pattern of STP. Two specific tornado outbreaks are then examined through this methodology, which allows the events to be placed into their climatological context with more nuance than typical proximity sounding-based approaches would allow.