Understanding tornado intensity in supercells and quasi-linear convective systems

Marion, Geoffrey R

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https://hdl.handle.net/2142/110726 Copy

Description

Title

Understanding tornado intensity in supercells and quasi-linear convective systems

Author(s)

Marion, Geoffrey R

Issue Date

2021-04-22

Director of Research (if dissertation) or Advisor (if thesis)

Trapp, Robert J

Doctoral Committee Chair(s)

Trapp, Robert J

Committee Member(s)

• Nesbitt, Steven
• Frame, Jeffrey
• Jewett, Brian

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Severe weather
• tornado
• tornadoes
• tornado intensity
• quasi-linear convective systems

Abstract

Identifying storms capable of producing strong to violent tornadoes remains a focus of severe weather research due to their potentially significant impacts on life and property. The three parts of this dissertation contribute to new understanding of the controls on, and operational anticipation of, such tornado intensity. In the first part, a theoretical framework linking tornadic storm updraft characteristics to tornado intensity is further explored. An application of Kelvin’s circulation theorem to pre-tornadic and tornadic circulations shows that initial tornadic circulations are necessarily constrained by the circulation of the parent near-surface mesocyclone. This implies that the two underlying characteristics of mesocyclonic circulation, namely width and intensity, will in turn control the tornadic circulation. Using idealized cloud-resolving model simulations of so-called “pseudostorms,” links between storm updraft intensity, updraft width, and cold pool intensity, as well as environmental vertical wind shear, are identified as contributors to tornado intensity. Meanwhile, cold pool depth is found potentially to impact tornado formation but may have little impact on tornado intensity. Additional pseudostorm simulations are performed without a heat sink or environmental winds, instead of using background rotation to develop updraft and near-surface rotation, to explore the impact of the updraft characteristics on near-surface vortex intensity. Both stronger and wider updrafts produce stronger near-surface rotation much more quickly than their weaker, narrower counterparts. Because of the linkages in real storms between the storm updraft and cold pool characteristics, it is determined that the storm updraft likely has the most significant influence on tornado intensity. The second part exploits the implementation of the high-resolution GOES-R series satellite and proposes a technique for the identification of storms supportive of strong tornadoes through estimation of storm updraft size by way of overshooting tops (OTs). Specifically, a method to quantify overshooting top area (OTA) is explored to identify storms with the greatest risk of forming strong to violent tornadoes. A comparison between maximum estimated tornado wind speeds and OTA yields a strong correlation (R2 = 0.544). These results show the potential of these quantifications to be used with real-time observations of tornadic storms, irrespective of storm mode, seasonality, or geographic location, allowing forecasters to determine which storms pose the highest risk of forming strong tornadoes. Finally, the third part focuses on tornadoes produced by quasi-linear convective systems (QLCSs), which generally are weak and short-lived, but have a high societal impact due to their proclivity to develop over short time scales, within the cool season, and during nighttime hours. Precisely why they are weak and short-lived is not well understood, although the preceding work suggests that QLCS updraft width may act as a limitation to tornado intensity. In this part, idealized simulations of tornadic QLCSs are performed with variations in hodograph shape and length as well as initiation mechanism to determine the controls of tornado intensity. Generally, the addition of hodograph curvature in these experiments results in stronger, longer-lived tornadic like vortices (TLVs). A strong correlation between low-level mesocyclone width and TLV intensity is identified (R2 = 0.61), with a weaker correlation in the low-level updraft intensity (R2 = 0.41). The tilt and depth of the updraft are found to have little correlation to tornado intensity. Comparing QLCS and isolated supercell updrafts within these simulations, the QLCS updrafts are less persistent, with the standard deviations of low-level vertical velocity and updraft helicity to be approximately 48% and 117% greater, respectively. A forcing decomposition reveals that the QLCS cold pool plays a direct role in the development of the low-level updraft, providing the benefit of additional forcing for ascent while also having potentially deleterious effects on both the low-level updraft and near-surface rotation. The negative impact of the cold pool ultimately serves to limit the persistence of rotating updraft cores within the QLCS.

Graduation Semester

2021-05

Type of Resource

Thesis

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http://hdl.handle.net/2142/110726

Copyright and License Information

Copyright 2021 Geoffrey Marion

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