The impact of midlevel shear vector orientation on the longevity of and streamwise vorticity current formation within simulated supercells with freeslip and semi-slip lower boundaries

Gray, Kevin Thomas

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https://hdl.handle.net/2142/121466 Copy

Description

Title

The impact of midlevel shear vector orientation on the longevity of and streamwise vorticity current formation within simulated supercells with freeslip and semi-slip lower boundaries

Author(s)

Gray, Kevin Thomas

Issue Date

2023-07-10

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

Frame, Jeffrey

Doctoral Committee Chair(s)

Frame, Jeffrey

Committee Member(s)

• Trapp, Robert
• Nesbitt, Stephen
• 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)

• Supercell thunderstorms
• Tornadoes

Abstract

Despite an increased understanding of environments favorable for tornadic supercells, it is still sometimes unknown why one environment produces many long-tracked tornadic supercells and another seemingly equally-favorable environment produces only short-lived nontornadic supercells. One relatively unexplored environmental parameter that may differ between such environments is the degree of backing or veering of the midlevel shear vector, especially considering that such variations may not be captured by traditional supercell or tornado forecast parameters. The midlevel shear vector orientation may determine where precipitation fallout occurs within a supercell, and thus the location of downdrafts and surges of outflow. Streamwise vorticity currents (SVCs) may develop along outflow and other intra-storm boundaries and these recently identified features are hypothesized to strengthen low-level mesocyclones and possibly increase the chance of tornadogenesis. We investigate the impact of the 3-6 km shear vector orientation on simulated supercell evolution by systematically varying it across 19 idealized simulations. We found that the orientation of the 3-6 km shear vector dictates where precipitation loading is maximized in the storms, and thus alters the storm-relative location of downdrafts and outflow surges. When the shear vector is backed, outflow surges generally occur northwest of an updraft, produce greater convergence beneath the updraft, and do not disrupt the inflow as much, meaning that the storm is more likely to persist longer and produce more tornado-like vortices (TLVs). When the shear vector is veered, outflow surges generally occur north of an updraft, produce less convergence beneath the updraft, and sometimes undercut it with outflow, often causing it to tilt at low levels, sometimes leading to storm dissipation. These storms are shorter lived and thus also produce fewer TLVs. Our simulations indicate that the orientation of the 3-6 km shear vector may impact supercell longevity and hence the time period over which tornadoes may form. The same suite of 19 simulations is analyzed to determine how SVC formation and characteristics may differ between storms. SVCs develop on the cold side of left-flank convergence boundaries (LFCBs) in these simulations and their updraft-relative positions are partially dependent on downdraft location. The duration and mean depth of SVCs do not differ significantly between simulations. The duration of SVCs that precede TLVs does not differ from other SVCs, and the mean depth of SVCs preceding TLVs is slightly greater than that of other SVCs. Trajectories initialized within SVCs reveal two primary airstreams, one that flows through an SVC for the majority of its length, and another that originates in the modified inflow within the forward flank and then becomes entrained into the SVC. Vorticity budgets calculated along trajectories reveal that the first airstream exhibits significantly greater streamwise vorticity than the second airstream. The vorticity budgets also indicate that stretching of streamwise vorticity is the dominant contributor to the large values of streamwise vorticity within the SVCs. TLV formation does not require the development of an SVC beforehand; 44\% of TLVs in the simulations are preceded by SVCs. When an SVC occurs, it is followed by a TLV 53\% of the time, indicating that not all SVCs lead to TLV formation. Another suite of simulations, three with a freeslip lower boundary and three with a semi-slip lower boundary, is examined to determine the effects of surface drag on supercell updrafts, internal boundaries, and SVCs. Low-level updrafts are weaker in simulations with a semi-slip lower boundary, but more steady, whereas updrafts in simulations with a freeslip lower boundary are stronger, but more pulse-like. LFCBs are more common in the freeslip simulations and forward-flank convergence boundaries (FFCBs) are more common in the semi-slip simulations. The freeslip lower boundary supports development of large streamwise vorticity at the lowest model level within outflow between the downdraft and updraft, while a semi-slip lower boundary yields antistreamwise vorticity in this region, somewhat limiting SVC intensity. The initial streamwise vorticity contributes the majority of the streamwise vorticity available for stretching in an SVC along an FFCB, while baroclinically-generated streamwise vorticity may double the initial streamwise vorticity available for stretching in an SVC along an LFCB. Vorticity produced by surface drag does not influence SVC development along an FFCB as trajectories through such an SVC remain at altitudes above the layer influenced by friction. Surface drag may reduce streamwise vorticity and increase crosswise vorticity in an SVC along an LFCB owing to trajectories that pass very near the surface. Overall, the streamwise vorticity in SVCs within simulations using a freeslip lower boundary is much greater than that in those using a semi-slip lower boundary and caution should be taken when interpreting model output from freeslip simulations.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Kevin Gray

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