Entrainment in simulated supercell thunderstorms

Jo, Enoch

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https://hdl.handle.net/2142/116205 Copy

Description

Title

Entrainment in simulated supercell thunderstorms

Author(s)

Jo, Enoch

Issue Date

2022-07-12

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

Lasher-Trapp, Sonia G

Doctoral Committee Chair(s)

Lasher-Trapp, Sonia G

Committee Member(s)

• Trapp, Robert J
• Sriver, Ryan L
• van den Heever, Susan C

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)

• Entrainment
• Supercells
• Cloud microphysics

Abstract

Supercell thunderstorms are associated with significant weather hazards such as heavy rainfall, hail and tornadoes which can lead to loss of life and destruction of property. Entrainment is one process that can modulate the intensity of supercells, and a better understanding of its impact could help improve thunderstorm forecasts and especially the precipitation they produce. For the first time, various mechanisms of entrainment during the mature stage of idealized supercell thunderstorms have been examined, including within different thermodynamic environments or different vertical wind profiles. Entrainment is calculated directly as fluxes of air over the surface of the updraft core; passive fluid tracers are used to assess the resulting dilution of the air ingested by the storm. Model microphysical rates are used to compare the impacts of entrainment on the efficiency of condensation/deposition of water vapor on hydrometeors within the core, and ultimately, upon precipitation that falls to the ground. Results show that two entrainment mechanisms on the core surface reduce the ability of the storms to produce precipitation within the updrafts: overturning “ribbons” of horizontal vorticity which translate vertically with time (and heretofore not ever recognized), and disorganized turbulent eddies. A third mechanism, the storm-relative airstream, actually supplies additional moisture into the storm updraft to enhance precipitation production. Although the total entrainment increases with increasing vertical wind shear, the fractional entrainment decreases, leading to an increase in undiluted air within the storm core and an increase in condensation efficiency. However, due to the increase in hydrometeors detrained aloft and the enhanced evaporation during their fall, the precipitation efficiency as gauged by the surface rainfall decreases with increasing vertical wind shear, as suggested in past studies. Decreasing the tropospheric relative humidity enhances the evaporation of falling precipitation, and therefore also decreases the precipitation efficiency. When the humidity of the air is decreased at lower altitudes, the moist air supplied by the storm-relative airstream is reduced, which diminishes the strength of the supercell updraft and the precipitation that it produces. These results are a start toward a more comprehensive quantification of the impact of entrainment on precipitation in supercell thunderstorms, which may in turn increase the accuracy of numerical modeling forecasts of their hazards (e.g., severe winds, flash flooding).

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Preamble: The majority of Chapters 3 and 4 within this dissertation are available in peer-reviewed publications, and the content of Chapter 5 will be submitted to a peer-reviewed journal soon after the time of the dissertation deposit. Chapter 3 (Lasher-Trapp et al. 2021) and Chapter 4 (Jo and Lasher-Trapp 2022) can be found in the American Meteorological Society’s Journal of the Atmospheric Sciences. Copyright notices from the American Meteorological Society: “The AMS grants its Authors the following permissions: Permission to reuse any portion of the Author’s Work, without a fee, in future lectures, press releases, or Works of the Author's own, provided that the AMS citation and the AMS copyright notice are included.” “© Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this Work, please contact permissions@ametsoc.org. Any use of material in this Work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC § 108) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. All AMS journals and monograph publications are registered with the Copyright Clearance Center (https://www.copyright.com). Additional details are provided in the AMS Copyright Policy statement, available on the AMS website (https://www.ametsoc.org/PUBSCopyrightPolicy).”

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