The effects of entrainment in the developing and rotating stages of supercell thunderstorms

Engelsen, Bryan Nicholas

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https://hdl.handle.net/2142/101620 Copy

Description

Title

The effects of entrainment in the developing and rotating stages of supercell thunderstorms

Author(s)

Engelsen, Bryan Nicholas

Issue Date

2018-07-19

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

Lasher-Trapp, Sonia

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Entrainment
• Supercell
• Clouds
• Dilution
• Thermals
• Thunderstorms
• Mixing
• Rotation
• Developing

Abstract

Entrainment, the process by which turbulent clouds introduce dry air from outside the cloud inward via overturning eddies at the cloud edge, can decrease the cloud buoyancy, and the water and/or ice mass it contains, limiting both cloud and precipitation development. Numerous studies have shown that growing cumulus clouds entrain air primarily as a result of the overturning thermal circulation near their tops, but have focused upon cumuli in environments with minimal vertical wind shear. Little attention has been given to investigating the entrainment into developing thunderstorms growing in environments with strong vertical wind shear, or to how rotating updrafts in some thunderstorms (i.e. supercells; produced in environments with specific characteristics of the vertical wind shear) might alter the amount of entrainment they experience. In the current study, idealized, 3D, high-resolution numerical simulations of supercell thunderstorms are used to evaluate entrainment and its effects during the developing and rotating stages of the storms. Entrainment is quantified using an algorithm that first estimates the sub-grid scale edge of the 3D cloud core, defined with specific condensate and vertical velocity thresholds, and then calculates the mass flux into that core. As entrainment proceeds in time, the resulting dilution of the core condensate is tracked. Multiple realizations in the same storm environment are created by altering the storm forcing type (heat flux versus “warm bubble”), the horizontal area over which the forcing is applied, and the vertical wind shear. Results show that vertical wind shear can greatly enhance the entrainment rate into the developing storms, being nearly twice as much locally but even exceeding hundreds of percent more when integrated over the entire storm in time. Similar to past studies, the proportionality of entrainment and dilution in the developing stages of the storms depends upon the local properties of the entrained air and the occurrence of multiple thermals. The method of initiating simulated storms with a “warm bubble” was also found to be detrimental to representing the turbulent eddies required for accurate simulations of entrainment, in the developing stages. Surprisingly, entrainment in the rotating stages of the storms decreased slightly but not substantially, counter to theoretical predictions.

Graduation Semester

2018-08

Type of Resource

text

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http://hdl.handle.net/2142/101620

Copyright and License Information

Copyright 2018 Bryan Engelsen

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