Quantifying entrainment and dilution in numerical simulations of developing thunderstorms

Allen, Luke Robert

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https://hdl.handle.net/2142/108526 Copy

Description

Title

Quantifying entrainment and dilution in numerical simulations of developing thunderstorms

Author(s)

Allen, Luke Robert

Issue Date

2020-07-21

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

Lasher-Trapp, Sonia G

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
• Thunderstorms
• Developing
• Dilution
• Clouds
• Cumulus
• Cumulonimbus

Abstract

Entrainment is the process by which turbulent clouds draw environmental air inward, and this process has long been understood to dilute the cloud, i.e., reduce the cloud liquid water content, buoyancy, and updraft velocity. In cumuli, entrainment results from overturning thermal circulations near cloud top. Past observational and numerical modeling studies have shown that in environments with significant vertical wind shear, the down-shear circulation is stronger. It has been hypothesized that greater entrainment (and thus dilution) occurs in stronger shear environments, but this relationship has not been suitably demonstrated with direct calculations of entrainment. This lack of understanding has consequences on the numerical weather prediction of convective initiation and development, as well as climate modeling. Here, entrainment is quantified within six different high-resolution numerical simulations of developing thunderstorms in environments with different wind profiles to understand the role of vertical wind shear in modulating cumulus entrainment. Entrainment is directly quantified using the mass flux across a defined cloud core surface. Greater entrainment consistently occurs in simulated storms developing in environments with greater vertical wind shear, due to stronger overturning thermal circulations near the cloud tops, and greater storm-relative inflow on the down-shear side of the cloud. Evaluations of the dilution produced by entrainment in the developing storms is often obscured by successive thermals in the sheared environments, often rising through the remains of previous thermals that modified the air entrained by the later thermals. The storms growing in strong-shear environments are also broader, so a smaller fraction of the cloud is affected by each individual entrainment event. Precipitation production appeared to be more related to the time the cloud had been present, and the aid of successive thermals, than entrainment and dilution. Thus, while more entrainment occurs in the higher-shear environments, and stronger forcing is required for the storms to develop, the enhanced interaction of successive thermals in such environments, as well as their enhanced core widths, can enable the development of deep convective storms.

Graduation Semester

2020-08

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/108526

Copyright and License Information

Copyright 2020 Luke Allen

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