Low-level curvature shear and supercell thunderstorm behavior
Brooks, Harold Edward
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Permalink
https://hdl.handle.net/2142/20165
Description
Title
Low-level curvature shear and supercell thunderstorm behavior
Author(s)
Brooks, Harold Edward
Issue Date
1990
Doctoral Committee Chair(s)
Wilhelmson, Robert B.
Department of Study
Atmospheric Science
Discipline
Atmospheric Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Language
eng
Abstract
Tornadoes pose threats to lives and property, and accurately issuing warnings of their occurrence represents a challenge to operational meteorologists. One tool that will become increasingly important in responding to this challenge is Doppler radar which is used to detect rotation over several kilometers within thunderstorms in a region known as the mesocyclone. Preliminary studies indicate the use of Doppler radar to issue tornado warnings based on mid-level mesocyclones yields a 50% false alarm rate, indicating that only half of storms with strong mid-level rotation have strong low-level rotation. Recent observational and theoretical work has implied that a possible discriminating factor between storms that have strong low-level rotation and those that do not is the amount of storm-relative curvature shear in the lowest 3 km of the atmosphere.
In this thesis we examine the link between mid-level and low-level rotation using a numerical storm model. A case study with different amounts of curvature shear is presented, followed by a series of simulations using idealized wind profiles in which the storm-relative curvature shear is varied. All of the simulated storms produce strong mid-level rotation. A set of simulations with strong storm-relative low-level environmental curvature shear produce flows resembling those observed in low levels of tornadic thunderstorms by Doppler radar. Rectilinear shear (straight line hodograph) cases produce no organized rotation at low levels. This result implies that information about the environmental winds in the vicinity of thunderstorms can be used to improve the tornado warning process.
Low pressure is seen near the surface in the inflow region of the storms with low-level mesocyclonic circulations. It is hypothesized that this perturbation low pressure is an important part of the mechanism by which curvature shear environments are conducive to producing intense low-level rotation. Another effect of the low-level curvature shear is to intensify growing updrafts in the early stages of supercell storms. The enhanced curvature produces a more helical updraft within the storm. The intensification of the updraft is consistent with the tendency of helical flows to resist mixing.
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