ADDITIONAL APPLICATIONS OF LINEAR THEORY USED IN DISCRIMINATING BETWEEN THUNDERSTORM TYPE Jonathan G

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ADDITIONAL APPLICATIONS OF LINEAR THEORY USED IN
DISCRIMINATING BETWEEN THUNDERSTORM
TYPEJonathan GarnerUniversity of
Nebraska-OmahaDept. of Physics
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Storm Type-Why is it Important?

• Severe weather can occur with any type of
  convective storm, but certain storms are more
  likely than others to produce severe weather
  (Weisman and Klemp 1986)
• Allows the forecaster to ascertain the overall
  severe thunderstorm potential (Moller 2001)

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Multicell Thunderstorms

• System advects with the mean flow
• New updrafts form at the gust front
• Mature cells propagate toward the rear of the
  cold pool

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Supercell Thunderstorms

• Long-lived thunderstorm (gt 1-hr)
• High correlation between vertical velocity and
  vertical vorticity
• Most supercells produce some kind of severe
  weather (e.g., large hail, destructive straight
  line winds, or tornadoes)
• A majority of all strong and violent tornadoes
  are produced by supercells

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Supercell Thunderstorms-Forecast Techniques
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Supercell Thunderstorms-Vertical Wind Shear

• Vertical wind shear plays a dominant role in
  thunderstorm organization.
• As vertical wind shear increases, so does the
  strength and longevity of the updraft
• Shape and size of the hodograph

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Supercell Thunderstorms-Non-Linear Theory
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Supercell Thunderstorms-Non-Linear Theory
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Supercell Thunderstorms-Linear Theory
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Theoretical Correlation Coefficient-
(Davies-Jones 1984)
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Storm-Relative Helicity (SRH)

• Inner product between streamwise vorticity and
  storm-relative winds
• SRH increases as the magnitude of streamwise
  vorticity and storm-relative winds increase, and
  the angle between their direction decreases
• SRH is highly dependent on storm-motion

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TAU

• A scalar product between streamwise vorticity and
  the normalized storm-relative winds similar to
  SRH
• The first component of TAU evaluates the strength
  of the storm-relative inflow (called V C)
• The second component is the product between V
  C and streamwise vorticity
• The result is an evaluation of the magnitude of
  streamwise vorticity which contributes to net
  updraft rotation

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TAU
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TAU
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TAU- What does it describe?

• When V C approaches or exceeds 1.0, and
  streamwise vorticity is present, net updraft
  rotation is likely
• When V C ltlt 1.0, updraft regeneration is
  more discrete, thus, individual updrafts may
  briefly rotate, but sustained net updraft
  rotation is more unlikely

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TAU- Data and Methodology

• 452 proximity soundings taken at 00 UTC between
  1999 and 2004
• Soundings are representative of approximate
  inflow sector
• All seasons included
• Severe weather occurrences were broken up into
  supercell and non-supercell events
• Lenient criteria were applied in order to quickly
  generate a large data set

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Relationship between TAU and SRH
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Relationship between TAU and SRH
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TAU- Correlation with other Parameters
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Skill Scores- Supercells vs. Non-Supercells
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Summary of Skill Scores- Supercells vs.
Non-Supercells
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SRH and TAU- Linear Relationship

• TAU and SRH can be used interchangeably in
  anticipating storm-type
• The linear relationship between TAU and SRH
  allows an approximation of SRH (TAU) to be
  computed through TAU (SRH)

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7/31/04-Southwest Minnesota
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8/2/04-Southwest South Dakota
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Conclusions

• Storm-relative inflow is critical to supercell
  development
• Parameters which incorporate storm-relative winds
  in the inflow layer are best at discriminating
  between supercell and non-supercell environments

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Conclusions

• V C evaluates the balance between the
  updraft and storm-relative inflow
• Discrete updraft development, often observed in
  multicellular systems, is more likely when V
  C ltlt 1.0
• Continuous updraft propagation is likely when V
  C approaches or exceeds 1.0

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Conclusions

• TAU is a measure of the potential magnitude of
  streamwise vorticity that an updraft will ingest
• Skill scores indicate that a TAU value gt 2.5
  (x 10-3 s-1) favors supercell thunderstorm
  development
• TAU is closely correlated with SRH

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Acknowledgments
Brian Thalken and Dave Keller