Predicting Supercell Motion Using A New Hodograph Technique

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Predicting SupercellMotion Using Hodograph
Techniques
Matthew J. Bunkers WFO Rapid City, SD Last
Updated 2/4/2002
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Based onPredicting Supercell Motion Using A New
Hodograph Technique
Matthew J. Bunkers, UNR Brian A. Klimowski,
UNR Jon W. Zeitler, HGX Richard L. Thompson,
SPC Morris L. Weisman, NCAR
by
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Objectives of Study

• Develop a dynamically based method that
  consistently predicts the motion of both right-
  and left-moving supercells (using only a
  hodograph)
• Compare the new method with existing methods of
  predicting supercell motion
• Recommend a preferred method for predicting
  supercell motion

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Supercell Motion Myths

• All supercells move to the right of the mean wind
  (not truecan move to the left of the mean wind!)
• If a storm is moving to the right of the mean
  wind, it is a supercell (not truecould just be a
  multicell storm)

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Justification forthis Study

• Some currently used methods fail under certain
  situations (because they are not Galilean
  invariant)
• Most supercells (gt 90) produce severe weather
  (i.e., hail, flooding, winds, tornadoes)
• Nearly all strong or violent tornadoes are
  produced by supercells

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Justification (Continued)

• Supercell motion is needed to evaluate
  storm-relative helicityhelping to discern
  tornadic potential
• Anvil-level storm-relative flow may be important
  in distinguishing among HP, CL, and LP supercells
• Most methods do not address the motion of
  left-moving supercells

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Importance of Galilean Invariance

• Next, idealized hodographs are used to illustrate
  how Galilean invariance applies to predicting
  supercell motion methods based on the mean wind
  are not Galilean invariant
• 1st slide cyclonic supercell moves slower and
  to the right of the mean wind (typical)
• 2nd slide cyclonic supercell moves faster and
  to the right of the mean wind (northwest flow)
• 3rd slide cyclonic supercell moves slower and
  to the left of the mean wind (rare)

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Upper-Right Quadrant
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Lower-Right Quadrant
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Upper-Left Quadrant
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Supercell Motion Prediction Methods

• Maddox (1976)30R75
• Colquhoun (1980)inflow outflow
• Davies and Johns (1993)30R75 and 20R85the JDL
  method
• Weisman (1996)COMET Program module
• Davies (1998)modification of DJ93 above
• Rasmussen and Blanchard (1998)offset from 0-4 km
  AGL shear
• Bunkers et al. (1998, 2000)this study

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Our Method

• A modification of Weisman (1996) and Weisman and
  Klemp (1986)
• Based on the internal dynamics of the
  supercellcalled the ID method
• Galilean invariant and shear-relative
• Observationally, dynamically, and theoretically
  based on studies from the 1940s to present
  (consistent pattern to supercell motion)

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The ID Method

• Uses the following physical concepts
• Advection of the storm by the mean wind
• Interaction of the convective updraft with the
  sheared environment to promote rotation and
  propagation
• Other external factors, including atmospheric
  boundaries and orography, are not accounted for

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The ID Method

• Following is a graphical depiction
• Plot the hodograph
• Plot the mean wind
• Draw the vertical wind shear
• Draw a line perpendicular to the vertical wind
  shear that passes through the mean wind
• Locate storm motion

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Equation for theCyclonic Supercell
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Data Used in Study

• 260 right-moving (cyclonic) supercells
• 30 left-moving (anticyclonic) supercells
• Data gathered from previous studies and the
  northern High Plainsprimary sources include
• Davies and Johns (1993)
• Brown (1993)
• Thompson (1998)

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Data Used (Continued)

• Most data gathered from 3 hours from 0000 UTC
  using radiosondes
• Some cases utilized WSR-88D, profilers, and
  averaged soundings
• Atypical hodographs were defined as those with
• 0-6 km AGL mean wind lt 10 m/s, or
• a surface wind with a northerly component and gt 5
  m/s

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Optimizing the ID Method

• Several iterations were performed to minimize the
  error in predicting supercell motion, with the
  final results being
• 0-6 km AGL non-pressure weighted mean wind
• 7.5 m/s deviation from the mean wind
• 0-0.5 km to 5.5-6 km mean shear vector

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Results (260 hodographs)
ID Method compared individually to others
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Typical Hodograph
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Results (148 Typical Hodographs)
ID Method compared individually to others
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Atypical Hodograph
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Results (77 Atypical Hodographs)
ID Method compared individually to others
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Australian Hodograph
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Importance of Storm Motion

• Research and operational studies have focused on
  Storm Relative Helicity (SRH) as a measure of
  supercell rotation and tornadic potential
• To determine SRH, Storm Motion must be known, or
  estimated (by definition)
• Following are some examples illustrating the
  variability of SRH, and the stability of the
  0-6km vertical wind shear

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Supercell-Helicity Relationship
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Supercell-Shear Relationship
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Summary

• The ID method, which is based on the theory for
  supercell propagation, is superior to the other
  proposed methods evaluated for all hodographs in
  this study (by 1 m/s)
• This method offers even more improvement in
  anticipating supercell motion and storm-relative
  parameters for atypical hodographs

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Summary (continued)

• The ID method allows for the prediction of
  left-moving supercells (unlike most other
  methods)
• When the 06-km vertical wind shear exceeds 30
  m/s, supercells become more likely (assuming
  convective initiation)
• The Eta model changed on April 21, 2000 to use
  the Bunkers et al. (2000) method for supercell
  motion input to SRH calculations

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Complications in Predicting Storm Motion

• Cold pool/shear interactions (internal)
• storm acceleration with time
• Boundaries, merging storms (external)
• e.g, drylines, fronts, outflows
• Orographic influences (external)
• Deeper or shallower storms (internal)
• e.g, mini-supercells, supercells over higher
  terrain, elevated supercells

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Complications in Predicting Storm Motion
(continued)

• If the shear is confined to the low levels, the
  supercell may become outflow-dominated
• stronger gust-front lifting less ventilation
  aloft
• If the shear is marginal and the CAPE is large,
  erratic movement may occur
• watch for boundaries/convergence zones
• new cell growth can dominate storm motion

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Complications in Predicting Storm Motion

• If the shear is exceptionally large, significant
  deviations from the mean wind may occur

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Bunkers and Zeitler (2000)Highly Deviant
Supercells, 20thSLS

• Even the ID Method fails to accurately predict
  the motion of some supercells (i.e., error gt 5
  m/s)
• A number of factors could account for these
  highly deviant supercells
• unrepresentative wind profile
• inappropriate mean wind layer
• exceptionally strong vertical wind shear
• weak mid-level vertical wind shear

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Bunkers and Zeitler (2000)Highly Deviant
Supercells, 20thSLS

• Focused on exceptionally strong vertical wind
  shear and weak mid-level vertical wind shear
• Expanded the dataset to 339 cases
• 245 (72) predictions had a mean absolute error
  of 2.7 m/s (Dataset 1)
• 94 (28) predictions had a mean absolute error of
  7.3 m/s (Dataset 2)

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Bunkers and Zeitler (2000)Highly Deviant
Supercells, 20thSLS

• Dataset 2 was split into 3 partitions
• Weak 08-km vertical wind shear
• Stronger gust front lifting (outflow dominated)
• Strong 08-km vertical wind shear
• Updraftshear interactions more important
  (supercell processes dominated)
• Strong 03-km shear/Weak 48-km shear
• Combination of gust front lifting and
  updraftshear interactions

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Recommendations

• Use the ID method as a starting point to predict
  supercell motion
• Determine if a shallower or deeper mean wind than
  0-6 km is warranted
• Identify boundaries and orography that may
  influence supercell motion
• Understand that the supercell motion will change
  with time
• Examine the distribution of the vertical wind
  shear
• Be aware of your environment!