A Preliminary Investigation of Supercell Longevity

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A Preliminary Investigation of Supercell Longevity

• MATTHEW J. BUNKERS, JEFFREY S. JOHNSON, JASON M.
  GRZYWACZ, LEE J. CZEPYHA, and BRIAN A. KLIMOWSKI
• NWS Rapid City, SD
• 6th High Plains Conference
• Dodge City, KS (10/9/2002 10/11/2002)

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Objectives

• Study long-lived supercell characteristics,
  evolution, demise, and environments (lifetimes ?
  4 h)
• Compare and contrast with those of short-lived
  supercells (lifetimes ? 2 h)
• How long will supercells last on any given day?

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Background

• Kinematics (shear, SRH, storm-relative winds)
• Thermodynamics (CAPE, CIN, LCL, relative
  humidity)
• Large-scale environment (boundaries, forcing
  mechanisms, moisture/instability axes, storm
  mergers/interactions convective mode)

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Data

• 42 long-lived supercell eventswith the majority
  across the northern High Plains
• 43 short-lived supercell eventsaverage lifetime
  of all supercells ? 2 h per event
• 22 moderate-lived supercell events (average
  lifetime between 2 and 4 h)
• Sounding data ( a few RUC soundings)

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Long-Lived SC Tracks
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General Observations(Long-lived supercells)

• 88 of the long-lived supercells were isolated
• All severe 75 produced severe hail and wind
  58 produced tornadoes
• Motion generally constant throughout lifetime

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Example 1
Start

• CL ? HP

End
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Example 2

• Warm front

End
Start
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Example 3
Start
End

• Elevated ?
• surface-based

Start
End
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Example of Low-End Event
Just north of BIS.
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Characteristics of Long-Lived Supercell Demise

• 70 weakened and/or dissipated
• 20 evolved into bows (some via mergers)
• 10 merged with other storms and lost identity
• -----------------------------------------------
  --
• Short-lived supercells 45, 45, 10

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Kinematic Results

• Deep-layer vertical wind shear significantly
  stronger for long- vs. short-lived supercells (?
  0.0001)
• Mean 0?8-km bulk shear (Long-lived) 36.6 m s-1
• Mean 0?8-km bulk shear (Short-lived) 21.3 m s-1
• 8-km storm-relative wind significantly stronger
  for long- vs. short-lived supercells (? 0.0001)
• Mean 8-km SRW (Long-lived) 20.5 m s-1
• Mean 8-km SRW (Short-lived) 12.1 m s-1

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BIS
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Contingency Table Results(long- vs. short-lived
SCs only!)

• Optimal 0?8-km bulk shear 30 m s-1
• POD 0.86, FAR 0.12, CSI 0.77
• Optimal 4?8-km bulk shear 10 m s-1
• POD 0.83, FAR 0.26, CSI 0.64
• Optimal 0?3-km SRH 200 m2 s-2
• POD 0.60, FAR 0.38, CSI 0.44

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10 km
10 km
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Thermodynamic Results

• Long-Lived Supercells
• MLCAPE 1415 J kg-1 ? 643
• MLCIN -42 J kg-1 ? 53
• MLBRN 13 ? 8
• MLLCL 1448 m ? 502
• Short-Lived Supercells
• MLCAPE 1623 J kg-1 ? 1241
• MLCIN -68 J kg-1 ? 69
• MLBRN 31 ? 46
• MLLCL 1771 m ? 565

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Large-Scale Environments

• Long-lived supercells often moved parallel to a
  moisture or instability axis, or moved at a
  similar speed as an instability axis
• Supercells were not observed to re-orient
  themselves along boundaries (change direction),
  but they often occurred in close proximity to
  theme.g., the BIS case

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One Possible Setting
8 km
SC motion
L
sfc

• 0?8-km bulk shear gt 25-30 m s-1
• 8-km SR wind gt 13-15 m s-1
• MLCAPE gt 800-1000 J kg-1
• MLCIN lt 75-100 J kg-1
• MLBRN 10 to 25
• MLLCL lt 1500-2000 m

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One Possible Setting
8 km
SC motion
L
sfc

• 0?8-km bulk shear gt 25-30 m s-1 (22)
• 8-km SR wind gt 13-15 m s-1 (6)
• MLCAPE gt 800-1000 J kg-1 (2483)
• MLCIN lt 75-100 J kg-1 (8.9)
• MLBRN 10 to 25 (12)
• MLLCL lt 1500-2000 m (1167)

BIS values from earlier radar example
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Summary

• Long-lived supercells (? 4 h)
• typically occur in strong shear environments,
  with strong storm-relative upper-level flow
• typically relatively isolated (vs. short-lived)
• produce considerable severe weather
• Supercell motion and boundary orientation may
  play a key role in supercell longevity
• External factors may act to limit supercell
  longevity in an otherwise favorable setting

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Acknowledgments

• The COMET Program
• The NOAA Central Library
• Wendy Abshire, COMET
• Dave Carpenter, NWS RAP
• Charlie Knight, NCAR
• Steve Williams, NCAR

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Additional Information

• 21st Conference on SLS, 655-658.
• matthew.bunkers_at_noaa.gov
• Thank you for your attentiontime for questions.