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A Preliminary Investigation of Supercell Longevity

A Preliminary Investigation of Supercell Longevity. M ATTHEW J. B UNKERS , J EFFREY S. J OHNSON , J ASON M. G RZYWACZ , L EE J. C ZEPYHA , and B RIAN A. K LIMOWSKI NWS Rapid City, SD 6 th High Plains Conference Dodge City, KS (10/9/2002 – 10/11/2002). Objectives:.

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A Preliminary Investigation of Supercell Longevity

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  1. 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)

  2. 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?”

  3. 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)

  4. Data: • 42 long-lived supercell events—with the majority across the northern High Plains • 43 short-lived supercell events—average 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)

  5. Long-Lived SC Tracks

  6. 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

  7. Example #1 Start • CL  HP End

  8. Example #2 • Warm front End Start

  9. Example #3 Start End • Elevated  • surface-based Start End

  10. Example of Low-End Event: Just north of BIS.

  11. 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%

  12. 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

  13. BIS

  14. 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

  15. 10 km 10 km

  16. 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

  17. 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 them…e.g., the BIS case

  18. One Possible Setting: 8 km SC motion • 08-km bulk shear > 25-30 m s-1 • 8-km SR wind > 13-15 m s-1 • MLCAPE > 800-1000 J kg-1 • MLCIN < 75-100 J kg-1 • MLBRN = 10 to 25 • MLLCL < 1500-2000 m L sfc

  19. One Possible Setting: 8 km SC motion • 08-km bulk shear > 25-30 m s-1 (22) • 8-km SR wind > 13-15 m s-1 (6) • MLCAPE > 800-1000 J kg-1 (2483) • MLCIN < 75-100 J kg-1 (8.9) • MLBRN = 10 to 25 (12) • MLLCL < 1500-2000 m (1167) L sfc BIS values from earlier radar example

  20. 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

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

  22. Additional Information: • 21st Conference on SLS, 655-658. • matthew.bunkers@noaa.gov • Thank you for your attention—time for questions.

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