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: 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?”“How long will supercells last on any given day?”

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 )

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)Sounding data (+ a few RUC soundings)

Long-Lived SC Tracks

General Observations: (Long-lived supercells) 88% of the long-lived supercells were “isolated”88% of the long-lived supercells were “isolated” All severe; 75% produced severe hail and wind; 58% produced tornadoesAll severe; 75% produced severe hail and wind; 58% produced tornadoes Motion generally constant throughout lifetimeMotion generally constant throughout lifetime

Example #1 CL  HP Start End

Example #2 Warm front Start End

Example #3 Elevated  surface-based End Start

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

Characteristics of Long-Lived Supercell Demise: 70% weakened and/or dissipated70% weakened and/or dissipated 20% evolved into bows (some via mergers)20% evolved into bows (some via mergers) 10% merged with other storms and lost identity10% merged with other storms and lost identity Short-lived supercells: 45%, 45%, 10%Short-lived supercells: 45%, 45%, 10%

Kinematic Results: Deep-layer vertical wind shear significantly stronger for long- vs. short-lived supercells (  = ) –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 (  = ) –Mean 8-km SRW (Long-lived) = 20.5 m s -1 –Mean 8-km SRW (Short-lived) = 12.1 m s -1

BIS

Contingency Table Results: (long- vs. short-lived SCs only!) Optimal 0  8-km bulk shear = 30 m s -1Optimal 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 -1Optimal 4  8-km bulk shear = 10 m s -1 POD = 0.83, FAR = 0.26, CSI = 0.64 Optimal 0  3-km SRH = 200 m 2 s -2Optimal 0  3-km SRH = 200 m 2 s -2 POD = 0.60, FAR = 0.38, CSI = 0.44

10 km

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

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

One Possible Setting: SC motion 0  8-km bulk shear > m s -1 8-km SR wind > m s -1 MLCAPE > J kg -1 MLCIN < J kg -1 MLBRN = 10 to 25 MLLCL < m sfc 8 km

One Possible Setting: SC motion 0  8-km bulk shear > m s -1 (22) 8-km SR wind > m s -1 (6) MLCAPE > J kg -1 (2483) MLCIN < J kg -1 (8.9) MLBRN = 10 to 25 (12) MLLCL < m (1167) sfc 8 km BIS values from earlier radar example

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

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

Additional Information: 21 st Conference on SLS, Thank you for your attention—time for questions.