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.

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, SD6th 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 How long will supercells last on any given day?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 88% of the long-lived supercells were “isolated”“isolated”

• All severe; 75% produced severe hail All severe; 75% produced severe hail andand wind; 58% produced tornadoes wind; 58% produced tornadoes

• Motion generally constant throughout Motion generally constant throughout lifetimelifetime

Example #1

• CL HPStart

End

Example #2

• Warm front

StartEnd

Example #3

• Elevated surface-based

End

End

Start

Start

Example of Low-End Event:

Just north of BIS.

Characteristics of Long-Lived Supercell Demise:

• 70%70% weakened and/or dissipated weakened and/or dissipated

• 20%20% evolved into bows (some via mergers) evolved into bows (some via mergers)

• 10%10% merged with other storms and lost merged with other storms and lost identityidentity

--------------------------------------------------------------------------------------------------• Short-lived supercellsShort-lived supercells: : 45%45%, , 45%45%, , 10%10%

Kinematic Results:

• Deep-layer vertical wind shear significantly stronger for long- vs. short-lived supercells ( = 0.0001)– Mean 08-km bulk shear (Long-lived) = 36.6 m s-1 – Mean 08-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

0

5

10

15

20

25

30

35

40

5 15 25 35 45 55

0-8-km Bulk Shear (m s-1)

8-k

m S

RW

(m

s-1

)Long-Lived (42)

Short-Lived (43)

Moderate-Lived (22)

0

5

10

15

20

25

30

35

40

5 15 25 35 45 55

0-8-km Bulk Shear (m s-1)

8-k

m S

RW

(m

s-1

)Long-Lived (42)

Short-Lived (43)

Moderate-Lived (22)

0

5

10

15

20

25

30

35

40

5 15 25 35 45 55

0-8-km Bulk Shear (m s-1)

8-k

m S

RW

(m

s-1

)Long-Lived (42)

Short-Lived (43)

Moderate-Lived (22)

BIS

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

• Optimal 0Optimal 08-km bulk shear = 30 m s8-km bulk shear = 30 m s-1-1

POD = 0.86POD = 0.86, FAR = 0.12, CSI = 0.77, FAR = 0.12, CSI = 0.77

• Optimal 4Optimal 48-km bulk shear = 10 m s8-km bulk shear = 10 m s-1-1

POD = 0.83POD = 0.83, FAR = 0.26, CSI = 0.64, FAR = 0.26, CSI = 0.64

• Optimal 0Optimal 03-km SRH = 200 m3-km SRH = 200 m22 s s-2-2

POD = 0.60POD = 0.60, FAR = 0.38, CSI = 0.44, FAR = 0.38, CSI = 0.44

VRM VRM

-25

-15

-5

5

15

25

-5 5 15 25 35 45

u (m s-1)

v (

m s

-1)

Long-Lived Supercells (42)

Short-Lived Supercells (43)

10 km10 km

Thermodynamic Results:

• Long-Lived SupercellsMLCAPE = 1415 J kg-1 643MLCIN = -42 J kg-1 53MLBRN = 13 8MLLCL = 1448 m 502

• Short-Lived SupercellsMLCAPE = 1623 J kg-1 1241MLCIN = -68 J kg-1 69MLBRN = 31 46MLLCL = 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

• 08-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

sfc

8 km

One Possible Setting:

SC motion

• 08-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)

sfc

8 km

BIS valuesfrom earlierradar 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:

• 21st Conference on SLS, 655-658.

• [email protected]

• Thank you for your attention—time for questions.