A Comprehensive Study of Cool Season Tornadoes in the Southeast United States Alicia C. Wasula July...

A Comprehensive Study of A Comprehensive Study of Cool Season Tornadoes in Cool Season Tornadoes in

the Southeast United the Southeast United StatesStates

Alicia C. WasulaAlicia C. Wasula

July 13, 2005July 13, 2005

Committee MembersCommittee Members

• Lance BosartLance Bosart• John MolinariJohn Molinari• Dan KeyserDan Keyser• Chris ThorncroftChris Thorncroft• Russ SchneiderRuss Schneider• Morris WeismanMorris Weisman

The ProblemThe Problem

• Nocturnal peak in tornado Nocturnal peak in tornado occurrence during cool season along occurrence during cool season along Gulf coastGulf coast

• Difficulty warning public Difficulty warning public increased increased injury and loss of life (e.g., Fike 1993)injury and loss of life (e.g., Fike 1993)

• Difficult to discriminate between true Difficult to discriminate between true threats and null events in real timethreats and null events in real time

GoalsGoals

• Provide evidence that this Provide evidence that this phenomenon existsphenomenon exists

• Provide possible physical explanations Provide possible physical explanations via:via:– Climatological analysis of low-level windsClimatological analysis of low-level winds– Composite analysis of southeast US Composite analysis of southeast US

tornado episodestornado episodes– Case study: 22-23 February 1998Case study: 22-23 February 1998

• Relate results to tornadogenesis Relate results to tornadogenesis mechanismsmechanisms

OutlineOutline

• Historical BackgroundHistorical Background• Review of low-level mesocyclone-Review of low-level mesocyclone-

genesis mechanismsgenesis mechanisms• ClimatologyClimatology• Composite resultsComposite results• Case Study: 22-23 February 1998Case Study: 22-23 February 1998• ConclusionsConclusions• Future WorkFuture Work

BackgroundBackground

• Cool season nocturnal tornado peak Cool season nocturnal tornado peak exists along Gulf coast (e.g. Knupp exists along Gulf coast (e.g. Knupp and Garinger 1993, Brooks et al. and Garinger 1993, Brooks et al. 2003)2003)– Peak exists during early morning hours Peak exists during early morning hours

~ 1200 UTC~ 1200 UTC• Can occur in low CAPE/high shear

environments• Return Flow air trajectory effects PBL

thermodynamics


• Cool season nocturnal tornado peak exists along Gulf coast (e.g. Knupp and Garinger 1993, Brooks et al. 2003)

• Can occur in low CAPE/high shear Can occur in low CAPE/high shear environmentsenvironments

• Return Flow air trajectory effects PBL thermodynamics

0-2

km

posi

tive m

ean

sh

ear

(x 1

00

-2 k

m p

osi

tive m

ean

sh

ear

(x 1

0 -3-3 s s -

-

11))Convective Available Potential Energy (CAPE, Convective Available Potential Energy (CAPE, J/kg)J/kg)

Johns et Johns et al. 1993al. 1993

CAPE vs. Shear for Cold Season Tornado CAPE vs. Shear for Cold Season Tornado CasesCases


• Cool season nocturnal tornado peak exists along Gulf coast (e.g. Knupp and Garinger 1993, Brooks et al. 2003)

• Can occur in low CAPE/high shear environments

• Return Flow air trajectory effects PBL Return Flow air trajectory effects PBL thermodynamicsthermodynamics

Types of Return Flow (Crisp Types of Return Flow (Crisp and Lewis 1992)and Lewis 1992)

Previous Composite Study: Peninsular Previous Composite Study: Peninsular Florida Tornado Environments Florida Tornado Environments

(Hagemeyer and Schmocker 1991)(Hagemeyer and Schmocker 1991)

1) 1) ThermodynamicsThermodynamics

Dry Season

Wet Season

ww

All non-TCAll non-TCtornadoestornadoes±± 2 h of 00/12Z 2 h of 00/12Z1980-19881980-1988

Dry seasonDry seasonww’ = ~5’ = ~5°C°C

Wet seasonWet seasonww’ = ~1’ = ~1°C°C

Previous Composite Study: Peninsular Previous Composite Study: Peninsular Florida Tornado Environments Florida Tornado Environments

(Hagemeyer and Schmocker 1991)(Hagemeyer and Schmocker 1991)

Dry Season

Wet Season

2) 2) ShearShear

uu vv

All non-TCAll non-TCtornadoestornadoes±± 2 h of 00/12Z 2 h of 00/12Z1980-19881980-1988

Question #1: How does Question #1: How does buoyancy/shear relationship buoyancy/shear relationship

effect convective mode?effect convective mode?

High, Dry

High,Moist

Mod.

Low

0 10 20 30 40 50 60

Ordinary Multicell Supercell

Relationship of Buoyancy/Shear to Convective ModeRelationship of Buoyancy/Shear to Convective Mode

CAPE (J/kg)CAPE (J/kg)

Shear (m/s)Shear (m/s)

Question #2: Given that Question #2: Given that shear is sufficient for shear is sufficient for supercells, how does supercells, how does

hodograph shape determine hodograph shape determine supercell type?supercell type?

Splitting Storm in Sheared Splitting Storm in Sheared FlowFlow

Klemp (1987)Klemp (1987)

Straight HodographStraight Hodograph

Left & right moverLeft & right moverequally dominantequally dominant

Left mover fasterLeft mover faster

Source: A Convective Source: A Convective Storm MatrixStorm Matrixhttp://meted.ucar.edu/convectn/csmatrix/http://meted.ucar.edu/convectn/csmatrix/

Clockwise ¼ turn HodographClockwise ¼ turn Hodograph

Right moverRight moverdominantdominant

Left mover dissipatesLeft mover dissipates


Counterclockwise ¼ turn HodographCounterclockwise ¼ turn Hodograph

Left moverLeft moverdominantdominant

Right mover dissipatesRight mover dissipates


Question #3:How does Question #3:How does near-surface hodograph near-surface hodograph shape effect shape effect low-level low-level

mesocyclone generation?mesocyclone generation?

Source of low-level rotation: baroclinic generation of Source of low-level rotation: baroclinic generation of horizontal vorticity in FFDhorizontal vorticity in FFDstretched in updraftstretched in updraft

FFDFFD


Streamwise VorticityStreamwise Vorticity

V

VVVorticityStreamwise

•Maximized when vorticity vector and velocity vector are parallel Maximized when vorticity vector and velocity vector are parallel •0 when vorticity vector and velocity vector are perpendicular0 when vorticity vector and velocity vector are perpendicular

Wicker (1996)

Horizontal Horizontal VorticityVorticityVectorVector((HH))

StormStormMotionMotionVectorVector

Wicker (1996)

Vertical VelocityVertical Velocityand and Horizontal WindsHorizontal WindsAt 50 m, 110 minAt 50 m, 110 min

EnvironmentalEnvironmentalWindsWinds

Storm InflowStorm Inflow

Wicker (1996)

Pert. TemperaturePert. Temperatureand and Horizontal VorticityHorizontal VorticityAt 50 m, 110 minAt 50 m, 110 min

FFD FFD HH adds to env. adds to env. HH

HH Parallel to Parallel to inflow at FFDinflow at FFDLarge streamwiseLarge streamwise vorticityvorticity

X

UpdraftUpdraft

Previous work has shown Previous work has shown that supercells interacting that supercells interacting

with boundaries can with boundaries can produce long-lived produce long-lived

tornadoestornadoese.g., Maddox et al. 1980, e.g., Maddox et al. 1980, Markowski et al. 1998, Markowski et al. 1998, Rasmussen et al. 2000Rasmussen et al. 2000

Maddox et al. 1980Maddox et al. 1980

Along outflow boundary (C), Along outflow boundary (C), streamwise vorticity is streamwise vorticity is

maximizedmaximizedVV

ClimatologyClimatology

Tornado ClimatologyTornado Climatology

Southeast US DomainSoutheast US Domain

Storm DataStorm Data

• Tornado Database 1950-2001Tornado Database 1950-2001• Corrected for duplicate reportsCorrected for duplicate reports• Use only F2 or greater tornadoes for Use only F2 or greater tornadoes for

climatologyclimatology• All tornadoes considered for All tornadoes considered for

compositescomposites• Cool season tornadoes (November – Cool season tornadoes (November –

March)March)

By By monthmonth

By Hour (UTC) By Hour (UTC)

1000 m Wind Climatology1000 m Wind Climatology

Pilot Balloon Pilot Balloon StationsStations

Pilot Ballon (Pibal) DatasetPilot Ballon (Pibal) Dataset

• 1948-19571948-1957• 4x/day (0300, 0900, 1500, and 2100 4x/day (0300, 0900, 1500, and 2100

UTC)UTC)• Wind data for lowest ~ 3 kmWind data for lowest ~ 3 km• No thermodynamic dataNo thermodynamic data• No winds above cloud baseNo winds above cloud base

1000 m Wind Climatology1000 m Wind Climatology

0900-2100 UTC Meridional Wind (1000 m) vs. Latitude

R2 = 0.73

R2 = 0.28

R2 = 0.05

-3

-2

-1

0

1

2

3

4

5

6

7

25 27 29 31 33 35 37 39

Latitude (deg)

09Z

- 2

1Z V

-win

d d

iffe

ren

ce

(m/s

)

1000 m 09Z V-wind 21Z V-wind

9-21Z diff. trendline 09Z V-wind trendline 21Z V-wind trendline

Surface Wind ClimatologySurface Wind Climatology

Surface Surface StationsStations

Surface ObservationsSurface Observations

• Locally archived at University at Locally archived at University at AlbanyAlbany

• HourlyHourly• Cool seasons 1995-96 to 1999-2000Cool seasons 1995-96 to 1999-2000• Subset of stations for which dataset Subset of stations for which dataset

is completeis complete

AEX Windroses by Hour

0%

5%

10%

15%

20%360

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

00Z

06Z

12Z

18Z

Sample WindrosesSample Windroses

MEM Windroses by Hour

0%

5%

10%

15%360

2040

60

80

100

120

140160

180200

220

240

260

280

300

320340

00Z

06Z

12Z

18Z

Coastal StationCoastal Station

Inland Station Inland Station

CoastalCoastal Stations (n=16)Stations (n=16)

Inland Stations (n=17)Inland Stations (n=17)

HypothesisHypothesis

• More cooling / higher pressures over More cooling / higher pressures over land at nightland at night

• Less cooling / lower pressures over Less cooling / lower pressures over ocean at nightocean at night

• Easterly geostrophic component at Easterly geostrophic component at surfacesurface

MORE COOLING AT NIGHT

H

LAND

OCEAN

LESS COOLING AT NIGHT

L

Vg

Eta NARR DatasetEta NARR Dataset

• 32 km reanalysis32 km reanalysis• November – March 1999-2000November – March 1999-2000• 0900-2100 UTC sea level pressure 0900-2100 UTC sea level pressure

differencedifference

Nov-Mar 1999-2000 09–21 UTC SLP Diff. (hPa) – Eta NARR

Summary: ClimatologySummary: Climatology

• Cool season tornado peak in southeast Cool season tornado peak in southeast USUS

• Nocturnal maximum is strongest near Nocturnal maximum is strongest near Gulf of Mexico coastGulf of Mexico coast

• 1000 m nocturnal LLJ is strongest near 1000 m nocturnal LLJ is strongest near Gulf coastGulf coast

• Surface winds along coastline back at Surface winds along coastline back at night due to more (less) cooling over night due to more (less) cooling over land (water)land (water)

CompositesComposites

Tornado Episode Tornado Episode CompositesComposites

• NCEP/NCAR 2.5NCEP/NCAR 2.5x 2.5x 2.5 Reanalysis dataset Reanalysis dataset• All tornado episodes 1950-2001All tornado episodes 1950-2001• ‘‘Episode-relative’ compositeEpisode-relative’ composite• Grouped by start time: 0000-0600, 0600-Grouped by start time: 0000-0600, 0600-

1200, 1200-1800, 1800-0000 UTC1200, 1200-1800, 1800-0000 UTC• Will show only TWO:Will show only TWO:

– DAY: 1800-0000 UTC (n=327)DAY: 1800-0000 UTC (n=327)– NIGHT: 0600-1200 UTC (n=130)NIGHT: 0600-1200 UTC (n=130)

DAYDAY NIGHTNIGHT

200 hPa height (m), isotachs (m s200 hPa height (m), isotachs (m s-1-1))

DAYDAY NIGHTNIGHT

1000 hPa height (m), 1000-500 hPa thck. (dam), 700 hPa RH (%)1000 hPa height (m), 1000-500 hPa thck. (dam), 700 hPa RH (%)

DAYDAY NIGHTNIGHT

700 hPa height (m), vertical motion (x 10700 hPa height (m), vertical motion (x 10-3-3 hPa s hPa s-1-1))

DAYDAY NIGHTNIGHT

850 hPa winds, 850 hPa 850 hPa winds, 850 hPa ee (K), 850-500 hPa lapse rate ( (K), 850-500 hPa lapse rate (C)C)

DAYDAY NIGHTNIGHT

CAPECAPE 590 J kg590 J kg-1-1 232 J kg232 J kg--

11

LCLLCL 965 hPa965 hPa 977 hPa977 hPa0-6 km shear0-6 km shear 19 m s19 m s-1-1 20 m s20 m s-1-10-2 km shear0-2 km shear 8 m s8 m s-1-1 8 m s8 m s-1-1

Surface Composite - Surface Composite - MethodologyMethodology

• Bin surface obs in 1Bin surface obs in 1° x 1° boxes ° x 1° boxes relative to first tornado reportrelative to first tornado report

• Calculate temp/dew point Calculate temp/dew point anomalies relative to monthly anomalies relative to monthly climo for that stationclimo for that station

• Composite u, v, PMSL, temp/dew Composite u, v, PMSL, temp/dew point anomalies for each grid boxpoint anomalies for each grid box

T’

PMSL

Surface Composite – All EventsSurface Composite – All Events

X

T’

Td’

vp

u

n

8°C Td’

10°C Td’

DAYDAY NIGHT NIGHT

X X

T’

Td’

v pu

nT’

PMSL

8°C Td’

10°C Td’

Summary: CompositesSummary: Composites

• Strong signal in spite of large sample size:Strong signal in spite of large sample size:– ULJ entrance region at 200 hPaULJ entrance region at 200 hPa– Vigorous upstream trough at 500 hPaVigorous upstream trough at 500 hPa– Southwesterly LLJ at 850 hPaSouthwesterly LLJ at 850 hPa

– Low-level Low-level ee ridge ridge

• Surface composites show 1Surface composites show 1stst tornado tornado occurs:occurs:– At strongest At strongest TT– On northern edge of moisture surge/southerly On northern edge of moisture surge/southerly

flowflow

Case Study:Case Study:22-23 February 22-23 February

19981998Central Florida Central Florida

Tornado OutbreakTornado Outbreak

Case OverviewCase Overview

• Nocturnal outbreak over central Florida Nocturnal outbreak over central Florida (~0000-0600 UTC)(~0000-0600 UTC)

• 42 fatalities/260 injuries42 fatalities/260 injuries• Ample shear/instability south of surface Ample shear/instability south of surface

front in central FLfront in central FL• System was disorganized over Gulf of System was disorganized over Gulf of

Mexico but rapidly intensified as it moved Mexico but rapidly intensified as it moved onshoreonshore

• Rapidly evolving threat + nocturnal nature Rapidly evolving threat + nocturnal nature made warning public difficultmade warning public difficult

Storm ReportsStorm Reports

+ Wind > 26 m s-1

Hail > 2 cm Tornado

TBW

MCO

GNV

1000 hPa hgt, 1000-500 hPa THCK 500 hPa HGHT, 1000 hPa hgt, 1000-500 hPa THCK 500 hPa HGHT, AVORAVOR

200 hPa HGHT, Isotachs 850 hPa HGHT, e, Isotachs

23 February 1998 0000 UTC

22/1815 22/1815 UTCUTC

23/0015 23/0015 UTCUTC

CAPE = 2891 J/kgLI = -9 CLCL = 962 hPa

TBW Sounding and Hodograph 23/0000 UTC

925

850 700

500

T

M

G

SST and SST’ 22 February SST and SST’ 22 February 19981998

m s-1

dBZ23/0156 23/0156 UTCUTC

m s-1

dBZ23/0336 23/0336 UTCUTC

m s-1

dBZ23/0515 23/0515 UTCUTC

QuestionQuestion

• Why did the disorganized convective Why did the disorganized convective line intensify so rapidly after making line intensify so rapidly after making landfall on the Florida peninsula?landfall on the Florida peninsula?

• 23/0000 UTC Surface Analysis• Lightning for ½ hour Ending 23/0000 UTC

GNV/MCO MeteogramsGNV/MCO Meteograms

e (

K)

e (

K)

T

M

G

Surface FrontogenesisSurface Frontogenesis

22/2100 UTC22/2100 UTC 23/0000 UTC23/0000 UTC

Surface FrontogenesisSurface Frontogenesis

23/0200 UTC23/0200 UTC 23/0400 UTC23/0400 UTC

Surface VorticitySurface Vorticity

23/0000 UTC23/0000 UTC 23/0400 UTC23/0400 UTC

Maddox et al. 1980Maddox et al. 1980

Summary: Case StudySummary: Case Study

• Costliest/deadliest tornado outbreak Costliest/deadliest tornado outbreak in FL historyin FL history

• Convective intensity related to SST’ in Convective intensity related to SST’ in Gulf of MexicoGulf of Mexico

• Rapid supercell development onshore Rapid supercell development onshore related to diabatically induced frontrelated to diabatically induced front– Inferred enhanced ascent + backed Inferred enhanced ascent + backed

windswinds• Ample instability/strong shear south Ample instability/strong shear south

of front (TBW sounding)of front (TBW sounding)

Summary: Case StudySummary: Case Study

• Surface vorticity at front increased rapidly Surface vorticity at front increased rapidly as convective line moved onshoreas convective line moved onshore

• Hypothesis: Storms ingested inflow from Hypothesis: Storms ingested inflow from highly helical (i.e. large streamwise highly helical (i.e. large streamwise vorticity) air as they came onshore at front vorticity) air as they came onshore at front rapid dev. of low-level rotation rapid dev. of low-level rotation

• Numerical models (e.g., Eta) failed to Numerical models (e.g., Eta) failed to capture shallow but strong surface frontcapture shallow but strong surface front

ConclusionsConclusions

Conclusions:Conclusions:

• Nocturnal cool season tornado Nocturnal cool season tornado phenomenon along Gulf coast existsphenomenon along Gulf coast exists

• Climatologically favorable 0-1 km shear Climatologically favorable 0-1 km shear profile exists along coastline at night:profile exists along coastline at night:– Nocturnal LLJ maximized at coast at ~ 1000 mNocturnal LLJ maximized at coast at ~ 1000 m– Surface winds ‘back’ along coastline at nightSurface winds ‘back’ along coastline at night

• This shear profile is enhanced when mid-This shear profile is enhanced when mid-latitude system moves across southeast latitude system moves across southeast US (e.g. isallobaric wind w/surface low to US (e.g. isallobaric wind w/surface low to west west enhanced backing in warm air + enhanced backing in warm air + strong LLJstrong LLJ

Conclusions cont’d:Conclusions cont’d:

• Composites show common features:Composites show common features:– ULJ entrance region at ~ 200 hPaULJ entrance region at ~ 200 hPa– Vigorous upstream trough at 500 hPaVigorous upstream trough at 500 hPa– LLJ and LLJ and ee ridge at 850 hPa ridge at 850 hPa– Surface cyclone to westSurface cyclone to west– Presence of highly anomalous (> 8Presence of highly anomalous (> 8C) C)

surface moisturesurface moisture– Surface front or boundary nearbySurface front or boundary nearby– Stronger features during nocturnal Stronger features during nocturnal

episodesepisodes

Schematic Cool Season Southeast US Tornado Setup

Conclusions cont’d:Conclusions cont’d:

• Case study showed features similar Case study showed features similar to composite:to composite:– Strong upper-level system to westStrong upper-level system to west– Strengthening surface front (diabatically Strengthening surface front (diabatically

generated even after sunset)generated even after sunset)– Anomalous moisture & LLJ at 850 hPaAnomalous moisture & LLJ at 850 hPa

Cold Season (n=75)Cold Season (n=75) Warm Season (n=69)Warm Season (n=69)Johns et al. (1993) Johns et al. (1993)

Cold Season Cold Season Warm Warm SeasonSeason

(Southeast)(Southeast) (Midwest)(Midwest)

• Lower CAPELower CAPE• Less steep lapse Less steep lapse

ratesrates• Higher shearHigher shear• Highly anomalous Highly anomalous

low-level moisturelow-level moisture

• Higher CAPEHigher CAPE• More steep lapse More steep lapse

raterate• Lower shearLower shear

Future WorkFuture Work• More detailed investigation of null eventsMore detailed investigation of null events

– Composite based upon high risk outlook locationComposite based upon high risk outlook location– What are good discriminators between null What are good discriminators between null

event/true threat?event/true threat?• CAPE distribution (elevated vs. sfc. based)CAPE distribution (elevated vs. sfc. based)• 0-1 km shear profile (degree of low-level backing)0-1 km shear profile (degree of low-level backing)• Presence/orientation of sfc. boundariesPresence/orientation of sfc. boundaries

• What about other types of severe weather What about other types of severe weather (e.g., high wind, hail, flash flooding)?(e.g., high wind, hail, flash flooding)?

• How do ingredients necessary for southeast How do ingredients necessary for southeast US nocturnal tornado episode compare to US nocturnal tornado episode compare to warm season Great Plains tornado episode?warm season Great Plains tornado episode?– More moisture at low-levelsMore moisture at low-levels– Lower CAPE, higher 0-6 km shearLower CAPE, higher 0-6 km shear

AcknowledgementsAcknowledgements

• LanceLance• Committee: Dan Keyser, John Committee: Dan Keyser, John

Molinari, Chris Thorncroft, Russ Molinari, Chris Thorncroft, Russ Schneider, Morris WeismanSchneider, Morris Weisman

• COMETCOMET• Grant Participants: Russ Schneider, Grant Participants: Russ Schneider,

Steve Weiss, Bob Johns, Geoff Steve Weiss, Bob Johns, Geoff Manikin, Pat WelshManikin, Pat Welsh

Acknowledgements (cont’d)Acknowledgements (cont’d)

• Celeste, Diana, Lynn, SharonCeleste, Diana, Lynn, Sharon• Grad Students, past and presentGrad Students, past and present• Sheryl (Honikman) Thorp, Eyad Sheryl (Honikman) Thorp, Eyad

Atallah, Kristen Corbosiero, Mike Atallah, Kristen Corbosiero, Mike Notaro, Teresa Bals-Elsholz, Dan Notaro, Teresa Bals-Elsholz, Dan Meade, Tom Galarneau, Anantha Meade, Tom Galarneau, Anantha AiyyerAiyyer

• My Family: my parents and brotherMy Family: my parents and brother• Tom and CatherineTom and Catherine

A Comprehensive Study of Cool Season Tornadoes in the Southeast United States Alicia C. Wasula July...

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