The Structure and Climatology of Boundary Layer Winds in

the Southeast United States and its Relationship to Nocturnal

Tornado EpisodesAlicia C. Wasula and Lance F. Bosart

University at Albany/SUNY, Albany, NY

Russell Schneider, Steven J. Weiss, and Robert H. JohnsStorm Prediction Center, Norman, OK

Geoffrey S. ManikinNOAA/NWS/NCEP/EMC, Camp Springs, MD

Patrick WelshNOAA/NWSFO Jacksonville, FL

Research Supported byCOMET grant#S99-19133

• High frequency of overnight/early morning tornadoes in southeast US

• Particularly strong signal near Gulf Coast• Relatively high number of fatalities at night

32 N

F2 or Greater 1950-2001East of 94 W, South of 36.5 N

Month

Tim

e (U

TC

)

20

20

5040

30

4030

20

20

20

30

6050

20

10

2010

10

10

10

20

10

20

40

30

3020

60

50

8070

40

40

50

10090

60

30

20

20

4030

20

50

APR MAY JUN JUL AUG SEP OCT DEC JANNOVFEB MARJAN

6-9

6-9

9-12

12-15

18-21

15-18

0-3

21-0

3-6

• High frequency of overnight/early morning tornadoes in southeast US

• Particularly strong signal near Gulf Coast• Relatively high number of fatalities at night

32 N

9

9

12

12

129

99

12

1212

12

99

9

18 15

18159

9

66

6

6

12

15

3

9

3

6

6

6

9

3

3

3

3 39

9

9

3

12

12

12

12

2118

18

15

15

15

1518

91212

18

2124

12

15

18

15

15

1215

96

15

15

9

96

12

15

12

9

9

F2 or Greater 1950-2001East of 94 W, South of 32 N

Month

Tim

e (U

TC

)

APR MAY JUN JUL AUG SEP OCT DEC JANNOVFEB MARJAN

6-9

6-9

9-12

12-15

18-21

15-18

0-3

21-0

3-6

Motivation

1. What is the climatology of surface and boundary layer winds in the southeast US?

2. How do they vary diurnally?

3. What does the boundary layer wind structure look like during tornado episodes?

Data Sources

• Historical pilot balloon (pibal) wind data– 1948-1957, 4x/day, wind data for lowest ~3km

• Hourly surface data (1995-2000)

• Eta regional reanalysis (32 km)

Pibal Stations

1000 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

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

R2 = 0.03R2 = 0.37

R2 = 0.62

-1

0

1

2

3

4

5

6

7

8

25 27 29 31 33 35 37 39

Latitude (deg)

Spe

ed (m

/s)

1000 m 9-21 diff. 09Z Speed 21Z Speed

9-21Z diff. trendline 09Z Speed trendline 21Z Speed trendline

Surface Stations

COASTAL

INLAND

FL East Coast

FL West Coast

FL Central

60 m

AEX Windrose

Coastal Stations

15%

20%

25%

30%

35%

East South West

%

Day=15-18Z

Dusk=00-03Z

Night=06-09Z

Dawn=12-15Z

0

240 120

E

S

W

Inland Stations

15%

20%

25%

30%

35%

East South West

%

Day=15-18Z

Dusk=00-03Z

Night=06-09Z

Dawn=12-15Z

0

240 120

E

S

W

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

Summary: Climatology

• Surface winds back along coast at night

• 1 km southerly low-level jet helps increase shear at night

• Question: What happens during tornado episodes?

Composite HodographsComposite Hodograph (Pibals)

0

2

4

6

8

10

12

14

-5 0 5 10 15 20 25

u (m/s)

v (m

/s)

All

024

1506

3006

50023

100022

150017

200015

250011

300010

0 m500 m1000 m1500 m2000 m2500 m3000 m

Composite Hodograph

0

2

4

6

8

10

12

14

16

-5 0 5 10 15 20 25

u (m/s)

v (

m/s

) 15Z

03Z

21Z

3-7Z n=415-19Z n=521-01Z n=12

Criteria:•Within 167 km Radius•Up to 4 h prior to first report

Surface Composite - Methodology

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

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

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

Surface Composite – All EventsT’

PMSL

X

75th 25th Percentile

8°C Td’

10°C Td’

DAY NIGHT

X X

25th

75th Percentile

T’

PMSL

8°C Td’

10°C Td’

Conclusions

• Pibal Climo: Evidence of southerly nocturnal LLJ near Gulf coast at 1 km

• Surface Climo: Higher SLP over land vs. water force nocturnal easterly component along coast

• Composite hodographs: Veering wind profile, stronger winds in low-levels at night (small sample size due to much missing data)

• Surface composite: First tornado occurs at highest temp. gradient, on edge of moisture surge, in region of most backed surface winds