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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
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