1. The Structure and Climatology of Boundary Layer Winds in the Southeast United States and its Relationship to Nocturnal Tornado Episodes Alicia C. Wasula and Lance F. Bosart University at Albany/SUNY, Albany, NY Russell Schneider, Steven J. Weiss, and Robert H. Johns Storm Prediction Center, Norman, OK Geoffrey S. Manikin NOAA/NWS/NCEP/EMC, Camp Springs, MD Patrick Welsh NOAA/NWSFO Jacksonville, FL Research Supported by COMET grant #S99-19133

2. 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

3. 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

4. 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?

5. 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)

6. Pibal Stations

7. 1000 m Wind Climatology

10. Surface Stations COASTAL INLAND FL East Coast FL West Coast FL Central 60 m

11. AEX Windrose

12. Coastal Stations

13. Inland Stations

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

15. 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?

16. Composite Hodographs 0 m 500 m 1000 m 1500 m 2000 m 2500 m 3000 m Composite Hodograph 0 2 4 6 8 10 12 14 16 -50510152025 u (m/s) v (m/s) 15Z 03Z 21Z 3-7Z n=4 15-19Z n=5 21-01Z n=12 Criteria: Within 167 km Radius Up to 4 h prior to first report

17. 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

18. Surface Composite – All Events T’ PMSL X 75 th 25th Percentile 8°C T d ’ 10°C T d ’

19. DAY NIGHT X X 25 th 75th Percentile T’ PMSL 8°C T d ’ 10°C T d ’

20. 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