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Alicia C. Wasula and Lance F. Bosart University at Albany/SUNY, Albany, NY

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

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Alicia C. Wasula and Lance F. Bosart University at Albany/SUNY, Albany, NY

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  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 • What is the climatology of surface and boundary layer winds in the southeast US? • How do they vary diurnally? • 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

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

  9. AEX Windrose

  10. Coastal Stations

  11. Inland Stations

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

  13. 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?

  14. Composite Hodographs 0 m 500 m 1000 m 1500 m 2000 m 2500 m 3000 m Composite Hodograph • Criteria: • Within 167 km Radius • Up to 4 h prior to first • report 16 14 12 15Z 10 v (m/s) 8 03Z 6 21Z 4 3-7Z n=4 2 15-19Z n=5 0 21-01Z n=12 -5 0 5 10 15 20 25 u (m/s)

  15. 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

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

  17. T’ PMSL DAY NIGHT X X 8°C Td’ 25th 75th Percentile 10°C Td’

  18. 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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