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Severe Hail Detection Using Reflectivity Elevations and Freezing Levels

Severe Hail Detection Using Reflectivity Elevations and Freezing Levels. Rod Donavon National Weather Service Grand Forks, ND. The views expressed are those of the author and do not necessarily represent those of the National Weather Service. Where is the Grand Forks (FGF) CWA?.

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Severe Hail Detection Using Reflectivity Elevations and Freezing Levels

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  1. Severe Hail Detection Using Reflectivity Elevations and Freezing Levels Rod Donavon National Weather Service Grand Forks, ND The views expressed are those of the author and do not necessarily represent those of the National Weather Service.

  2. Where is the Grand Forks (FGF) CWA?

  3. Severe Hail Detection • During the 2001 and 2002 Convective Seasons, the staff at FGF have had great success issuing Severe Thunderstorm Warnings based on 50-55 dBZ Reflectivity Max Heights (AGL) and Freezing Level Heights (MSL). • Average Lead Time during this period was 20.7 minutes, an increase of 6 minutes over 1996-2000 • POD increased from 0.854 to 0.919 • FAR lowered from 0.299 to 0.177

  4. Reflectivity All Tilts Product • Allows Radar Operator to view all available elevation slices within the current Volume Scan in Real Time. • Radar Operator can view storm structure and make decisions before derived products become available. • Can give idea of updraft strength and hydrometeor potential based on reflectivity heights.

  5. VCP 11: Radar Elevation Angles

  6. Data Used For Study • Storm Data to help identify events • Model Soundings and NCDC Freezing Level Charts • Archived Radar Data • Correlated 50-55 dBZ reflectivity max heights and associated hail reports with FZLs • Max Reflectivity Heights obtained using elevation scans and not with cross sections.

  7. Quality Control of Data • Core Dumps were not used. Looked for a core maintained for previous 2 volume scans. • Elevated Convection excluded (Future) • Hail report discarded if report likely not largest hail with storm at reported time. • Hail report discarded if unable to associate with a storm.

  8. Results For All Hail Sizes

  9. Results for 0.75-1.00 Inch Hail

  10. The Numbers Game Establishing a Warning Criteria (TEST) Freezing Level HeightsWarning Criteria 8000-11500 ft MSL Multiply FZL by 2 12000 ft MSL Multiply FZL by 2.1 = 26000 ft AGL 12500 ft MSL Multiply FZL by 2.25 = 28500 ft AGL 13000 ft MSL Multiply FZL by 2.4 = 31000 ft AGL 13500 ft MSL Multiply FZL by 2.6 = 35000 ft AGL 14000 ft MSL Multiply FZL by 2.7 = 38000 ft AGL

  11. Central Iowa April 18, 2002

  12. What is the Criteria for 4/18/02? Warning Criteria = 28500 ft Freezing Level on 4/18/02 = 12500 ft MSL +-200 ft

  13. Results ForCentral Iowa 4/18/02 • 21 Hail events using 10 mi/15 min Rule • 50-55 dBZ Reflectivity Max Heights using All Tilts Product Non-Severe = < 30000 ft AGL 0.75-1.00 in Hail = 30000-36800 ft AGL 1.25-2.00 in Hail = 37000-41000 ft AGL

  14. Stats for Using TEST Severe Criteria • POD = 1.00 • FAR = 0.087 2 Counties**Severe Hail after crossing adjacent county border** • Missed Events = 0 • Mean Lead time = 16.5 min **Lead times only used for developing storms**

  15. Issue The Warning Now?

  16. Using System During ExplosiveDevelopment Freezing Level: = 12000 ft Warning Criteria = 26000 ft

  17. Cross Section at 2045Z 2.00 inch hail reported at 2050Z. Later produces 4.50 inch hail.

  18. Can Hail Size Be Estimated?

  19. Adding 1.25-2.00 Inch Hail

  20. Adding >2.00 Inch Hail

  21. Can Hail Size Be Estimated Cont.Central Iowa 5/16/99: FZL = 12700 ft MSL 50-55to 41000 ft 2.50 inch hail 50-55 to 38500 ft 1.75 inch hail

  22. Hail Size Estimation 50-55 dBZ to 24000 ft Pea sized Hail

  23. Hail Size Estimation15 Min Later: Second Storm 50-55 dBZ to 41000 ft 1.75 inch hail

  24. DoesHorizontal Storm Size Matter? No! It is Updraft Strength and the Height of the 50-55 dBZ Core that counts! 50-55 dBZ to 41500 ft: 2.75 inch hail

  25. How Does The Iowa Data Compare to FGF Data?

  26. Hail Size Potential Related to FZL?

  27. Potential Errors • Height differences between Elevation Angles at greater distances from the RDA New VCP gamma/77 to help (Fall 2003) • Cone of Silence • Superrefraction of radar beam - overestimation of target height • Subrefractionof radar beam - underestimation of target height

  28. Subrefraction vs Superrefracton Subrefraction Scenario – inverted V sounding with moist layer aloft Superrefraction Scenarios – Temp Inversions, thunderstorm outflows and Cold Frontal Passages

  29. Most Hail Reports are Estimated

  30. Conclusions • Hail size potential increases as the 50-55 dBZ reflectivity max height increases. • Useful tool in the warning process. Severe Potential detected 0.5-1.5 volume scans before derived products. • False Alarm Rates (FAR) are low • Updraft Strength at FZLs below 10000 ft rarely can support hail greater than 1.00 inch. • Rare to have >1.00 inch hail when FZLs are >14000 ft. Too much melting occurs.

  31. Conclusions Cont. • System works very well for Explosive Development and Pulse Severe • Criteria likely to work well for much of the Northern Plains. Southern Plains remains to be determined

  32. Future Additions • Will continue to add to database • Convert FZL from MSL to AGL to test usefulness for higher elevations • Elevated Convection Comparison. Will severe criteria to be lower? • Compare data from other Regions. Currently NWS San Angelo, TX working on a comparison. • Possibility of Detecting Downburst Winds with 50-55 dBZ core dump

  33. Questions or Comments? Email: rodney.donavon@noaa.gov

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