Severe Hail

Information about Severe Hail

Published on February 12, 2008

Author: Tomasina

Source: authorstream.com

Content

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?:  Where is the Grand Forks (FGF) CWA? Severe Hail Detection:  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 Reflectivity All Tilts Product:  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. VCP 11: Radar Elevation Angles:  VCP 11: Radar Elevation Angles Data Used For Study:  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. Quality Control of Data:  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. Results For All Hail Sizes:  Results For All Hail Sizes Results for 0.75-1.00 Inch Hail:  Results for 0.75-1.00 Inch Hail The Numbers Game Establishing a Warning Criteria (TEST):  The Numbers Game Establishing a Warning Criteria (TEST) Freezing Level Heights Warning 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 Central Iowa April 18, 2002:  Central Iowa April 18, 2002 What is the Criteria for 4/18/02?:  What is the Criteria for 4/18/02? Freezing Level on 4/18/02 = 12500 ft MSL +-200 ft Warning Criteria = 28500 ft Results For Central Iowa 4/18/02:  Results For Central 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 Stats for Using TEST Severe Criteria:  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** Issue The Warning Now?:  Issue The Warning Now? Using System During Explosive Development:  Using System During Explosive Development Freezing Level: = 12000 ft Warning Criteria = 26000 ft Cross Section at 2045Z:  Cross Section at 2045Z 2.00 inch hail reported at 2050Z. Later produces 4.50 inch hail. Can Hail Size Be Estimated?:  Can Hail Size Be Estimated? Adding 1.25-2.00 Inch Hail:  Adding 1.25-2.00 Inch Hail Adding >2.00 Inch Hail:  Adding >2.00 Inch Hail Can Hail Size Be Estimated Cont. Central Iowa 5/16/99: FZL = 12700 ft MSL:  Can Hail Size Be Estimated Cont. Central Iowa 5/16/99: FZL = 12700 ft MSL 50-55 to 41000 ft 2.50 inch hail 50-55 to 38500 ft 1.75 inch hail Hail Size Estimation:  Hail Size Estimation 50-55 dBZ to 24000 ft Pea sized Hail Hail Size Estimation 15 Min Later: Second Storm:  Hail Size Estimation 15 Min Later: Second Storm 50-55 dBZ to 41000 ft 1.75 inch hail Does Horizontal Storm Size Matter?:  Does Horizontal 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 Slide25:  How Does The Iowa Data Compare to FGF Data? Hail Size Potential Related to FZL?:  Hail Size Potential Related to FZL? Potential Errors:  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 Subrefraction of radar beam - underestimation of target height Subrefraction vs Superrefracton:  Subrefraction vs Superrefracton Subrefraction Scenario – inverted V sounding with moist layer aloft Superrefraction Scenarios – Temp Inversions, thunderstorm outflows and Cold Frontal Passages Most Hail Reports are Estimated:  Most Hail Reports are Estimated Conclusions:  Conclusions Useful tool in the warning process. Severe Potential detected 0.5-1.5 volume scans before derived products. 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. Hail size potential increases as the 50-55 dBZ reflectivity max height increases. False Alarm Rates (FAR) are low Conclusions Cont.:  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 Future Additions:  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 Questions or Comments?:  Questions or Comments? Email: [email protected]

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