Evaluating the Reliability of VIL Density for Determining Severe Hail in Iowa

1. Evaluating the Reliability of VIL Density for Determining Severe Hail in Iowa Penny Zabel

2. Overview • Introduction • Data Analysis - Estimation by VIL Density - Baron Radar Algorithm • Results • Future

3. Vertically Integrated Liquid • VIL=∑ 3.44 x 10-6[(zi+zi+1)/2]4/7 dh • zi and zi+1 are reflectivity values at top and bottom of layer with depth dh. • VIL is the equivalent liquid water content value derived from radar reflectivity data. (American Meteorological Society, 2000)

4. Cone of Silence (American Meteorological Society, 1993)

5. VIL Density • VIL Density = VIL / Echo Top • Makes VIL independent of height. • Reduces distance error in radar measurements

6. Typical VIL and VIL Density VIL Density VIL 82 g/m^2 4.25 g/m^3

7. Within Cone of Silence VIL Density VIL 23 g/m^2 5.25 g/m^3

8. How Can VIL Density Estimate Hail Size? • Reflectivity is proportional to the diameter of a target to the sixth power. • Reflectivity increases exponentially as target size increases. • VIL increases exponentially.

9. Drop Size is Everything! (American Meteorological Society, 1993)

10. Data • 110 Hail reports • 2 events each from spring, early summer, and late summer. • Recorded VIL, echo top, and calculated VIL Density for each report.

11. Hail Size vs. VIL

12. Hail Size vs. VIL Density

13. Average VIL Density • 0.75”-0.99” 4.39 g/m3 • 1.00”-1.99” 4.61 g/m3 • 2.00”or greater 5.30 g/m3

14. Data Summary • A VIL density of 4.0 g/m3 correctly identified 93% of all hail reports. • This is a higher VIL density threshold than previous papers.

15. Data Summary • Amburn and Wolf 3.5g/m3 90% (Amburn and Wolf, 1996) • Baumgardt and King 3.5g/m3 90% (Baumgardt and King, 2002) • Troutman and Rose 3.5g/m3 81% (Troutman and Rose, 1997) • Roeseler and Wood 3.5g/m3 72% (Roseler and Wood, 1997)

16. Baron Estimated Hail

17. Baron Estimated Hail cont.

18. Regional Adaptation

19. Wet Bulb Zero Height • Low wet bulb zero height means more of the energy that is back-scattered can be from frozen instead of liquid targets. • A low wet bulb zero height also often signifies less instability, and smaller updrafts.

20. Overestimate Hail Size • Low wet bulb zero height may lead to high VIL density, but small hail. • April 8, 1999 WBZ 7,000 ft. • VIL density of 4.25 g/m3 • Hail size 0.75”

21. Underestimate Hail Size • High wet bulb zero height may lead to large hail with small VIL density. • July 2, 1999 WBZ 13,400 ft. • VIL density of 3.50 g/m3 • Hail size 1.75”

22. VIL is not the only thing • VIL Density is an indicator, but should be used in conjunction with other radar signatures of severe hail. • Three Body Scatter Spike • Bounded Weak Echo Region

23. BWER BREF 1

24. BWER BREF 2

25. BWER BREF 3

26. BWER

27. Results • A greater VIL density is needed for large hail in Iowa than in the south where other studies have been done. • Wet bulb zero height is one factor in this.

28. Results (cont.) • Distance does not significantly affect calculations based on VIL density. • When using Baron algorithm to estimate hail size, meteorologists should keep in mind differences in atmospheric conditions.

29. Future • Examine more data to determine any stronger correlations. • Examine the possibilities of updating the radar algorithm to include a factor for wet bulb zero height.

30. Thanks to:KCCI-TV*John McLaughlinGreg WilsonBaron Radar Systems

31. Questions?