evaluating the reliability of vil density for determining severe hail in iowa n.
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Evaluating the Reliability of VIL Density for Determining Severe Hail in Iowa. Penny Zabel. Overview. Introduction. Data Analysis. - Estimation by VIL Density. - Baron Radar Algorithm. Results. Future. Vertically Integrated Liquid. VIL= ∑ 3.44 x 10 -6 [(z i +z i+1 )/2] 4/7 dh.

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Presentation Transcript
  • Introduction
  • Data Analysis

- Estimation by VIL Density

- Baron Radar Algorithm

  • Results
  • Future
vertically integrated liquid
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)

cone of silence
Cone of Silence

(American Meteorological Society, 1993)

vil density
VIL Density
  • VIL Density = VIL / Echo Top
  • Makes VIL independent of height.
  • Reduces distance error in radar measurements
typical vil and vil density
Typical VIL and VIL Density

VIL Density


82 g/m^2

4.25 g/m^3

within cone of silence
Within Cone of Silence

VIL Density


23 g/m^2

5.25 g/m^3

how can vil density estimate hail size
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.

Drop Size is Everything!

(American Meteorological Society, 1993)

  • 110 Hail reports
  • 2 events each from spring, early summer, and late summer.
  • Recorded VIL, echo top, and calculated VIL Density for each report.
average vil density
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
data summary
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.

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)

wet bulb zero height
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.
overestimate hail size
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”
underestimate hail size
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”
vil is not the only thing
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







  • 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.
results cont
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.
  • 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.