Evaluating the Reliability Evaluating the Reliability of VIL Density for of VIL Density for

Determining Severe Hail Determining Severe Hail in Iowain Iowa

Penny ZabelPenny Zabel

OverviewOverview

• Data Analysis- Estimation by VIL Density- Baron Radar Algorithm

• Results• Future

• Introduction

Vertically Integrated LiquidVertically 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 SilenceCone of Silence

(American Meteorological Society, 1993)

VIL DensityVIL Density

• VIL Density = VIL / Echo Top

• Makes VIL independent of height.

• Reduces distance error in radar measurements

Typical VIL and VIL DensityTypical VIL and VIL Density

VIL VIL Density

82 g/m^2 4.25 g/m^3

Within Cone of SilenceWithin Cone of Silence

VIL VIL Density

23 g/m^2 5.25 g/m^3

How Can VIL Density Estimate How Can VIL Density Estimate Hail Size?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!Drop Size is Everything!

(American Meteorological Society, 1993)

DataData• 110 Hail reports

• Recorded VIL, echo top, and calculated VIL Density for each report.

• 2 events each from spring, early summer, and late summer.

Hail Size vs. VILHail Size vs. VIL

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0 25 50 75 100VIL (g/m^2)

Hai

l Siz

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Hail Size vs. VIL DensityHail Size vs. VIL Density

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0 1 2 3 4 5 6 7VIL Density (g/m^3)

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Average VIL DensityAverage 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 SummaryData Summary

• This is a higher VIL density threshold than previous papers.

• A VIL density of 4.0 g/m3 correctly identified 93% of all hail reports.

Data SummaryData Summary

• Amburn and Wolf 3.5g/m3 90%

• Baumgardt and King 3.5g/m3 90%

• Troutman and Rose 3.5g/m3 81%

• Roeseler and Wood 3.5g/m3 72%

(Amburn and Wolf, 1996)

(Baumgardt and King, 2002)

(Troutman and Rose, 1997)

(Roseler and Wood, 1997)

Baron Estimated HailBaron Estimated Hail

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VIL Density (g/m^3)

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Hail Size

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ail S

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Baron Estimated Hail cont.Baron Estimated Hail cont.

Regional AdaptationRegional Adaptation

Hail SizeHail Size BaronBaron SuggestedSuggested

0.75”0.75” 4.00 g/m4.00 g/m33

1.00”1.00” 3.25 g/m3.25 g/m33 4.25 g/m4.25 g/m33

2.00”2.00” 4.75 g/m4.75 g/m33 4.50 g/m4.50 g/m33

3.00”3.00” 5.50 g/m5.50 g/m33 5.00 g/m5.00 g/m33

Wet Bulb Zero HeightWet 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 SizeOverestimate 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 SizeUnderestimate 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 thingVIL 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

BWERBWERBREF 1BREF 1

BREF 2BREF 2

BWERBWER

BREF 3BREF 3

BWERBWER

BWERBWER

ResultsResults

•Wet bulb zero height is one factor in this.

• A greater VIL density is needed for large hail in Iowa than in the south where other studies have been done.

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.

FutureFuture

• 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.

Thanks to:Thanks to:

KCCI-TVKCCI-TV*John McLaughlin*John McLaughlin

Greg WilsonGreg WilsonBaron Radar SystemsBaron Radar Systems

Questions?Questions?