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

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# Evaluating the Reliability of VIL Density for Determining Severe Hail in Iowa - PowerPoint PPT Presentation

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

Penny Zabel

Overview
• Introduction
• Data Analysis

- Estimation by VIL Density

• Results
• Future
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

(American Meteorological Society, 1993)

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

VIL Density

VIL

82 g/m^2

4.25 g/m^3

Within Cone of Silence

VIL Density

VIL

23 g/m^2

5.25 g/m^3

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)

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.
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
• 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
• 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
• 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
• 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 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
BWER

BREF 1

BWER

BREF 2

BWER

BREF 3

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.
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.
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.