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

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

Vertically Integrated Liquid VIL=∑ 3.44 x [(z i +z i+1 )/2] 4/7 dh z i and z i+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 VIL Density 82 g/m^24.25 g/m^3

Within Cone of Silence VIL VIL Density 23 g/m^25.25 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 Recorded VIL, echo top, and calculated VIL Density for each report. 2 events each from spring, early summer, and late summer.

Hail Size vs. VIL

Hail Size vs. VIL Density

Average VIL Density 0.75”-0.99” 4.39 g/m ”-1.99” 4.61 g/m ”or greater 5.30 g/m 3

Data Summary This is a higher VIL density threshold than previous papers. A VIL density of 4.0 g/m 3 correctly identified 93% of all hail reports.

Data Summary Amburn and Wolf 3.5g/m 3 90% Baumgardt and King 3.5g/m 3 90% Troutman and Rose 3.5g/m 3 81% Roeseler and Wood 3.5g/m 3 72% (Amburn and Wolf, 1996) (Baumgardt and King, 2002) (Troutman and Rose, 1997) (Roseler and Wood, 1997)

Baron Estimated Hail

Baron Estimated Hail cont.

Regional Adaptation Hail Size BaronSuggested 0.75” 4.00 g/m ” 3.25 g/m g/m ” 4.75 g/m g/m ” 5.50 g/m g/m 3

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

BREF 2 BWER

BREF 3 BWER

BWER

Results 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.) 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.

Thanks to: KCCI-TV *John McLaughlin Greg Wilson Baron Radar Systems

Questions?