Transcript Evaluation of the Baron Radar Hail Detection Algorithm

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[(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
82 g/m^2
VIL Density
4.25 g/m^3
Within Cone of Silence
VIL
23 g/m^2
VIL Density
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.
Hail Size vs. VIL
Hail Size (in)
6
5
4
3
2
1
0
0
25
50
VIL (g/m^2)
75
100
Hail Size vs. VIL Density
Hail Size (in)
5
4
3
2
1
0
0
1
2
3
4
5
VIL Density (g/m^3)
6
7
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
(Roseler and Wood, 1997)
72%
Hail Size (in)
Baron Estimated Hail
5
4
3
2
1
0
0
1
2
3
4
5
VIL Density (g/m^3)
6
7
Baron Estimated Hail cont.
Estimated Hail Size
6
5
4
3
2
1
0
0
1
2
3
Hail Size
4
5
6
Regional Adaptation
Hail Size Baron
Suggested
0.75”
4.00 g/m3
1.00”
3.25 g/m3
4.25 g/m3
2.00”
4.75 g/m3
4.50 g/m3
3.00”
5.50 g/m3
5.00 g/m3
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
BWER
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.
Thanks to:
KCCI-TV
*John McLaughlin
Greg Wilson
Baron Radar Systems
Questions?