Evaluating the Reliability of VIL Density for
Determining Severe Hail in Iowa
Penny Zabel
Department of Geological and Atmospheric Sciences,
Iowa State University, Ames, Iowa
Mentors: John McLaughlin and William Gallus
ABSTRACT
This study examines the use of vertically integrated liquid density as an indicator for the size of
hail in thunderstorms in Iowa. Cases were studied from different parts of the severe weather
season. VIL density was compared to reported hail size to determine any correlation. It was
found that VIL density is related to hail size, and for Iowa, a good threshold for severe hail for
storms with a moderate wet bulb zero level is 4.0 g/m3. It was also determined that when using
the Baron radar system algorithm that estimates hail size, attention should be paid to the wet bulb
zero level. A low wet bulb zero height can lead to a high VIL density with smaller hail, and a high
wet bulb zero height can cause an underestimation in hail size based on VIL density.
______________________________
1. Introduction
In recent years, studies have been
conducted throughout the country to
determine if any correlation exists
between vertically integrated liquid
(VIL) density and hail size, and the
probability of detecting the presence of
severe hail (0.75” or larger) in storms.
VIL is a measure of how much liquid
water is in a vertical column of air, and
is dependent on factors of the airmass
the storm has formed in. VIL density is
VIL divided by the echo top height of
the storm.
In the past, VIL has been used
operationally to estimate hail size, but
due to the dependence on the airmass
and the variability with distance from the
radar, VIL is being phased out, and VIL
density is now being used to detect the
presence and size of hail.
Figure 1 is two radar images
obtained from KCCI-TV. The first of
the two images shows VIL values for a
typical severe thunderstorm cell. The
second image is the VIL density for the
same storm.
VIL values and VIL
density values are of similar relative
magnitude in these images, with VIL
slightly higher.
This study also attempts to test the
Baron radar algorithm currently in use to
estimate the maximum hail size
1
82 g/m^2
(a)
4.25 g/m^3
(b)
FIG. 1. (a) VIL values of a cell on July 2, 1999, approximately 70 miles from the radar. (b) VIL
Density values as calculated by the Baron radar for same time and position as figure1. Both
figures 1 and 2 are typical of Iowa storms containing hail.
possible within storms based on the
VIL Density.
This study examines 6 hail events
across western and central Iowa and
analyzes differences due to time of year
and distance from the radar, as well as
atmospheric conditions. The radar data
used came from the Baron radar display
system from KCCI-TV in Des Moines
which utilizes the WSR-88D data. Null
cases were not included in this study.
2. Previous Studies
In one of the first VIL density
studies, Amburn and Wolf (1996)
studied 221 storms in the Tulsa,
Oklahoma area, and found that a VIL
Density of 3.5 g/m3 verified 90% of
severe hail cases. This is similar to a
paper done by Baumgardt and King
(2002) in LaCrosse, Wisconsin. They
also found 3.5 g/m3 to verify 90% of
their 70 cases (Baumgardt and King,
2002). Troutman and Rose (1997) found
a VIL density of 3.5 g/m3 to verify only
81% of cases in the Nashville warning
area.
The
Baron
Radar
algorithm
developed by Dr. Greg Wilson is based
on the results of a study published by
Roeseler and Wood (1997). This study
was based along the northwest coast of
the Gulf of Mexico. They found 3.5
g/m3 VIL density to only verify 72% of
all severe hail reports (Roeseler and
Wood, 1997).
Hart and Frantz (1998) did a study
on the effects of wet bulb zero height on
the size of hail. They found a VIL
density of 3.5 g/m3 to verify 79% of
severe hail cases. Hart and Frantz went
further and analyzed the atmospheric
conditions surrounding these cases.
They found 67% of the large hail cases
to have wet bulb zero heights between
7000 and 11000 ft. This is one of few
studies so far relating VIL and wet bulb
zero height (Hart and Frantz, 1998).
3. VIL and VIL Density
VIL is a function of reflectivity and
converts weather radar reflectivity data
into an equivalent liquid-water content
value (Amburn and Wolf, 1996).
VIL is defined as:
VIL = ∑3.44*10-6 [(Zi + Zi+1)/2]4/7dh
where Zi and Zi+1 are the radar
reflectivity at the bottom and top of the
layer dh.
2
Reflectivity is measured by the back
scattering of energy back to the radar
after being directed into the storm. It is
assumed that all the reflectivity
measured by the beam is from liquid
water.
Reflectivity is highly dependent on
the size of the target object in the radar
equation.
It is proportional to the
diameter of the object to the 6th power.
This will cause an overestimation of
reflectivity and liquid water content
when hail is present because hail is
much larger than the largest raindrops.
This will in turn result in an increased
VIL value.
23 g/m^2
(a)
VIL Density is defined as:
VIL Density = VIL / Echo Top *
1000
Multiplication by 1000 is needed to
get units of g/m3 rather than kg/m3. VIL
Density is used to account for the
difference in vertical height of storms.
By dividing by the echo top, VIL is
normalized and helps to eliminate
problems due to the cone of silence and
if the storm is far from the radar.
Cone of silence errors result when
the storm is close to the radar. The radar
can not scan directly above itself, and
thus leaves a gap in data for a circle
approximately 25 miles in diameter.
Figure 2 shows images of VIL and
VIL density for a storm that is
approximately 10 miles away from the
radar. Typically, VIL and VIL density
values will be of similar magnitude.
These images show a much larger VIL
density then VIL, due to the entire
column not being sampled. VIL density
eliminates this error.
5.25 g/m^3
(b)
FIG. 2. (a)VIL values for a storm close to the
radar site (b) VIL density values for the same
time and position.
Figure 3 is a plot of hail size vs. VIL
for all reports in this study. There is no
significant evidence from this graph that
VIL and hail size are strongly correlated.
Most cases had larger hail with higher
VIL, but there are many outliers. This
could be due partly to range from the
radar.
Figure 4 shows hail size vs. VIL
density. Here, the correlation is much
more obvious. There is a definite trend
toward higher VIL densities with
increasing hail size with only a few stray
points.
3
Hail Size (in)
VIL vs. Hail size
5
4
3
2
1
0
0
50
100
VIL (g/m^2)
FIG. 3. VIL and associated hail size of all hail reports in this study.
Hail Size (in)
VIL Density Vs. Hail size
5
4
3
2
1
0
0
1
2
3
4
5
6
7
VIL Density (g/m^3)
hail size of all hail reports in this study.
FIG. 4. VIL density and associated
4. Methodology
Radar information was obtained
from WSR-88D radar data archived by
KCCI-TV for 6 hail cases from 1998
through 2002. The cases were chosen to
have two cases from spring, early
summer, and late summer, all parts of
the Iowa severe weather season. The
number of cases was limited due to the
availability of saved radar data and the
lack of severe hail events in the 2002
season. Hail reports were obtained from
the National Climactic Data Center
website and the Storm Prediction Center
website.
VIL, echo top, and VIL density from
the WSR-88D were recorded, and hail
size as estimated by the Baron algorithm
was also recorded for each report of hail
within the range of the radar, as well as
the distance of each report from the
radar. The beam height was then
estimated using a VCP 11 chart.
Measurements were recorded for the
best estimate of when the hail likely
4
occurred within a 10 minute window of
the report time. The reports are from the
public, and trained storm spotters. There
is no guarantee of the accuracy of all
these reports, and the report may not
represent the largest hail that occurred.
Hail smaller than ¾ in. was not
considered in this study due to lack of
reports.
The following are the dates and
atmospheric conditions for the studied
storms.
Date
April 8, 1999
April 18, 2002
May 30, 1998
July 7, 2001
Sept. 7, 2001
Sept. 18, 2002
Wet Bulb
Freezing
Zero height Level
(ft)
(ft)
6,895
10,032
8,520
10,925
11,067
13,556
13,392
13,392
10,863
10,863
11,016
12,872
TABLE 1. Wet bulb zero height and freezing
level for days in which hail was studied based
on soundings from Omaha, NE.
All atmospheric information was
obtained from a radiosonde database
access site (RAOB, 2002) using the
Omaha soundings for the time nearest
the event. The raw sounding data was
processed using RAOB (Shewchuk,
1998).
5. Results
The results of this research are
similar to those found by previous
studies. There definitely seems to be a
relationship between VIL and echo top,
and VIL density for estimating hail size.
Other studies have found 3.5 g/m3 to
be a good threshold for severe hail. This
study has shown that for storms in Iowa,
a VIL density of 4.0 g/m3 is a better
indicator of possibly severe hail. Only 8
severe hail reports out of the 111 studied
had a VIL density of less than 4.0 g/m3.
This works out to 93% of the cases being
verified by 4.0 g/m3 VIL density or
greater. There were no severe hail
reports in the studied cases with a VIL
density of less than 2.75 g/m3.
Table 2 shows the average VIL
density for hail sizes. As is expected,
the average VIL density increases with
hail size.
Hail size
0.75”-0.99”
1.00”-1.99”
2.00” and greater
VIL density
4.39 g/m3
4.6 g/m3
5.30 g/m3
TABLE 2. Average VIL density per hail sizes.
It was also found that there is a
preferred wet bulb zero level for the
accuracy of VIL density. On April 8,
1999, the wet bulb zero height was less
than 7000 ft. On this day, there was a
case of a VIL density of 5.25 g/m3 with
only 1” hail.
The opposite was also true. On July
2, 1999, the wet bulb zero height was
greater than 13,000 ft. This resulted in
one report of 1.75” hail, where the VIL
density was only 3.5 g/m3.
The effects of wet bulb zero were not
studied extensively in this research, but
some theories were maintained:
When the wet bulb zero height is
very low, more of the targets reflecting
the radar beam may be frozen rather than
liquid drops. This will cause a high VIL
density calculation.
One reason for small hail when the
VIL is high is that a low wet bulb zero
height can signifies a relatively stable
atmosphere or low top convection,
where hail does not have enough below
freezing area to get large. The stable
atmosphere would lead to small updrafts,
and hinder the growth of hail. The
opposite can also be assumed.
5
The algorithm and thresholds used
by the Baron Radar system are based on
studies done in the southeast United
States. This area has high freezing
levels, and wet bulb zero levels.
The
algorithm
consistently
overestimated hail size for every day of
storms that was studied. Figure 5a is the
same scatter plot as figure 4, but with the
estimated hail size added. Figure 5b
shows the estimated hail size versus
actual hail size for all the cases. It can
be seen here that hail size is being
overestimated.
If the radar is being used in a cooler
climate, like Iowa, where the wet bulb
zero level is not going to be as high as in
the southeast United Stated, adjustments
6. Conclusions
It has been shown that VIL density
can be a good indicator for the size of
hail in Iowa and other parts of the
country.
In Iowa, 4.00 g/m3 appears to be a
good threshold for severe hail. Other
VIL density suggestions are noted in
Table 3.
Hail size
0.75” – 0.99”
1” – 1.99”
2” – 2.99”
3” and greater
VIL density
4.00 g/m3
4.25 g/m3
4.50 g/m3
5.00 g/m3
Hail Size (in)
TABLE 3. Hail size and suggested VIL density
thresholds based on results of this study.
5
4
3
2
1
0
0
1
2
3
4
5
6
7
VIL Density (g/m^3)
Estimated Hail Size
(a)
6
5
4
3
2
1
0
0
1
2
3
4
5
6
Hail Size
(b)
FIG. 5. (a) Blue points are hail size and associated VIL density for all cases. Estimated
hail size for VIL density are in purple. (b) Blue points represent hail size plotted
against the estimated hail size. The red line represents the perfect reliability line, while
the green line is the estimated size decr eased by ¼ inch.
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may need to be made to the estimated
hail size computed by the system. In the
case of Iowa, the results of this study
indicate that the estimated hail size
needs to be reduced by ¼ inch (Fig. 5b).
Whether using the Baron system or
not, a meteorologist also needs to be
aware of atmospheric conditions. Low
wet bulb zero heights can lead to
overestimations of hail size based on
VIL, and high wet bulb zero heights can
lead to underestimations.
Other factors that need to be kept in
mind are the speed and tilt of the storm.
If the storm is moving fast, the top of the
storm may already have moved out of
the grid column before the whole storm
is sampled. This means some of the
reflectivity from the vertical storm
column would be measured into the grid
space next to it.
The same can happen if the storm is
strongly tilted. The radar is sampling in
a vertical column and part of the storm
may be in the next grid box.
It is important to remember that VIL
density is only one radar indicator of
severe hail. Operational meteorologists
must also be sure to look for other
signatures of hail on radar.
The three-body scatter spike is one
possible tell-tale sign of hail. This is
caused when the hailstones are larger
than about 1 in. in diameter. Mei
scattering starts to effect the radar beam,
and the hail stone reflects a portion of
the radar energy down to the ground.
The ground then backscatters it to the
hail stone, and back to the radar. This
causes a delay in the amount of time it
takes for the energy to return to the
receiver, and the radar will assume this
is caused by rain at a greater distance
than what it really is.
Figure 6 is a good example of a
three-body scatter spike. The radar is
located to the east of this storm. There is
a long spike out the left side of the
highest reflectivity area.
(a)
(b)
FIG. 6. (a) A Three body scatter spike on
radar. The blue box shows the VIL, VIL
density, and max estimated hail size for this
storm. (b) An enlargement of the box in the
corner of figure 6a.
Another easy signature to pick out is
the bounded weak echo region, or
BWER. This results from very strong
updrafts on the edge of a storm. It
typically looks like a small hole or donut
in the upper levels of the radar scans.
Looking at the lowest level radar (figure
7a), and a level in the middle of the
storm (figure 7b), there will be an area
of weaker reflectivity above where some
stronger reflectivity was. Higher up in
the storm (figure7c) there will be the
area of low reflectivity where the strong
updraft is likely creating large hail.
Surrounding this will be stronger returns
7
BREF 1
BREF 2
(a)
(b)
BREF 3
(c)
FIG. 6. Three levels of a storm showing a BWER
where rain and hail are escaping the
updraft.
Acknowledgements. The author thanks
John McLaughlin for all his help with
this research, as well as Dr. William
Gallus, Greg Wilson, and the numerous
people who helped with editing.
REFERENCES
Amburn, S., and P. Wolf, 1996: VIL density as a
hail indicator. 18th Conf. on Severe Local
Storms. San Francisco, CA, Amer. Meteor.
Soc., 581-585.
American Meteorological Society, 1993: Nexrad
Information Dissemination Services (NIDS).
Unysis Corporation, 455.
______, 2000: Glossary of Meteorology.
American Meteorological Society, 855.
Baumgardt, D. A., and C. King, 2002:
Verification of the WSR-88D build 9.0 hail
algorithm over the upper Midwest.
[Available online at http://
www.crh.noaa.gov/arx/hailstudy.html.]
Edwards, R., and R. Thompson, 1998:
Nationwide comparisons of hail size with
WSR-88D vertically integrated liquid water
(VIL) and derived thermodynamic sounding
data. Wea. Forecasting. 13, 277-285.
Hart, P.A., and K. Frantz, 1998: A comparison of
VIL density and wet bulb zero height
associated with large hail over north and
central Georgia. SR/SSD 98-30.
National Climactic Data Center, cited 2002:
Extreme weather and climate events.
[Available online at http://
lwf.ncdc.noaa.gov/oa/climate/severeweather
/extremes.html.]
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RAOB, cited 2002: Radiosonde database access.
[Available online at
http://raob.fsl.noaa.gov/]
Roeseler, C.A., and L. Wood, 1997: VIL density
and associated hail size along the northwest
Gulf coast. Preprints, 28th Conf. On Radar
Meteorology, Austin, TX, Amer. Meteor.
Soc, 434-435.
Shewchuk, J., 1998: RAOB Version 4.0.
Environmental Research Services.
Storm Prediction Center, cited 2002: Storm
reports. [Available online at
http://www.spc.noaa.gov/climo/.]
Troutman, T., and M. Rose, 1997: An
examination of VIL and echo top associated
with large hail in middle Tennessee.
SR/SSD 97-15.
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