Transcript An Examination of Hail Cases from Eastern New York and

Preparing for a “Change” in Severe
Hail Warning Criteria in 2010
Brian J. Frugis
NWS WFO Albany, NY
NROW XI November 4-5, 2009
Motivation for Study
• NWS ER will be changing severe hail criteria
from 0.75” to 1.00” on January 1, 2010
• All current hail studies for the region are based
on 0.75”
– VIL of the Day
– VIL Density
– Common warning techniques using dBZ heights
relative to the -20° C level
• New thresholds and techniques are needed to
accurately warn for the new hail criteria
What was done
• Examined radar returns for each report of
severe hail (0.75” or larger) from the major
severe cases of hail from the summer of
2009
– July 16th and June 15th hail events
• Compiled data and compared averages of
each parameter that was examined
Data for Study
Total Number of Hail Reports
70
Hail Reports Measuring 0.75”
20
Hail Reports Measuring 1.00”
25
Total Number of Report less than 1.00” 33
(Penny and Nickel)
Total Numbers of Reports 1.00” or
37
Greater (Quarters and Larger)
3 additional hail reports were rejected due to probable errors in locations
and/or times
Storm Reports for Study
Parameters Studied
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50 dBZ Echo Top
55 dBZ Echo Top
60 dBZ Echo Top
65 dBZ Echo Top
Vertically Integrated Liquid (VIL)
Storm Echo Tops (ET)
VIL Density
Height of 50 dBZ Echo Top above -20° Celsius
isotherm
Why dBZ heights?
• In a warning situation, the
height of the 50+ dBZ
level in correlation with
the -20° Celsius level is
often examined. The
increasing amount of
height above this level
increases the level of
confidence of severe hail
in a particular
thunderstorm.
Why VIL Density?
• VIL Density = (VIL/ET) * 1000
• Units are in g m-3
• This helps “normalize” the VIL to produce
a common values for thunderstorms
producing hail, independent of the
particular air mass.
Previous Studies on VIL Density
• Previous studies have been done examining VIL
Density for the ALY CWA
– VIL density values of 3.50 g m-3 identified 82% of severe
hail based on a 0.75” criteria in a paper by Blaes et. al
in 1998
– They also determined that a VIL density of 3.70 g m-3
would depict 91% of severe hail cases based on a
proposed 1.00”criteria (based on 154 total reports)
– However, this study also mentioned that this isn’t
reliable since there would also be a False Alarm Rate
(FAR) of 48% based on the data set used
• There were limited 1.00” reports in their database
• It was recommended that additional studies be done
Methodology
• Used FSI 4D Storm Investigator to get heights of different dBZ
thresholds
• Also used 4 panels displays of the lowest tilts from adjacent radar
sites (BGM, OKX, TYX and CXX)
• Used D2D gridded VIL and ET displays to get specific values at
times of each report
Limitation of Radar Location
• Radar Location
– The KENX radar’s
cone of silence made
judging echo tops
difficult for storm’s
close to the RDA
• A solution to this was to
view other radars, such
as KTYX, KBGM,
KCXX, and KOKX.
Limitation of Radar Location
• Radar Beam
Blockage
– Portions of the
Catskills (whose
terrain reaches
heights of 4180
feet) can block the
lowest tilts of the
radar beam
• This could give
inaccurate VIL
values for storms
in these areas
Problems with Hail Reports
– Size is not always accurately measured
• Spotters don’t always use rulers and some reports
are from the untrained public
– Location and time
• Some locations and times listed on hail report may
be estimated and some spotters may not have
given their exact latitude and longitude
– Have tried to include only the reports that made sense
based on radar for this study
• Some hail reports may have occurred in
unpopulated areas or in places where spotters
were not available
Truncation and Rounding Errors
• Estimating
ETs of
various
levels is
difficult
higher up
and further
away from
the storm
0.75” vs. 1.00” Hail
0.75” (Penny)
Hail
1.00”
(Quarter)
Hail
Difference
Avg Height of
50 dBZ Echo
Top
25.7 kft
27.9 kft
2.2 kft
Avg Height of
50 dBZ Echo
Top above
-20° Celsius
Isotherm
5.4 kft
6.8 kft
1.4 kft
Avg VIL
(kg/m3)
44 kg/m3
49 kg/m3
5 kg/m3
Severe vs. Non-Severe
(based on new warning criteria)
0.75-0.88” (Penny 1.00”+
or Nickel) Hail
(Quarter or
Larger) Hail
Difference
Avg Height
of 50 dBZ
Echo Top
26.1 kft
32.6 kft
6.5 kft
Avg Height
of 50 dBZ
Echo Top
above -20°
Celsius
Isotherm
5.8 kft
10.0 kft
4.2 kft
Avg VIL
(kg/m3)
45 kg/m3
52 kg/m3
7 kg/m3
Severe vs. Non-Severe
(based on new warning criteria)
0.75-0.88” (Penny 1.00”+
or Nickel) Hail
(Quarter or
Larger) Hail
Difference
Avg Height
of 55 dBZ
Echo Top
21.0 kft
27.7 kft
6.7 kft
Avg Height
of 60 dBZ
Echo Top
15.9 kft
22.5 kft
6.6 kft
Avg Height
of 65 dBZ
Echo Top
11.6 kft
18.6 kft
7.0 kft
Storm Echo Top vs. VIL
VIL Density
Storm Echo Top (kft)
50.0
45.0
40.0
35.0
30.0
25.0
20.0
20
25
30
35
40
45
50
55
60
3
VIL (g/m )
Severe Hail
Non-Severe Hail
Based on a severe hail criteria of 1.00”
65
70
VIL Density Threshold
Probability of Detection
(POD)
False Alarm Rate (FAR)
Based of a VIL
Density of 3.50 g/m3
81%
76%
Based on a VIL
Density of 3.60 g/m3
81%
61%
Based on a VIL
Density of 3.70 g/m3
76%
48%
Based on a VIL
Density of 3.80 g/m3
70%
48%
Based on a VIL
Density of 4.00 g/m3
54%
36%
VIL Density as an Indicator of
Severe Hail
• Not a good correlation between increasing VIL
Density and severe hail in these cases as there
was still a high FAR
– May be based on small data set of only 70 severe
reports
• Previous data set by Blaes et al. did not have a
large number of hail cases over 1.00”
– Their study had large FAR of 48% for a value of 3.70”
(despite POD of 91%) showing low correlation for VIL
Density and severe hail based on a 1.00” criteria
• This study also had a FAR of 48% and POD of only 76%
Summary
• The 50 dBZ ETs are in general about 10 kft
above the -20° Celsius level
– Average height is around 32.6 kft
• The dBZ echo tops (for 50, 55, 60 and 65 dBZ)
are about 6-7 kft higher in a severe over a nonsevere storm based on the new criteria
• No particular values of VIL Density seem to
correlate well in our area for severe hail
– At least based off this limited sample
Future Work
• Compile statistics from other events from
2009 as well as previous years
– Will differences between severe and nonsevere storms be similar?
• Create a new “VIL of the Day” nomogram
for operational use
• Attempt to see if VIL Density has a better
correlation with a larger database
Acknowledgements
• Many thanks to Joe
Villani and Tom
Wasula for their help
and guidance on this
project!
Questions?
Any questions or comments?
[email protected]