Transcript TOP CWA Golf Ball size Hail Study

TOP CWA Golf Ball Size Hail Study
Bill Gargan WFO TOP, KS
Why I conducted this research?
• To find techniques to help forecasters, while
analyzing storms on radar, to generate more
accurate warnings and severe weather
statements for golf ball size hail.
• Golf ball size hail can cause considerably
more damage than quarter size hail.
DATA
• Archived reflectivity (Zr) and storm relative
velocity (SRM) data was obtained from both
the KTWX and KEAX radars. Archived
radiosonde data from the KTOP upper air
observing site was used to obtain the height
of both the melting and -20 degree C levels.
• 174 golf ball size hail reports were obtained
from Storm Data across the TOP County
Warning Area from 1999 to 2008.
The following components analyzed
from Zr and radiosonde data
• Maximum Zr at -20 degree C. (From HDA, most severe
hail growth occurs with temperatures near -20C or
colder. )
• Maximum Zr value and corresponding AGL height.
• 2Vr (rotation).
• Storm Top Divergence.
• 50 dBZ Height vs the melting level height. (Donovan 1”)
• Percentage of storms with identifiable BWER
• Percentage of storms with three-body scatter
signature.
Data collection methodology
• The radar data was analyzed up to 2 volume
scans before the time of a golf ball size hail
report and up to one volume scan after the
time of a golf ball size hail report. For all the
radar parameters, I looked at the maximum
values within the time frame.
• Some observation errors are inherent to this
study based on the over and under estimation
of hail size by the storm spotters, and potential
errors in the time of the report.
The following components analyzed
from Zr and radiosonde data
• Maximum Zr at -20 degree C
• Maximum Zr value and corresponding AGL
height.
• 2Vr (rotation).
• Storm Top Divergence vs the melting level height.
• 50 dBZ Height vs the melting level height.
• Percentage of storms with identifiable BWER
• Percentage of storms with three-body scatter
signature.
The following components analyzed
from Zr and radiosonde data
• Maximum Zr at -20 degree C
• Maximum Zr value and
corresponding AGL height.
•
•
•
•
•
2Vr (rotation).
Storm Top Divergence.
50 dBZ Height vs the melting level height.
Percentage of storms with identifiable BWER
Percentage of storms with three-body scatter
signature.
The following components analyzed
from Zr and radiosonde data
• Maximum Zr at -20 degree C.
• Maximum Zr value and corresponding AGL
height.
• 2Vr (rotation).
•
•
•
•
Storm Top Divergence vs the melting level height.
50 dBZ Height vs the melting level height.
Percentage of storms with identifiable BWER
Percentage of storms with three-body scatter
signature.
Mean 2Vr = 57 KTS
The following components analyzed
from Zr and radiosonde data
• Maximum Zr at -20 degree C
• Maximum Zr value and corresponding AGL
height.
• 2Vr (rotation).
• Storm Top Divergence.
• 50 dBZ Height vs the melting level height.
• Percentage of storms with identifiable BWER
• Percentage of storms with three-body scatter
signature.
Storm Top Divergence (STD)
• Divergence was observed at or near the top of
the storm using SRM.
• STD = |∇(Vro,-Vri)|= |Vro – ( -Vri)|
Mean STP X = 109 KTS, SDV= +/- 25 KTS, X-STD=(84 KTS, 134 KTS)
The following components analyzed
from Zr and radiosonde data
•
•
•
•
Maximum Zr at -20 degree C
Maximum Zr value and corresponding AGL height.
2Vr (rotation).
Storm Top Divergence.
• 50 dBZ Height vs the melting level
height.
• Percentage of storms with identifiable BWER
• Percentage of storms with three-body scatter
signature.
Mean X = 39.1, STD = +/-5.9, 2STD = +/-11.8, X-STD = (45,33), X-2STD = (51,27)
50 dBZ Height vs. Melting Level
• Recorded the approximate 50 dBz height for
each storm where Golf Ball Size hail was
observed.
• Plotted the 50 dBz height vs the height of the
melting level.
The following components analyzed
from Zr and radiosonde data
•
•
•
•
•
Maximum Zr at -20 degree C
Maximum Zr value and corresponding AGL height.
2Vr (rotation).
Storm Top Divergence.
50 dBZ Height vs the melting level height.
• Percentage of storms with
identifiable BWER
• Percentage of storms with threebody scatter signature.
Examining Storm Structure
• I noted all the storms that had either a Three
Body Scatter Spike (TBSS) and/or Bounded
Weak Echo Region (BWER).
Example of a BWER
BWER result
• 10 percent of the storms that produced Golf
Ball size hail had a definable BWER’s.
• Almost every storm examined had a WER.
• BWER detection is often dependent on
distance from radar and beamwidth
TBSS example
TBSS results
• 15 percent of the storms that produced Golf
Ball size Hail had a definable TBSS.
• However, TBSS cannot be seen easily for many
azimuths of the storm, relative to the RDA
location, i.e. TBSS could be difficult to see
where it would be placed in an area already
covered by precipitation.
Final Result
• The best correlation was noted between the 0
deg C height and 50 dBZ height. A polynomial
can be fitted to provide a 50 dBZ height given
the observed (SND) or forecasted (RUC) 0 deg
C height.
Another Finding
• Observing the height of the 50 dBZ core along
with STD may provide you with a technique to
rule out a storm’s potential for producing golf
ball size hail.
STD vs 50 dBZ Height
• Less than 3 percent of all Golf Ball size hail
reports occurred with 50 dBZ heights less than
30,000 feet and STD less than 80 KTS.
• Less than 6 percent of all Golf Ball size hail
reports occurred with 50 dBz heights less than
30,000 feet.
Conclusion
• The 50 dBz height vs the melting level gives
the best correlation from this study may and
help the radar operator to determine when a
severe storm may produce Golf Ball size hail.
• Increase confidence in determining when a
storm is capable of, or unlikely, to produce golf
ball size hail and may lead to more accurate
hail size inclusion in SVS’s.
• Point soundings from the time and location of
observed golf ball size hail from using NARR
grib data would have given more accurate
melting levels.
Future Research
• To use NARR grib data, instead of the 12Z or 00Z TOP
upper air data, to determine melting height for both
the location and time of golf ball size hail report.
• Use the NARR grib data to determine how well the
Maximum Expected Hail Size (MEHS) algorithm
performs.
• Currently examining Radar and Environmental
Conditions associated with Baseball size hail in the
NWS Topeka County Warning Area
• Also, one of several co-PI’s working on a four inch and
larger hail study across the plains states.