a case study of a well-defined bow echo with bookend vortices

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A CASE STUDY OF A WELL-DEFINED BOW ECHO WITH BOOKEND VORTICES
Mark Richards and Shannon White
NOAA/National Weather Service
Nashville, Tennessee
Stephen Goss
NOAA/National Weather Service
Storm Prediction Center
Norman, Oklahoma
Tennessee and Paducah, Kentucky. This resulted in a
generally southerly wind, gradually increasing in speed
with height, through the lowest 500 mb across middle
Tennessee. The 0000 UTC Nashville sounding (not
shown) indicated only marginal instability and weak vertical wind shear. Surface dewpoints were in the low to
mid 60s (F) across the area. However, manual surface
analyses from 0400 UTC, 0800 UTC, and 1200 UTC
9 June 1996 (Figs. 1,2 and 3) showed a wave oflow pressure moving across middle Telmessee along a frontal
boundary. This boundary/low pressure wave served as a
focusing mechanism, aiding the development of a lone
thunderstorm in northern Alabama along the frontal
boundary just ahead of the low pressure wave. As the
storm moved into middle Tennessee, it rapidly intensified
and developed into a well-defined bow echo. This rapid
intensification, along with the presence of a surface
boundary, weak vertical wind shear, and small-scale
cyclonic rotation at the end of the bow echo, are the four
main ingredients often associated with the formation of
non-supercellular tornadoes (Lee and Wilhelmson 1996).
Abstract
An analysis of a bow-echo event with associated bookend
vortices is presented. The evolution of the cell is followed
from a weak thunderstorm in north Alabama to a welldefined bow echo which caused widespread damage in
middle Tennessee. The presence of bookend vortices is
shown and is linked to the tornadic damage found in
areas east of Nashville. The synoptic situation along with
the bookend vortices is a condition often associated with
the development of non-supercellular tornadoes. Study
findings can be used byforecasters to aid in the warning
of tornadoes in similar events in the future.
1. Introduction
Early on the morning of 9 June 1996, a thunderstorm
moving through middle Tennessee developed rapidly into
a well-defined bow echo (Fujita 1981). The storm eventually caused widespread damage in Wilson County, just
east of Nashville. Although most of the damage that
occurred was the result of "straight-line" winds near the
apex of the bow, there were eyewitness accounts of funnel
clouds and short-lived tornadoes. A post-storm survey
conducted by local emergency management and National
Weather Service personnel revealed small areas of FO to
F1 (Fujita 1981) tornado damage embedded within the
widespread straight-line wind damage.
As the bow evolved, WSR-88D storm-relative velocity
products revealed weak rotation at both ends of the
bow-Dne end rotating cyclonically and the other anticyclonically. Furthermore, significant wind shear was
also noted near the apex. It is hypothesized that the tornadic damage observed during the post-storm survey was
the result of brief gustnadoes/tornadoes associated with
the cyclonically rotating ''bookend'' vortex at the western
edge of the bow echo, and/or with the area of strong shear
near the bow's apex. Radar characteristics of the bow
echo will be discussed, as well as the wind damage associated with these radar characteristics.
3. Radar Characteristics and Damage
Initially, as the thunderstorm moved out of north
Alabama, the WSR-88D showed the storm to be quite
benign, with reflectivity values in the 30-40 dBZ range
and vertically integrated liquid (VIL) values of only 5-10
kg m 2• However, the Rapid Update Cycle (RUC) surface
analyses (on PC-GRIDDS) revealed the boundary (mentioned previously) lying across middle Tennessee with the
associated wave of low pressure providing an area of
enhanced lift favorable for thunderstorm development
and intensification. Indeed, at around 0710 UTC, VIL values associated with the thunderstorm began to increase;
additionally, the composite reflectivity showed a 55-60
dBZ core aloft and the beginnings ofthe bow-echo structure. The 0.5 elevation base reflectivity, however,
remained quite weak. Forecasters realized at this time
that the thunderstorm was intensifYing significantly, as
the strengthening updraft suspended a core of rain aloft.
Additionally, a reflectivity cross section was cut through
the developing storm, further revealing the suspended
reflectivity core. The cross section showed a layer
between 10 and 20 thousand feet containing reflectivity
values greater than 50 dBZ.
0
2. Synoptic/Mesoscale Environment
The 0000 UTC 9 June 1996 upper-air analysis (not
shown) showed a relatively deep 850-mb to 500-mb closed
low near the Mississippi River between Memphis,
23
24
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Fig. 1. Surface analysis 0400 UTC 9 June 1996.
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Fig. 2. Surface analysis 0800 UTC 9 June 1996.
Volume 21 Number 4
September 1997
25
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Fig. 3. Surface analysis 1200 UTe 9 June 1996.
By 0736 UTC, the composite reflectivity (Fig. 4) displayed a 65 to 70 dBZ reflectivity core. Additionally, close
inspection showed a reflectivity void behind the apex of
the developing bow echo, indicating the presence of a
rear inflow notch/downdraft. However, the corresponding VIL and echo tops remained below severe thresholds
based on VIL density criteria (Stewart 1992; Amburn
and Wolf 1996). By the next volume scan (not shown), a
fairly well-defined bow-echo feature was observed at the
lowest elevation slices, while a high reflectivity core
remained suspended aloft. The velocity data also began
to show increasing wind speeds. While attached velocity
Figs. 7 and 8 do not show substantial winds, this is misleading due to the fact that the storm is moving almost
perpendicular to the radar beam. Estimating wind
speeds using the angle between the storm motion vector
and the radar beam yielded speeds greater than 100
!mots. At this time, the first reports of damage were
received at the National Weather Service Office (NWSO)
N ashville (numerous trees and power lines were downed
by strong winds).
Over the next 30 to 45 minutes, the storm continued
to strengthen. The 0827 UTC composite reflectivity
(Fig. 5) and 0833 UTC base reflectivity (Fig. 6) depicted a well-defined bow echo over Wilson County.
Corresponding storm-relative velocity products (Figs.
7 and 8) and base velocity products (not shown) indicated significant shear near the apex of the bow, and
the existence of rotating vortices at the ends of the bow
(as indicated on Figs. 7 and 8). These vortices were
referred to as "bookend" vortices by Weisman (1993 ).
The post-storm survey of the area through which the
bow echo moved revealed 20 houses that sustained damage, four barns destroyed, several horses and cows killed,
and numerous trees and power lines downed.
Furthermore, the survey indicated that the majority of
the damage was caused by straight-line winds near the
apex of the bow. However, a couple of areas within the
damage swath appeared to contain damage from tornadic-type rotation, and eyewitness reports verified this
observation.
Post-storm analysis of the WSR-88D storm-relative
velocity data indicated that the significant shear near the
apex of the bow, and the area of rotation associated with
the cyclonically-rotating bookend vortex at the edge of
the bow (mentioned previously) appeared to correspond
to the location of the tornadic damage revealed during
the storm survey.
4. Summary
TIllS case illustrates widespread straight-line wind
damage associated with a bow echo, as well as weak,
short-lived, non-supercellular tornadoes along the gust
front, and near bookend vortices (in particular, the
cyclonically rotating vortex). Throughout this event, echo
tops remained below 35 thousand feet and VIL values
hovered generally in the 30 to 35 range. The warnings
issued were based almost solely on the bow-echo shape
and the presence of suspended high-reflectivity cores.
National Weather Digest
26
.
-. ."11'..-e
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CMP REF
37
12 4 NM
. 54 NM
136 / 13 9 / 9 6 137: 36
RO A' KOH X 3 6 / 14/
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SMITH
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191313
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KOH X 191313
COFFEE
Fig, 4, WSR-88D composite reflectivity, 0736 UTC 9 June 1996.
CM P REF
37
124 NM .54 NM
136 / 139 / 96 138 : 27
RDA ' KO HX 36 / 14 /
676 FT 86 / 33 /
JACI(SO
49N
46W
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CLA MAX = 72 DBZ
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Fig, 5, WSR-88D composite reflectivity, 0827 UTC 9 June 1996.
Q1 5 TVS 191313
PROD RCV D' R
KOH X 19136 . 54
R
RPS
1.5
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September 1997
SIMPS O
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BASE REF
124 NM . 54 NM RES
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Fig. 6. WSR-88D base reflectivity at 0.5 elevation, 0833 UTe 9 June 1996.
0
REL VEL MAP 56 SR M
124 NM
. 54 NM
06 / 09 / 96 08 ' 33
RDA ' KOH X 36 / 14 / 49 N
676 FT 86./ 33 ./ 46W
i:"-=--~~=r===~"--===;:= ELEV=
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OVL ' AN M TV AT '
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Q1 5 SRM 192 4
PROD RCVO ' R
KOH X 1929 1 . 1
R
RPS
0 .5
Fig. 7. WSR-88D storm-relative velocity at 3.4 0 elevation, 0827 UTe 9 June 1996.
I
,. 1
28
National Weather Digest
REL UEL MAP 56 SRM
124 NM
.54 NM
66 / 69 / 96 68'33
RDA ' KOHX 36 / 14 / 49N
676 FT 86 / 33 / 46W
.~=-_~=P"<===== ELEU =
2 . 4 DEG
MODE A /
21
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MAX= -52 KT 56 KT
CL~ SRM ' 192DEG
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ND
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6 .5
Fig. 8. WSR-88D storm-relative velocity at 2.4° elevation, 0833 UTe 9 June 1996.
The WSR-88D performed well, and even indicated the
cyclonic and anticyclonic rotations at the ends of the bow
echo. However, as expected, the WSR-88D did not indicate any TVS-like (Tornado Vortex Signature) circulations during tIns event (any tornadic circulations were
likely very small spatially, and very shallow vertically).
Obviously, any tornadoes that occur with non-supercellular storms will remain difficult to resolve directly using
the WSR-88D velocity products. However, the shear areas
near the apex ofthe bow as well as areas of rotation associated with the bookend vortices appeared to be the catalyst for the development of weak, short-lived tornadoes
during this event. Past research of other cases supports
these findings (Burgess and Smull 1990; Przybylinski
and Schmocker 1993).
With more and more cases indicating that tornadoes or gustnadoes can, and often do, form in the
absence of mesocyclonic rotation (Brady and Szoke
1988; Wakimoto and Wilson 1989), the question of
what type of warning to issue - tornado or severe
thunderstorm - can be raised. Further research will
have to be conducted to determine what radar signatures could indicate tornadic activity, in addition to the
straight-line wind damage normally expected during
severe bow-echo events. In this case, areas of weak
rotation at the ends of the bow, and strong shear near
the apex of the bow appeared to correspond with the
locations of observed tornadic damage. Perhaps these
observations, along with the identification of environments favorable for the formation of non-supercellular
tornadoes, can be used in future events to identify a
storm's tornadic potential.
Acknowledgments
The authors thank Darrell Massie, NWSO Nashville,
Tennessee, and Bob Johns, NWS Storm Prediction
Center, Norman, Oklahoma, for their comments and
review of the manuscript. Thanks is also extended to
Dedric Walker, NWSFO Oxnard, California for supplying
references to the authors. The authors also thank Henry
Steigerwaldt, Science and Operations Officer, NWSO
N ashville, Tennessee for his extensive review and assistance during tills study.
Authors
Mark Richards is currently a forecaster at the
National Weather Service Office in Nashville, Tennessee.
He issues forecasts for the public and aviation communities as well as a wide variety of severe weather statements and warnings. His interests lie in Doppler radar
and microscale meteorology. He earned a Bachelor of
Science degree in meteorology from Florida State
University in 1988.
Shannon Whlte is currently at the National Weather
Service Office in Nashville, Tennessee. She is a meteorologist intern who works with the upper air program and
aids forecasters in both public and aviation forecasting.
Her interests lie in precipitation and cloud physics stud-
Volume 21 Number 4
29
September 1997
ies. She earned her Bachelor of Science degree in meteorology from North Carolina State University in 1994.
Stephen Goss works in the Hazardous Weather
Update Unit of the NWS Storm Prediction Center in
Norman, Oklahoma. He writes updates for hazardous
weather events across the nation and handles media
inquiries for those events. He is interested in severe
thunderstorms and tornadoes, specifically tornadogenesis. He earned a Bachelor of Science degree in meteorology from The Pennsylvania State University in 1993.
References
Amburn, S. A. and P. Wolf, 1996: VIL density as a hail
indicator. Preprints, 18th Conf On Severe Local Storms,
San Francisco, Amer. Meteor. Soc., 581-585.
Brady, R H. and E. Szoke, 1988: The landspout - a common type of northeast Colorado tornado. Preprints, 15th
Conf on Severe Local Storms, Baltimore, Amer. Meteor.
Soc., 312-315.
Burgess, D. W and B. F. Smull, 1990: Doppler radar observations of a bow echo associated with a long-track severe
windstorm. Preprints, 16th Conf on Severe Local Storms,
Kananaskis Park, Canada, Amer. Meteor. Soc., 203-208.
Fujita, T. T., 1981: Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38,
1511-1534.
Lee, B. D. and R B. Wilhelmson, 1996: The numerical
simulation of non-supercellular tornadogenesis.
Preprints, 18th Conf on Severe Local Storms, San
Francisco, Amer. Meteor. Soc., 408-412.
Stewart, S. R, 1992: An empirical forecasting technique
for predicting pulse-type thunderstorm gusts using radar
derived vertically integrated liquid (VlL) and the penetrative downdraft mechanism. A Non-Thesis., University
of Oklahoma School of Meteorology, 83 pp.
Przybylinski, R W and G. K. Schmocker, 1993: The evolution of a widespread convective windstorm event over
central and eastern Missouri. Preprints, 13th Conf on
Weather Analysis and Forecasting, Vienna, Virginia,
Amer. Meteor. Soc., 461-465.
Wakimoto, R M. and J. W Wilson, 1989: Non-supercell
tornadoes. Mon. Wea. Rev., 110,707-718.
Weisman, M. L., 1993: The genesis of severe, long-lived
bow echoes. J. Atmos. Sci., 50, 645-670.
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