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Tornadoes in Nonmesocyclone Environments with Pre-existing
Vertical Vorticity
along Convergence Boundaries
James M. Caruso
National Weather Service, Wichita, Kansas
Jonathan M. Davies
Private Meteorologist, Wichita, Kansas
Published in the NWA Electronic Journal of Operational Meteorology
1 June 2005
(updated 12 December 2005)
1. Introduction
Tornadoes resulting from stretching of pre-existing vertical vorticity
along convergence boundaries are not unusual in the warm season in the
central plains of the United States. These tornadoes are
associated mainly with nonmesocyclone processes (Brady and Szoke 1989),
sometimes called "nonsupercell" (Wakimoto and Wilson 1989), with the
tornadoes often called "landspouts" (Bluestein 1985). In some
cases, tornadoes in such environments can reach significant intensity
(defined here as F2 or greater on the Fujita scale), particularly if
instability is large. One example is the nonmesocyclone tornadoes
that hit the Denver, Colorado area on 15 June 1988 (Wakimoto and Wilson
1989); two of the four tornadoes caused F2 and F3 damage. Another
example is a case examined in this study with an F2 intensity
nonmesocyclone tornado near Wellington, Kansas on 27 August 2004 that
lasted nearly 20 minutes. So, it seems clear that some
nonmesocyclone tornadoes can present a significant threat to property
and lives, suggesting that terminology such as "landspout" (interpreted
by many to mean "weak tornado") may at times be misleading or
inappropriate. Nonmesocyclone tornadoes are typically more
difficult to anticipate and forecast than supercell/mesocyclone
tornadoes, which have been more widely researched.
This paper will review a common mesoscale setting that
appears to be favorable for nonmesocyclone tornadoes resulting from
pre-existing vertical circulations along wind shift boundaries.
Discussion and examples will also focus on low-level thermodynamic
characteristics that appear to be associated with such tornadoes and
their environments, as well as radar identification of boundaries that
play a role in such events. Three cases involving nonmesocyclone
tornadoes during 2002-2004 in south central Kansas will serve as events
for examination. It is hoped that this study will increase
situational awareness for mesoanalysts and warning meteorologists
regarding some nonmesocyclone tornado environments.
2. Nonmesocyclone tornado
environments
A common surface setting associated with nonmesocyclone tornadoes
identified informally by Davies (2003) involves a slow-moving or
stationary wind shift boundary (typically a weak front or trough
without a strong temperature contrast), often oriented northeast to
southwest (Fig. 1). Winds shifting
from southerly to westerly or northwesterly across the boundary
generate convergence and potential for pre-existing vertical vorticity
circulations (Brady and Szoke 1989; Wakimoto and Wilson 1989) or
misocyclones (Fujita 1981). Smaller scale thunderstorm outflow
boundaries or horizontal convective rolls intersecting the boundary can
also provide focus for vertical vorticity circulations.
An important thermodynamic feature of this setting is a surface heat
axis that parallels or intersects the main wind shift boundary,
generating an area of steep low-level lapse rates within the lowest 2
or 3 km above ground. In tornadic cases, this axis of steep
low-level lapse rates will typically overlap an area of low-level CAPE
(e.g., 0-3 km CAPE, Rasmussen 2003) somewhere along the boundary.
The presence of CAPE below 3 km suggests small convective inhibition
(CIN), and sufficient quality and depth of moisture to maintain
significant buoyancy for mixed-layer lifted parcels, indicating that
thunderstorm updrafts along the boundary will be significantly
surface-based. When combined with steep low-level lapse rates,
these thermodynamic characteristics (in addition to significant total
CAPE for deep convection) suggest potential for rapid low-level
accelerations beneath updrafts, resulting in enhanced vertical
stretching with little or no resistance to rising parcels. In
such environments, nonmesocyclone tornadoes can occur when a vigorous
developing updraft is juxtaposed with a pre-existing vertical vorticity
circulation along the main wind shift boundary. Tornado
development results from the vertical stretching of these
circulations, enhanced by the low-level thermodynamic
environment.
This process is shown in Fig. 2, reproduced
from Wakimoto and Wilson (1989). Nonmesocyclone tornadoes
typically develop early in a storm's life-cycle (e.g., Burgess et al.
1993) during the updraft stage, whereas mesocyclone tornadoes occur
during a supercell storm's mature stage when significant downdrafts
have organized around the main updraft (e.g., Lemon and Doswell
1979). Because nonmesocyclone tornadoes can develop early and
rapidly, mesoanalysis of pre-existing surface features where
thunderstorm development is expected is crucial to anticipate
them. Parameters such as large storm-relative helicity (SRH,
Davies-Jones et al. 1990) and low lifting condensation levels (LCL
heights, e.g., Rasmussen and Blanchard 1998) used in forecasting
supercell/mesocyclone tornadoes are generally not relevant to
nonmesocyclone tornadoes because the formation processes are
different. Slow boundary-relative storm motion is typical with
nonmesocyclone tornadoes, and as noted by Davies (2003), the
southern-most cells on radar are often most favored for
tornadoes. Settings similar to Fig. 1
can often support events lasting up to an hour or more, involving
multiple nonmesocyclone tornadoes.
The following sections will examine three recent cases of tornadoes
developing in nonmesocyclone environments over south central
Kansas. These cases will highlight the importance of mesoscale
boundaries and low-level thermodynamic characteristics.
Thermodynamic variables such as total CAPE and 0-3 km CAPE in this
study were computed using lowest 100 mb mixed-layer lifted parcels
(denoted as "ML"; e.g., MLCAPE) similar to Thompson et al.
(2003). The virtual temperature correction (Doswell and Rasmussen
1994) was used in thermodynamic computations.
3. 27 August 2004 case
Four tornadoes associated with a nonmesocyclone environment
occurred in south central Kansas on 27 August 2004 between 0000 UTC and
0100 UTC. The strongest tornado was
rated F2, and touched down about 3 km (2 mi) south of Wellington,
Kansas (Fig. 3), causing $250,000 damage
with a maximum path width of roughly 100 m (110 yds) and a path length
of 5 km (3 mi). This tornado lasted nearly 20 minutes and had a
large, full condensation funnel extending all the way to the ground
from a high cloud base, not the typical visual appearance of a
so-called "landspout".
The synoptic setting involved an upper level shortwave
trough (not shown) located upstream in Colorado, with the
tornadoes occurring in an environment of weak deep-layer shear, well to
the southeast of the shortwave feature and stronger winds aloft.
On the mesoscale, the surface map at 2300 UTC (Fig.
4)
showed
a weak quasi-stationary front oriented northeast to southwest across
south central Kansas and a weak low over north
central Oklahoma. The atmosphere was extremely unstable with MLCAPE of
3500 to 4000 J kg-1 (Fig. 5)
across
south
central Kansas. A Rapid Update Cycle (RUC, Benjamin et al. 2004) model
analysis sounding, similar to those used in Thompson et al. (2003) and
Davies (2004), is shown in Fig. 6 at 2300
UTC for Winfield, Kansas, 40
km east of the location where the F2 tornado occurred 90 min later.
This profile indicated that steep 0-2 km lapse rates (near 9.0 oC
km-1)
were present, along with deep low-level moisture, resulting in
substantial 0-3 km MLCAPE (around 90 J kg-1) and very little
convective inhibition (MLCIN). The environment was not
suggestive of supercell tornadoes, with only small 0-1 km SRH (< 50 m2s-2)
and high MLLCL heights (> 1600 m AGL).
Figure
7
shows parameter fields of 0-2 km lapse rate and 0-3 km MLCAPE generated
from the RUC model at 2300 UTC on 27 August 2004. Notice that
steep low-level lapse rates and low-level CAPE were juxtaposed in an
area over south central Kansas and north central Oklahoma, along the
quasi-stationary front where thunderstorms were developing. As
discussed in the prior section, this low-level thermodynamic setting
suggested some potential for rapid parcel ascent and enhanced low-level
stretching. The mesoscale and thermodynamic setting matched the
composite pattern in Fig. 1 rather well.
A visible satellite image at 2303 UTC (Fig. 8) showed
rapidly developing thunderstorms across south central Kansas in
a narrow northeast to southwest axis along the frontal zone. An outflow
boundary that intersected the main frontal zone at a break in this line
of storms can be seen in the radar reflectivity image in Fig. 9a, forming a "triple-point" (Lemon and
Quoetone 1995, and others). This boundary intersection likely served to
increase low-level convergence and pre-existing vertical vorticity on
the front northeast of the weak surface low. Figure 9b is a radar reflectivity loop
showing the back-building nature of the multicell storms over south
central Kansas and the evolution of the boundary intersection. A
4-panel display of base reflectivity from the KICT WSR-88D radar at
0046 UTC 28 August 2004 (Fig. 10) shows the
rapid updraft development (5.1 degree slice) that occurred over the
low-level boundary intersection convergence zone (0.5 degree
slice). The F2 tornado was in progress at this time south of
Wellington.
Figure 11 shows the base
velocity for the same time and slices as in Fig.
10. Base velocity
was in use instead of velocity from storm-relative motion (SRM) because
of very slow storm movement. A tight couplet
with maximum rotational velocity of 41 kt located at elevation angles
1.8
through 10.0 degrees (5,461 - 29,000 ft AGL) is seen in Fig. 11,
coinciding with the rapid updraft development over the low-level
boundary intersection convergence zone. The base velocity data
also indicated that the rotation began in the lower levels of the storm
and built upward into middle levels between 0029 UTC and 0046
UTC. Radar data provided no evidence of a pre-existing midlevel
mesocyclone or supercell structure prior to the tornado that occurred
from 0030 UTC to 0049 UTC. There were no indications of a weak
echo region (WER), bounded weak echo region (BWER), inflow notch,
rear-flank downdraft (RFD), or hook echo before the tornado was on the
ground. This raises the following question: Was the velocity
couplet in Fig. 11 a rapidly developing
supercell mesocyclone, or simply the radar sampling the increasing
circulation around the tornado? Most important, from an
operational perspective, this velocity couplet was not evident before
the tornado occurred, yet the low-level boundary intersection and
evolution was quite visible in radar imagery well in advance of the
tornado.
Developing and maintaining situational awareness regarding
mesoscale
features and thermodynamic environments that may have potential for
nonmesocyclone tornadoes can help forecasters react quickly in issuing
tornado warnings as radar features evolve, as described above.
Although it is difficult for WSR-88D radar to detect low-level
precursors (boundaries or pre-existing low-level circulations) beyond
ranges of roughly 80 km (45 nm) due to problems with the radar horizon
and beam width spreading (Warning Decision Training Branch 2002), this
case fortunately occurred relatively close to the radar site.
Even with no prior mesocyclone on radar, the boundary intersection and
low-level thermodynamic setting were features that could be noted and
assessed in advance of the tornado.
4. 9 July 2003 case
During the late afternoon and evening on 9 July 2003, severe
thunderstorms developed along a frontal
boundary oriented northeast-southwest in central Kansas (Fig. 12). Severe weather reports
included
eight
tornadoes, several severe wind gusts (50 kt or greater), and large hail
ranging from
4.5 cm (golfball size) to 11 cm (softball size). The strongest
tornado, rated F1, occurred about 8 km (5 mi) southeast of Cunningham,
Kansas, and had a path length of 8 km (5 mi) with a path width of
roughly 90 m (100
yards).
A surface heat axis at late afternoon (not shown) paralleled
the weak
frontal zone, intersecting it over south
central Kansas. Similar to the prior case, the atmosphere was quite
unstable with MLCAPE of 2500 to
3500 J kg-1 (not shown) across south
central Kansas. A RUC analysis sounding (Fig. 13)
at 2300
UTC at Kingman,
Kansas, located 27 km east-northeast of the location where the F1
tornado occurred 90 min later, suggested several characteristics
relevant to nonmesocyclone
tornado potential. These included very steep low-level lapse
rates (near 10.0 oC
km-1) along with a deep low-level moist layer that resulted
in
0-3 km MLCAPE (around 40 J kg-1) and negligible MLCIN, even
though MLLCL heights were quite high (> 2000 m
AGL). In addition, significant 0-6 km shear (38 kt) and 0-1 km SRH
(near 100
m2s-2, larger than in the prior case) suggested
that
the environment was also
marginally favorable for supercell tornadoes, making for a more
complicated environment setting. The RUC analysis fields in Fig. 14 show that low-level lapse rates and
low-level CAPE overlapped significantly in south central Kansas at 0000
UTC 10 July 2003, suggesting potential for rapid parcel ascent and
enhanced low-level stretching with updrafts along the frontal
boundary. Thunderstorms were in progress along this boundary at
0000 UTC, back-building to the south-southwest, with several tornadoes
occurring between 0000 UTC and 0100 UTC (Fig. 14).
Reflectivity and velocity images in Fig.
15a show the intersection of the northeast-southwest
front with a thunderstorm outflow boundary across western
Kingman County at 0034 UTC 10 July 2003 at the time the F1 tornado
noted earlier was occurring. In the reflectivity, a large thunderstorm
can be seen extending to the immediate east-northeast of the boundary
intersection. The thunderstorm exhibited some supercell
characteristics as it moved slowly southward across Reno, Kingman, and
Harper Counties during the late afternoon and early evening hours,
producing very large hail. As the thunderstorm pushed southward,
its outflow boundary moved southwestward, persistently intersecting the
main frontal zone (see radar reflectivity loop in Fig. 15b). The boundary intersection area (a
source of pre-existing vorticity) appeared to stay within the flanking
line structure of the supercell, and radar imagery suggested that a
series of strong updrafts developed over this boundary intersection
area in repetitive fashion, with resulting tornadoes. At the time
of the F1 tornado southeast of Cunningham, the SRM velocity data in Fig. 15a showed two low-level velocity
couplets near the tornado location. However, radar evidence of a
pre-existing midlevel mesocyclone was located several kilometers to the
northeast of the reported tornado.
This case is an excellent example of how nonmesocyclone tornadoes can
occur in conjunction with a supercell thunderstorm. It also
demonstrates a potential problem where a radar operator, who might be
unaware of the possibility of nonmesocyclone tornadoes, could be
focused on the "wrong" part of the storm (the midlevel
mesocyclone). This would make it more
difficult to understand or accept observed reports of funnel clouds or
tornadoes farther to the southwest. Also, a forecaster
thinking more in terms of supercell/mesocyclone tornado settings and
unaware of the
potential for nonmesocyclone tornadoes might be
confused and surprised when tornado reports began to materialize in an
environment with relatively high cloud bases and LCL heights (e.g.,
Rasmussen and Blanchard 1998).
5. 11 April 2002 case
Two tornadoes, both rated F0, occurred near peak heating during the
late afternoon of 11 April 2002 in south central
Kansas between 2200 and 2300 UTC in a nonmesocyclone environment. Figure 16 shows one of the tornadoes
southeast of Pretty Prairie,
Kansas, lasting between 15 and 20 minutes, emanating from a high,
flat
cloud base.
The surface map at 2300 UTC (Fig.
17)
showed a
weak quasi-stationary frontal boundary oriented northeast to southwest
across central Kansas, with a surface heat axis (not shown)
paralleling and intersecting the front in south central Kansas. The
atmosphere was
less unstable than in the prior cases, but still moderately
unstable with MLCAPE of 1000 to
1500 J kg-1 indicated by ETA analysis fields at 2100 UTC (Fig. 18) in an
environment with
weak deep-layer shear (< 30 kt, not shown). A RUC analysis
sounding (Fig. 19) at 2200 UTC for
Hutchinson,
Kansas (located about 40 km north-northeast of the Pretty Prairie
tornado) suggested several nonmesocyclone tornado environment
characteristics. Very steep low-level lapse rates (9.5 to 10.0 oC
km-1)
were present along with deep low-level
moisture that resulted
in small MLCIN and notable 0-3 km MLCAPE (around 40 J kg-1),
even though MLLCL heights were above 1500 m
AGL. The sounding also indicated that 0-1 km SRH was small (< 25 m2s-2),
suggesting
an unfavorable environment for supercell tornadoes. But combinations
of steep low-level lapse rates and substantial 0-3 km MLCAPE (Fig. 20) were maximized
along the frontal
boundary in south central Kansas where the tornadoes occurred,
suggesting increased potential for low-level stretching with updrafts
along the boundary. Like the prior cases, this case also closely
matched the
composite pattern for nonmesocyclone tornadoes in Fig. 1.
Figure 21a is the KICT 0.5 degree base reflectivity image at 2234
UTC when thunderstorms were in progress along the
northeast-southwest frontal zone. The yellow arrows in Fig. 21a indicate the
direction of the low-level wind at specific locations, and the
white ellipses highlight likely areas of pre-existing vertical
vorticity. Note that the lower
left ellipse indicates a wavelike inflection signature along the
front (e.g., Pietrycha and Manross 2003),
and the top two ellipses show the intersections of thunderstorm
outflow
boundaries with the front. The tornado in Fig.
16 was in progress at the time of Fig. 21a,
at the location where the purple arrow points to a new updraft
developing over one of these intersections. The back-building
nature of the thunderstorms and evolution of the frontal/outflow
boundary intersections can be seen in the radar reflectivity loop in Figure 21b. SRM velocity data during
and prior to the tornado (not shown) did not indicate significant
rotations, nor did it show any evidence of a pre-existing midlevel
mesocyclone. As with the prior cases, data suggests that the
low-level boundary intersection (a source of vertical vorticity), and
the thermodynamic environment (favorable for rapid low-level
stretching), were important factors supporting development of
nonmesocyclone tornadoes in this case.
6. Conclusion
All three of the tornado events in this study fit a composite pattern
supportive of nonmesocyclone tornadoes
identified
by Davies (2003) as shown in Fig. 1.
A weak, slow-moving or stationary surface front with little
temperature contrast, but a sharp wind shift from south or
southwest to northwest winds was evident in all cases
examined. The frontal wind shift boundary was
oriented northeast to southwest, and was
important in providing low-level convergence and pre-existing
low-level vertical vorticity. Along this wind shift boundary, very
steep low-level lapse rates were found
to overlap substantial low-level CAPE and total CAPE, likely enhancing
low-level
stretching of parcels entering storm updrafts on the boundary. In all
cases examined, nonmesocyclone tornadoes
occurred where new strong updrafts developed over the intersection of a
thunderstorm outflow boundary with the main frontal zone,
similar to the case presented in Lemon and Quoetone (1995).
Tornadogenesis appeared to occur early in the updraft life cycles when
low-level stretching occurred at the frontal/outflow boundary
intersections, enhanced by the low-level thermodynamic
environment. This is in contrast to supercell/mesocyclone
tornadoes, which normally occur during a supercell storm's mature stage
when downdrafts (e.g., the rear flank downdraft) are well
organized. Atmospheric instability varied from moderate to
extreme in the three cases studied.
Two of the three tornado cases examined (27 August 2004
and 11 April 2002) had
environments
with small SRH and weak deep-layer shear. This is in contrast to
supercell tornado environments where SRH is typically large, deep-layer
shear is strong, and LCL heights are low. The remaining tornado case (9
July
2003)
appeared to be a "hybrid" setting, fitting the composite pattern and
thermodynamic characteristics from Davies (2003), but also exhibiting
SRH and 0-6 km shear values that were marginally favorable for
supercells. The result was nonmesocyclone tornadoes associated with a
high-based supercell thunderstorm. The tornadoes appeared to
occur within the storm's flanking line instead of with the midlevel
mesocyclone.
As noted earlier, nonmesocyclone tornado
environments can support
events
lasting up to an hour or more, involving multiple tornadoes. Two of the
tornadoes documented in this study
lasted nearly 20 minutes in duration, with one causing significant
(F2) damage. This emphasizes that some nonmesocyclone tornadoes can
present a significant threat to life and property. Because
nonmesocyclone tornadoes are not preceded by midlevel or low-level
mesocyclones, WSR-88D velocity data is usually of limited use in the
traditional mode of tornado warning by radar for most tornadoes in
nonmesocyclone environments. However, at closer ranges (e.g.,
within 60-80 km of the radar), velocity and reflectivity data from the
WSR-88D can be useful for identifying and tracking
storm-scale/mesoscale boundaries to suggest areas of enhanced
pre-existing vorticity (see Figs. 9a, 15a, and 21a).
As seen in Pietrycha and Manross (2003), low-level circulations along
slow-moving frontal boundaries or troughs are often discernible in
velocity and reflectivity products when not too far from the radar
site.
Identification of such features, as well as recognition of the setting
and common
ingredients supportive of nonmesocyclone tornadoes (e.g.,
Fig.
1) can provide heightened short-term
awareness to forecasters and
warning meteorologists. Such knowledge may offer lead time for
deploying storm spotters to
areas of greatest concern,
helping to anticipate tornado reports that might otherwise be a
“surprise” without appropriate situational awareness. It may
also be possible to issue more rapid tornado warnings when a
combination of environment awareness, spotters, and radar are used
together (e.g., Cook et al. 2005).
Mesoanalysis is crucial for recognizing in advance the thermodynamic
ingredients and boundaries that can be conducive to nonmesocyclone
tornadoes. Prior to thunderstorm formation, environments with
potential for nonmesocyclone tornadoes can be diagnosed using careful
surface
mesoanalysis, satellite imagery, soundings (such as from Local Analysis
and Prediction System (LAPS) and RUC
model data), and
radar imagery (e.g., Pietrycha and Manross 2003, and Cook et al.
2005). Radar data, particularly high
resolution 8 bit base reflectivity and velocity products within 60 km
range and overlain with
surface data (such as
METAR plots), can be very useful for finding boundaries and boundary
intersections to suggest areas of enhanced pre-existing vertical
vorticity. Results from this study also suggest that assessing
combinations
of steep low-level lapse rates and positive low-level CAPE in the
vicinity of mesoscale wind shift boundaries through examination of the
overlapping fields may be helpful in assessing nonmesocyclone tornado
potential. It is important to remember that most
nonmesocyclone tornadoes occur in environments characterized by small
SRH and high LCL heights, contrasting with more "typical" supercell
tornado environments that are emphasized in forecaster training.
Further research is
planned regarding application of the concepts and parameters reviewed
this paper to future nonmesocyclone tornado events.
Acknowledgments:
The authors would like to thank Ken Cook and Paul
Howerton, both at NWS Wichita, for their review and comments on this
paper. Thanks is also extended to Dan Baumgardt at NWS La Crosse
for incorporating several experimental nonmesocyclone environment
parameters,
pertaining to research by Davies (2003), into the AWIPS Volume Browser.
Sincere thanks also go to Jim
Reed Photography and John
Brand for their tornado images used in this paper.
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