Supercell thunderstorms are
perhaps the most violent of all thunderstorm types, and are capable
of producing damaging winds, large hail, and weak-to-violent tornadoes.
They are most common during the spring across the central United
States when moderate-to-strong atmospheric wind fields, vertical
wind shear (change in wind direction and/or speed with height),
and instability are present. The degree and vertical distribution
of moisture, instability, lift, and especially wind shear have
a profound influence on convective storm type, including supercells,
multicells (including squall lines and bow
echoes), ordinary/pulse storms, or a combination of storm
types. Once thunderstorms form, small/convective-scale interactions
also influence storm type and evolution. There are variations
of supercells, including "classic," "miniature,"
"high precipitation (HP)," and "low precipitation
(LP)" storms. In general, however, the supercell class of
storms is defined by a persistent rotating updraft (i.e., mesocyclone)
which promotes storm organization, maintenance, and severity.
More information concerning environmental conditions and the structure
of classic and HP supercells is given below. WSR-88D Doppler radar
imagery showing the evolution of some supercell
events across Kentucky and south-central Indiana are available.
(30-60 minutes) storm; generally is non-severe but pulse severe
storm is possible; storm moves with mean wind; little or no vertical
wind shear/weak winds aloft in environment; chaotic hodograph
(Fig. 1); typical in summertime; buoyancy process
Multicell: Group of cells in different stages of development;
can be severe or non-severe; often move with the mean wind; show
discreet propagation with new cell growth on the unstable inflow
flank; weak-to-strong environmental wind shear/winds aloft; usually
a "straight-line" (unidirectional) hodograph indicating
speed and/or directional shear conducive for MCSs, squall lines,
and bow echoes (Fig.
1); gust front process
important (balance between convectively-induced low-level
cold pool strength and depth under the heavy rain and the ambient
low-level wind shear) to trigger new cells.
Supercell: Large severe storm occurring in a significant
vertically-sheared environment; contains quasi-steady, strongly
rotating updraft (mesocyclone); usually moves to the right (perhaps
left) of the mean wind; can evolve from a non-supercell storm;
moderate-to-strong vertical speed and directional wind shear in
the 0-6 km layer; usually a "curved" hodograph in the
lowest 0-3 km and a straight line above (Fig. 1);
dynamic process important resulting in a steady-state storm
Fig. 1: Hodograph
showing vertical wind shear for ordinary, multicell, and supercell
thunderstorms. Dots along hodograph line represent end point of
arrowheads of vectors (not shown) originating from (0,0) point
(x/y-axis intersection) that reveal wind speed and direction at
the indicated height (in km). For example, on supercell hodograph,
winds at 1 km altitude are from the southeast, stronger from the
south at 2 km, with winds increasing in speed (longer vectors
from (0,0) point to each dot) and veering to southwest at higher
altitudes. The longer the hodograph, the greater the vertical
wind shear. Not only length, but shape of hodograph is important.
For example, straight-line hodograph for multicells and curved
hodograph for supercells both indicate speed and directional shear.
However, curved hodograph indicates presence of a low-level wind
maximum (jet) which increases storm-relative flow into storm and
potential for supercell development. Supercells can evolve from
straight-line hodographs as well but are more common with curved
hodographs. In contrast, only weak shear is shown for ordinary
cells, although if high instability is present, then a severe
pulse storm can occur, with hail and/or brief damaging winds.
OF SUPERCELLS; ENVIRONMENTAL CHARACTERISTICS
- Supercells are not defined
by their depth or volume.
They can be large or small, high-topped or low-topped, and can
occur anywhere, including the Ohio Valley. They are most common
in the central United States. While supercells are not as common
as other convective types, they often produce violent weather.
- The interaction between updrafts
and the vertically-sheared environment strongly controls the
degree of organization and severity of convection. Supercells
and tornadoes are associated with moderate-to-strong vertical
wind shear (and helicity) and moderate-to-high CAPE (instability) (Fig. 2).
Rough total wind shear threshold for supercells is 40 kts
(20 m/s) in the 0-6 km layer. To determine this threshold,
look at the length of the hodograph (which includes speed and
directional shear) in this layer, and "lay out" the
hodograph along the x-axis to see if it exceeds 40 kts. If so,
supercells are quite possible; if not, supercells can still occur
given some shear and high CAPE values.
Fig. 2: Scatter plot
of strong and violent supercell tornadoes with respect to 0-2
km helicity (y-axis) values in m2/s2 and CAPE (x-axis) in J/kg.
Major tornado outbreaks typically associated with moderate-to-high
CAPE (1500-3500 J/kg) AND helicity (150-450 m2/s2). Isolated
to scattered tornadoes associated with low CAPE and high helicity
(upper left part of plot). Scattered tornadoes associated with
high CAPE and low helicity (lower right part of plot).
- Strong 0-6 km shear (long hodograph)
causes high helicity/high potential for supercell and mesocyclone
(rotating updraft) development, but NOT necessarily tornadoes.
Mesocyclone strength also is dependent on buoyancy. Tornado
development is dependent on dynamical structure in the storm.
Generally, a supercell/mesocyclone occurring in an environment
with significant low-level (0-2 km) curvature in the hodograph
(indicating the presence of a low-level jet) is conducive to
- Vertical wind shear causes
the development of dynamic processes in the storm which affect
the evolution, strength, longevity, and motion of the supercell. Explanation: 1) Environmental
shear results in a rotating updraft as horizontal vorticity is
titled vertically into the updraft. 2) The diagnostic pressure
equation states that rotation about a vertical axis (rotating
updraft) must be balanced by a pressure gradient force pointed
toward the center of rotation causing lowered pressure in the
middle-levels of a storm where the rotation/updraft is strongest.
3) This vertical pressure perturbation leads to an even stronger
updraft into the middle-levels, which in turn causes even more
rotation (due to vertical stretching) as the updraft speed increases
with height, which in turn can feed back and cause an even stronger
middle-level pressure perturbation. The deeper the environmental
wind shear, the more efficient the dynamic process should be.
- This dynamic process results
in an enhanced steady-state updraft; dynamic forces are as
important or even more so than buoyancy forces in supporting
updraft strength and rotation. The supercell actually
can "suck up" air and continue well into night
despite the loss of heating, weaker instability, and dissipation
of ordinary cells. The dynamic process also causes high (low)
pressure on the upshear/downdraft (downshear/updraft) side of
the storm, which results in storm tilt and a right movement of
the storm compared to the mean wind.
- Dynamic forces eventually
can cause the main updraft to split into 2 separate updrafts,
i.e., each supercell can develop both cyclonic (on the right
flank) and anticyclonic rotation (on the left flank) in the middle-levels. This can cause the storm to split into
2 separate cells, one moving right and the other left of the
mean wind. For a right (left) moving storm, the cyclonic rotation
is within the updraft (downdraft) and the anticyclonic rotation
is within the downdraft (updraft) with the tightest reflectivity
gradient on the south/east (north) side of the storm coincident
with the updraft. A classic example of a splitting storm occurred
28, 1996 over south-central Indiana. The right mover
evolved into a classic supercell that produced several tornadoes.
- Consider hodographs in evaluating
the potential for storm splitting and which cell will dominate.
A straight-line hodograph (unidirectional shear) is more conducive
for storm splitting than a curved hodograph in the lowest few
kilometers. Assuming a split occurs, a hodograph with significant
curvature (clockwise turning to the shear vectors) in the low-levels
promotes a strong right and weak left moving supercell.
- The storm relative inflow
direction and magnitude are very important. This determines which storm(s) will remain
strong/severe. For example, if 2 cells are aligned north-south,
both can remain strong despite ground-relative southerly
inflow if the storm-relative inflow has an easterly component.
Strong inflow speeds promote a stronger updraft strength and
more rotation. Strong middle-level storm-relative flow into the
supercell also seems to correlate with a strong mesocyclone capable
of tornadogenesis in the low-levels.
MECHANISMS IN SUPERCELLS
- Nearly all supercells produce
some sort of severe weather (large hail or damaging winds) but
only 30 percent or less produce tornadoes. Thus, one must try to differentiate a tornadic
supercell from a non-tornadic one.
- In the environment, strong
0-6 km shear (long hodograph) and ample buoyancy is needed to
generate a significant storm mesocyclone. Then, the supercell/mesocyclone occurring in an
environment with significant low-level (0-2 km) "curvature"
in the hodograph seems to be conducive to tornado development.
- However, tornado development
is dependent on the dynamical structure in the storm. There
must be a strong updraft and source of vertical vorticity for
strong mesocyclone and tornado development. Environmental horizontal
vorticity caused by ambient vertical wind shear is critical to
form a rotating updraft (mesocyclone). The environmental
vorticity may be crosswise or streamwise.
- However, tornado formation
appears to be related to a storm scale process: the
vertical tilting of baroclinically-induced horizontal vorticity.
This process occurs along an outflow boundary associated with
the forward flank downdraft (Fig. 3).
Along this boundary in or near the low-level hook region on radar,
a small-scale circulation occurs as warm environmental air rises
on the warm side of the boundary while cold air sinks and undercuts
on the cold side, which generates streamwise horizontal
vorticity along the boundary (note the sense of rotation in Fig. 3). This vorticity then is tilted and
rapidly accelerated vertically into the storm updraft as the
middle-level mesocyclone dynamically "sucks up" low-level
air, resulting in a more prominent low-level mesocyclone and
likely tornadogenesis. This process sometimes can be seen visually
as a tail cloud moving into the hook area from the east, and
may be visible on the WSR-88D reflectivity/velocity as
an outflow boundary or fine line echo.
- The streamwise vorticity associated
with this low-level process usually is NOT evident in the environment
(i.e., identifiable in a sounding). It is generated through the
storm's interaction with the environment. Thus, marginal ambient
wind shear may still support supercells and even tornadoes given
the presence of mesoscale/storm-scale interactions, which can
greatly increase the local wind shear, helicity, and therefore
mesocyclone strength and tornado potential. Once shear is
enhanced and maintained locally in the hook/weak echo region,
a series of mesocyclones and tornadoes are possible in the vorticity-rich
REFLECTIVITY SIGNATURES ASSOCIATED WITH SUPERCELLS
Fig. 3: Thunderstorm-scale
schematic of a supercell-environmental interaction, that can result in the
creation of vertical tilted baroclinically-induced horizontal vorticity.
This can lead to enhancement of the low-level mesocyclone and possibly
tornadogenesis. Text in schematic briefly describes this process.
- For "classic" supercells,
a low-level pendant or hook often is present on the right
rear side of the storm (Fig. 4).
Within the hook is a weak echo region (WER) signifying the
location of a strong rotating updraft (mesocyclone). The
hook is formed through the interaction of the forward flank and
rear flank downdrafts with the updraft area. The maximum reflectivity
(heavy rain and large hail) core usually is located just north
and/or east of the WER. In the downwind (weaker) portion of the
low-level reflectivity pattern, a "V-notch"
or "enhanced V" signature may be evident, indicating
blocking flow aloft causing some environmental air to move around
the storm. An actual supercell thunderstorm, as viewed by the KLVX WSR-88D
Doppler radar over north-central Kentucky, is shown in Fig.
4a. A vertical cross-section of a typical classic supercell
(along line C-D in Fig. 4)
is shown in Fig.
||Fig. 4: Plan
view of a typical classic supercell as viewed in
radar reflectivity data. Bottom (top) picture represents low-level
(upper-level) reflectivity. A weak echo region (WER) is noted in low-levels, a bounded
weak echo region aloft (BWER), with echo overhang above the BWER
overtop the low-level WER (i.e., storm tilt). A large area of
light precipitation and cloud extends well downwind in
the upper anvil portion of the storm.
4a: Low-level WSR-88D Doppler
radar image of an actual supercell thunderstorm over north-central
Kentucky on May 28, 1996. Dark red color represents very heavy rain and
hail. A hook echo is seen on the southwest flank of the storm, coincident
with a tornado on the ground at this time.
Fig. 5: Vertical
cross-section of a typical classic supercell along line C-D in Fig. 4. The
x-axis (y-axis) are horizontal (vertical) distance in km. Reflectivity
values in dBZ are shown within the storm. The low-level WER,
elevated BWER, echo overhang showing storm tilt, and downwind
anvil debris clouds clearly are evident.
- Above the WER, a Bounded
Weak Echo Region (BWER) (i.e., donut hole) may be present
at higher elevation angles (Figs. 4 and 5),
indicating overhang in the storm and the location of a strongly
rotating updraft. A persistent BWER is associated with
a significant mesocyclone.
- High reflectivity often caps
off the BWER above it.
The top part of the storm (echo top) is shifted over the low-level
reflectivity gradient or over the WER with possible significant
anvil debris extending downwind (Figs. 4 and 5).
- Heavy Precipitation (HP)
supercells: These exhibit similar features as
classic supercells. However, the low-levels frequently
show a broad high reflectivity pendent or Front Flank Notch (FFN)
(i.e., kidney bean shape) on the leading edge of the storm,
indicating the location of the WER and rotating updraft (Fig. 6). Mesocyclones for HP storms may be embedded
in heavy rain. HP supercells are not as isolated as "classic"
storms, and often may be embedded within squall lines and travel
along boundaries. HP supercells occur in environments with
rich low-level moisture and moderate-to-strong wind shear, and
are a threat for tornadoes, large hail, damaging winds, and flash
flooding. An example of an HP storm embedded within a squall
line occurred over south-central Kentucky on May
Fig. 6: Plan
view of radar base reflectivity in the low-levels (bottom picture)
and middle-levels (top picture) of a typical HP supercell. A
WER is present on the forward flank of the storm in low-levels
with echo tilt aloft overtop the low-level WER. Highest reflectivity values
in low-levels can resemble a kidney bean shape.
- Classic and HP supercells
sometimes can evolve into a bow echo as the rear flank downdraft or a rear
inflow jet causes the storm to accelerate outward, resulting
in a bowing storm with damaging straight-line winds (Fig. 7).
Fig. 7: Sequence
of basic plan view reflectivity schematics showing how a supercell
("A") can transition into a bow echo storm ("D") due to
development of a rear inflow jet and/or intense rear flank downdraft from
the HP storm.
- Most severe events occur
near the updraft/downdraft interface on the right rear (classic)
or front flank (HP) part of a storm. The strongest tornadoes often occur as the
BWER begins to collapse.
SIGNATURES ASSOCIATED WITH SUPERCELLS
- Mesocyclone: A small-scale solid body rotation closely
associated with a convective updraft. True supercell mesocyclones
(ones associated with tornadoes, e.g.,
Fig. 8) must meet or exceed established
thresholds for shear, vertical extent, and persistence. For
supercells, the following approximate criteria seem to well for
- Shear: Distance between the maximum inbound and maximum
outbound less than equal to 5 nm. Rotational velocity
Vr = [(max outbound velocity + max inbound velocity) ÷
2]: Severe thunderstorm warning: greater than about 20 kts (15 kts)
if the storm is less (greater) than 100 nm from the radar site.
Tornado warning: greater than about 40 kts (30-35 kts) if the storm
is less (greater) than 100 nm away. These values are only approximate,
so detailed consideration of storm structure, trends, and trained
spotter observations are very important as well.
- Vertical extent: Shear extends at least 8,000-10,000 ft
in the vertical (but shear may NOT extend this high up for low-top
storms or distant supercells that still can cause severe weather).
- Persistence: Coherent rotational signature persists
at least 2 volume scans.
8: WSR-88D storm-relative reflectivity image of
a tornado-producing mesocyclone near the town of Mt. Washington in
north-central Kentucky (southeast of Louisville) on May 28, 1996.
Red (green) colors denote radial winds directed away from (toward)
the radar located to the west (left) of the area shown. Thus, a
tight, cyclonic (counterclockwise) circulation is shown near Mt.
Washington. Just northeast of the town, the lighter shaded green
color represents storm-relative flow directed into the mesocyclone, which
appears to aid in tornado development and maintenance. The
mesocyclone is at the same time and position as the hook echo in the
reflectivity image in Fig. 4a above.
- CAUTION: Severe weather and non-supercell tornadoes
associated with squall lines and bow
echoes may still occur, despite these supercell criteria
not being met.
- Tornadoes are most likely
during the period of maximum mesocyclone core strength. Mesocyclones with the smallest diameters and
highest rotational velocities (Vr) extending over a deep layer
represent the greatest tornadic threat.
- Only about 30 percent or less
of mesocyclones that meet supercell criteria produce tornadoes,
although most all (90 percent or more) produce some
sort of severe weather.
- Mature idealized mesocyclone
rotational structure in WSR-88D storm-relative velocity data:
Low-levels: Usually see
cyclonic convergence (assuming the storm is close enough to the
RDA). Middle-levels: pure cyclonic rotation (maximum inbound/outbound
are on neighboring radials at the same distance from the radar
site). Upper-levels: cyclonic divergence. Storm Top:
pure divergence (maximum inbound/outbound are along same radial).
- Some mesocyclones produce
a single rotational core; others produce a series of cores in a periodic fashion. The first mesocyclone
core has a relatively long organizing and mature stage. However,
subsequent mesocyclones (if any) can develop and mature much faster
in the vorticity-rich convective environment, resulting in a
series of mesocyclones and a family of tornadoes.
- Multiple mesocyclones can
evolve as the rear flank
downdraft (mini-cold front) accelerates outward and catches up
with the forward flank downdraft (mini-warm/stationary front)
resulting in a convective-scale triple point occlusion at the
mesocyclone/updraft center. Thus, the original mesocyclone weakens while
a new one can spin up rapidly at the triple point (Fig. 9).
Fig. 9: Conceptual
model of mesocyclone core evolution. The "L" shows
the mesocyclone location with convective-scale cold and warm/stationary
fronts extending from the meso. The cold front is the leading edge of the
rear flank downdraft, while the warm/stationary front represents the
southern edge of the forward flank downdraft from rain-cooled air north of
the boundary. The bold lines are tornado tracks.
The insert shows tornado family tracks and the small square in
the insert is the region expanded in the schematic.
- The Tornado Vortex Signature
(TVS) is a strong, gate-to-gate (adjacent radials on the
WSR-88D Doppler radar) shear associated with tornadic scale rotation
that meets or exceeds established criteria for shear, vertical
extent, and persistence. Identification
of a low-level TVS suggests that a tornado may be occurring or may soon
develop assuming a favorable reflectivity pattern. However, even without an identified TVS,
identification and reflectivity and storm-relative velocity structure
is invaluable in assessing the need for a tornado warning.
FOR WARNING DECISIONS FOR SUPERCELLS:
- Always consider as much information
as possible, including 1) pre-storm environment; 2) radar reflectivity
structure and trends; 3) base and storm-relative velocity, including mesocyclone structure and
trends; 4) other pertinent WSR-88D products; 5) storm-scale interactions (within storm environment)
causing cell mergers, enhanced shear and rotation, etc.; and
6) spotter reports.
- Do not base a warning decision solely upon mesocyclone
strength. Consider the information
mentioned above. However, as a rule-of-thumb, if a supercell is identified,
including one with only a "weak"
mesocyclone, a severe thunderstorm warning should be issued; if a "moderate" or "strong" mesocyclone
is indicated and is supported by favorable reflectivity structure and the
presence of enhanced low-level storm-relative inflow, a tornado
warning should be strongly considered.
- Know conceptual models of storm
structure thoroughly. For example, even if velocity data are
hard to interpret (e.g., range folding, improper dealiasing,
weak mesocyclone at far ranges), but reflectivity structure or spotter
reports suggest a severe or tornadic storm, issue the appropriate
warning at once.