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A "squall line" refers
to a linearly-oriented zone of convection (i.e., thunderstorms).
Squall lines are common across the United States east of the Rockies,
especially during the spring when the atmosphere is most "dynamic."
A "bow echo" or "bowing line segment" is an
arched/bowed out line of thunderstorms, sometimes embedded within
a squall line. Bow echoes, most common in the spring and summer,
usually are associated with an axis of enhanced winds that
create straight-line wind damage at the surface. In fact, bow
echo-induced winds/downbursts account for a large majority of
the structural damage resulting from convective non-tornadic winds.
Transient tornadoes also can occur in squall lines, especially
in association with bow echoes. These tornadoes, however, tend
to be weaker and shorter-lived on average than those associated
with supercell
thunderstorms. Severe squall lines and bow echoes
are quite common in the Ohio Valley, including Kentucky. A sequence
of WSR-88D
Doppler radar images and discussions from some squall line/bow
echo events across Kentucky and south-central Indiana are available
to complement this document. Detailed squall line and bow
echo information is given below, including pre-storm environments,
and WSR-88D radar reflectivity, velocity, and mesocyclone characteristics.
PRE-STORM ENVIRONMENTS ASSOCIATED WITH SQUALL LINES/BOW ECHOES
Warm Season Events (Summer;
Weak Synoptic Forcing)
- SURFACE PATTERNS
- Generally east-west frontal boundary (genesis area frequently north of front).
- Strong surface convergence (near genesis area).
- High dewpoints pooled near front/genesis area; maximum values just south of front.
- Surface equivalent potential temperature (theta-e) values very high along derecho track.
- Bow echo often moves parallel to front with slight component toward warm sector.
- UPPER-LEVEL PATTERNS
- Straight or anticyclonically curved mid/upper-level flow near a ridge axis.
- Weak shortwave trough located near/upstream from genesis region.
- Moderate/strong warm advection at 850 and 700 mb present near genesis area with weaker advection downwind. Neutral or weak cold advection noted in mid/upper levels over and downwind of genesis area.
- 850 mb moisture very high and pooled just south of bow echo track; drier air can be present at 700 and 500 mb enhancing damaging wind potential.
- Bow echo moves generally along 850 and 700 mb thermal gradient (along or north of thermal/theta-e ridge axis).
- THERMODYNAMIC AND VERTICAL
WIND SHEAR PROFILE
- Long-lived warm season bow echo events associated with very unstable air mass.
- Average maximum convective available potential energy (CAPE) values in genesis area roughly 2400 J/kg with even greater instability downwind where average maximum CAPE is about 3500-4000 J/kg (range of 2500-6000 J/kg).
- Extreme instability due to pooling of moisture near front, so surrounding upper-air soundings may be unrepresentative of true local instability.
- Winds at 850 and 700 mb show good directional shear (veering) near genesis area, and often mainly speed shear parallel to storm track downwind.
Cool Season Events (Late Winter/Spring Strong Dynamic Forcing)
- SURFACE PATTERNS
- Strong, progressive low pressure system and associated warm and cold fronts.
- Squall line with embedded bowing line segments often located along or north of warm front southward across warm sector along or ahead of cold front.
- UPPER-LEVEL FEATURES
- Moderate/strong wind fields throughout atmosphere; 850 mb wind speeds 30-60 kts common with upper-level jet stream axis aloft nearby (often north and/or west of squall line).
- Wind fields stronger than in warm season bow echo events.
- Significant divergence/convergence fields and dynamical forcing (to produce strong lift) associated with convective development which overcomes possible limited moisture and instability.
- Environmental wind momentum
aloft may transfer downward causing damaging surface winds, especially
if no low-level inversion present.
- THERMODYNAMIC AND VERTICAL
WIND SHEAR PROFILE
- Cool season squall lines/bow echoes often associated with less instability than for warm season events.
- Degree of instability varies widely. CAPE values vary from less than 500 J/kg to over 2000 J/kg. Strong forcing/vertical shear compensates for limited instability.
- Layer of dry, potentially cold air (or cold advection/backing winds) often present in mid-level downdraft entrainment area (3-7 km layer) which impinges on squall line from upstream side, thereby enhancing damaging wind potential.
- Cool season squall lines/bow echoes associated with moderate/strong wind shear within the lowest 2.5 km layer (surface to 850 or 700 mb).
- Optimal conditions for bow echoes: linear shear profile with strong SPEED (limited directional) shear of 50 kts within lowest 2.5 km of atmosphere with minimal shear aloft.
TWO BASIC PATTERNS OF SQUALL LINE/BOW ECHO CONVECTIVE SYSTEMS
Progressive:
- Length of squall line/bow
echo relatively short and curved, and oriented perpendicular
to mean environmental wind. Line bulges/bows downwind which is
associated with downburst activity. Warm season bow echoes associated
with high instability and an east-west surface front often exhibit
a progressive pattern.
Serial:
- Length of squall line/bow echo usually extensive (much longer than progressive type) and oriented nearly parallel to mean environmental wind direction. Within squall line, a series of line echo wave patterns (LEWPs) and bow echoes often occur, resulting in damaging winds and possible transient tornadoes. High precipitation (HP) supercell characteristics sometimes can occur within organized, long-lived bowing line segments within serial squall lines. Dynamically-induced cool season events often exhibit a serial pattern.
REFLECTIVITY CHARACTERISTICS:
- During a bow echo's incipient
stage, a strong downburst may descend within or on the rear flank
of the convective echo, resulting in an initial bulging echo
pattern.
- Bow echoes exhibit a bulging/bowing
of the reflectivity gradient forward/downwind from the rest of
the squall line. Usually, a strong low-level reflectivity gradient
is present on the leading edge of intense convection indicating
strong convergence and a vertical updraft. See images below.
- Subtle weak echo regions
(WERs) may be present on the leading edge of the reflectivity
gradient marking the location of significant storm-relative inflow
and the updraft zone.
- Rear inflow notches (RINs)/weak
echo channels (WECs) frequently are noted behind the leading
intense convection, which usually are co-located with local enhancements
in the rear inflow jet (RIJ). See images below.
- Within an overall serial-type
squall line, there may be several bowing echo segments embedded.
- Bow echoes often are associated
with significant damaging surface winds (assuming a well-mixed
boundary layer) near the apex of the bow (i.e., along the RIJ),
and possible non-supercell tornadoes along or north (cyclonic
side) of the apex.
- The leading convective line
remains intense if the low-level cold pool beneath the convection
balances the ambient vertical wind shear, so that the outflow
boundary and intense updrafts remain on the leading edge of the
convective line. An outflow boundary propagating ahead of the
line may initiate new cells downwind but will eventually diminish
updrafts and the intensity within the main line.
- There may or may not be a
relatively large "stratiform" precipitation area (albeit
still some thunder and lightning) behind the leading convective
line depending on the amount of storm-relative elevated front-to-rear
flow. Squall lines that exhibit significant stratiform rainfall
behind the entire length of the line are referred to as symmetric,
while those with significant trailing rainfall only with the
northern portion of the line are referred to as asymmetric. Serial-type
"cool season" squall lines usually are associated with
more training stratiform precipitation than progressive "warm
season" events.
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LEFT: WSR-88D
Doppler radar low-level reflectivity data showing an intense bow echo
across north-central Kentucky. A strong reflectivity gradient is
present along the leading edge. Wind damage is pronounced along and near
the bow apex while transient tornadoes are possible just north of the
apex.
BOTTOM: Close-up reflectivity (left) and storm-relative map velocity (SRM; right) images of the bow echo. Organized bow echoes sometimes exhibit small-scale low ("L") and frontal structure. Wind damage is maximized along the bulged out cold/gust front, especially when a weak echo channel (WEC; left) is present behind the leading line associated with a strong rear inflow jet (RIJ; right). In this case, a tornado occurred near the triple point within the frontal structure just north of the bow apex. The black circle in SRM data identifies the mesocyclone that produced the tornado. |
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VELOCITY CHARACTERISTICS:
- Mid-altitude radial convergence
(MARC) signature may be evident in WSR-88D storm-relative velocity
map (SRM) data at an altitude of about 3-7 km. Strong (over 50
kts; 25 m/s), persistent, deep-layered radial velocity differences
(Vin + Vout along similar radials, i.e., MARC) within an area
of convection can signify entrainment of environmental air that
can enhance negative buoyancy through evaporation, resulting
in downdraft acceleration, i.e., a downburst. Rear inflow in
the system coupled with substantial MARC can accentuate the downburst.
This causes the onset of damaging surface winds and the development
of a low-level bow structure in reflectivity data. However, wind
damage can occur before significant low-level bowing appears.
Thus, the identification of spatially and temporally coherent
MARC in convective systems is crucial to anticipating subsequent
wind damage. MARC can precede the onset of surface wind damage
by up to 15-20 minutes. MARC is also very useful in anticipating
possible microbursts associated with severe pulse storms. Decreasing
WSR-88D vertically integrated liquid (VIL) values in conjunction
with significant MARC may signify a collapsing storm that is
about to produce a downburst.
- Local enhancements in the
rear inflow jet (RIJ) tend to develop along and behind axes of
bowing line segments, especially those associated with significant
trailing stratiform precipitation. Convective downdrafts can
intensify wind flow and damage associated with RIJs along the
leading bow apex. See images above.
- If the ambient wind shear
is moderate-to-strong, the RIJ tends to remain elevated up to
near the leading edge of the bow echo, then rapidly descends
at the updraft/downdraft interface causing significant wind damage.
Systems with elevated RIJs tend to be long-lived with rapid multicell
growth along the leading edge of the system.
- If the ambient shear is weak,
the RIJ tends to descend and spread out along and behind the
leading line, still with possible wind damage but less intense/shorter-lived
than for stronger sheared MCSs.
- Squall lines often contain
two main airflow streams relative to the moving convective system.
The first stream is rear-to-front associated with the RIJ. Above
this stream is storm-relative front-to-rear flow. This stream
has warm, moist origins ahead of the squall line, rises up rapidly
within the leading convection, then exhibits a much more gently
sloped ascent behind the line resulting in trailing stratiform
precipitation.
MESOCYCLONE CHARACTERISTICS:
- Cyclonic circulation (mesocyclone)
formation and possible tornadogenesis within squall lines usually
occur in association with bowing line segments given sufficient
forcing, instability, and wind shear.
- For leading line (bow echo)
tornadoes, the initial circulation typically develops as an area
of enhanced cyclonic-convergence in the lower portions of the
storm along/just north of the bow apex, then rapidly intensifies
and deepens in altitude, partly due to rapid vertical stretching
in the updraft. The circulation may be wrapped within precipitation.
The vortex eventually broadens and weakens as it propagates rearward
with respect to the leading line. Tornado occurrence is most
likely during the intensifying and deepening stage, when tight
shears and strongest rotational velocities exist. See images above.
- Tornadoes sometimes can be
tucked within a subtle weak echo region (WER) on the front forward
flank of an organized bow containing high precipitation (HP)
supercell characteristics. Tornadoes also are possible within
moderate-to-heavy precipitation under or near a rapidly rotating
comma head reflectivity signature.
- Convective cell or boundary
interactions with bow echoes may further enhance convergence
and vertical stretching allowing for more rapid development and
spin-up of mesocyclones/cyclonic circulations.
- Vortex evolution along an
organized, long-lived bow echo can be cyclic, i.e., the initial
circulation develops and intensifies, propagates along the bow,
and eventually weakens. However, new circulations can develop
quickly along the bow apex just to the south and/or east of the
initial vortex go through the same life cycle. This can result
in a series/family of transient tornadoes.
- Tornadoes associated with
bow echoes frequently are relatively short-lived and usually
of F0-F2 intensity. Tornadic damage usually is embedded within
and/or on the northern fringe of maximum straight-line wind damage
associated with the bow apex and rear inflow jet. While tornado
damage within bow echoes can be significant, the large majority
of damage from bow echoes is from straight-line winds.
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