Squall Lines Introduction Summary
2D Evolution / Weak-Moderate Shear Summary
2D Evolution / Moderate-Strong Shear Summary
Line-end Vortices Summary
Supercell Lines Summary
Squall Line Motion Summary
Bow Echoes Summary

Physical Processes Index

Fundamental Dynamics Summary
Cold Pool / Shear Interactions Summary
Early 2D Processes Summary
The Rear-Inflow Jet Summary
Line-end Vortices Summary
Coriolis Effects Summary
Bow Echoes Summary

Case Exercises Summary

Squall Lines Introduction Summary

• Squall lines may either be triggered as a line, or organize into a line from a cluster of cells

• For a given CAPE, the strength and longevity of an MCS increase with increasing depth and strength of the vertical wind shear

• The characteristic squall line life cycle is to evolve from a narrow band of intense convective cells to a broader, weaker system over time

• The time over which this evolution takes place depends strongly on the magnitude of the low-level vertical wind shear; stronger shear leads to longer-lived systems

• It is the component of low-level environmental shear perpendicular to the line orientation (line-normal shear) that is most critical for controlling squall line structure and evolution

• In general, stronger system cold pools require a larger magnitude of vertical wind shear to produce a stronger and longer-lived system

• In general, the higher the LFC, the more low-level shear required for the system’s cold pool to continue initiating convection

• For mid-latitude environments, this module classifies surface to 2-3 km AGL shear strengths as weak (< 10 m/s), moderate (10-18 m/s), and strong (> 18 m/s)

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In plane view:

• At 1-2 hours into its evolution (early stage), the squall line is composed of independent convective cells and there is low-level convergence along the leading edge of the developing system cold pool

• At 2-6 hours into its evolution (mature phase), the squall line has a nearly solid line of strong convective cells at the leading edge, an extensive surface cold pool, a region of enhanced stratiform precipitation separated from the leading cells by a weak echo channel, and strongly converging flow at the leading edge of the cold pool

• At 4-8 hours into its evolution (later stage), the squall line's leading edge convection weakens, the surface cold pool surges ahead of the system, the surface flow field weakens, and the stratiform precipitation may last for hours

• The MCS life cycle may repeat itself as a new round of convection is triggered at the leading edge of the cold pool or if an external forcing feature, such as a cold front, helps to continually retrigger convection

• The surface pressure field of a mature MCS (evolving in weak-moderate shear) is characterized by a pre-squall mesolow and a mesohigh with the surface cold pool. A wake low may be present at the back edge of the stratiform precipitation

• In general, the weaker the shear, the faster the evolution

In vertical cross section:

• In the early stage, cells may be upright or slightly tilted downshear, whereas the updrafts are tilted downshear. The anvil begins to spread (especially in the downshear direction)

• The next generation of cell updrafts remain more upright than those of the initial cells

• In the mature stage, new cells continue to trigger on the leading edge of the cold pool, advect rearward, and feed into the expanding area of enhanced stratiform precipitation

• The system updraft is now tilted upshear from the front-to-rear of the squall line (the anvil spreads upshear as well), and the rear-inflow jet develops at mid-levels, diverging when it reaches the surface

• A mature MCS viewed in cross section has a mesolow at mid-levels above the mesohigh with the cold pool, and a mesohigh at the top of the anvil

• In the later stage, the leading line convective cells become shallower and weaker and the system-scale updraft also becomes more shallow as it tilts further rearward

• The rear-inflow jet continues to descend and spread out at the surface well behind the leading edge of the cold pool as the gust front considerably outruns the precipitation

• Localized hail, downbursts, and small tornadoes are most likely during the early-to-mature stages, but enhanced system-scale winds associated with the rear-inflow jet are most likely to occur during the mature stage

• MCSs evolving in large CAPE, moderate low-level shear environments are capable of producing large swaths of damaging winds, usually during their mature stage

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In plane view:

• At 1-3 hours into its evolution (early stage), the squall line is a narrow line of strong convective cells (some of the cells may be supercellular) and there is strong low-level convergence along the leading edge of the developing system cold pool

• At 3-8 hours into its evolution (mature phase), bow-shaped segments may begin to develop. Compared to a weakly sheared system, the stratiform precipitation region is narrower and there is usually no area of enhanced stratiform precipitation

• The surface flow field is strongly convergent at the leading edge of the cold pool, which remains close to the leading edge of the precipitation

• At 6-12 hours into its evolution (later stage), the squall line's leading edge convection begins to weaken, the surface cold pool begins to move ahead of the system, and lighter precipitation begins to fall over a broader area behind the system

• The MCS lifecycle may repeat itself as a new round of convection is triggered at the leading edge of the cold pool or if an external forcing feature, such as a cold front, helps to continually retrigger convection

• The surface pressure field of a mature MCS (evolving in moderate-strong shear) is characterized by a pre-squall mesolow and a strong mesohigh with the surface cold pool. A weak wake low is also possible

• In general, the stronger the shear, the longer this evolution takes. A strongly sheared system may live for > 12 hours, as long as conditions continue to be favorable

In vertical cross section:

• In the early stage, the system has a much longer period of downshear-tilted structure than weakly sheared systems, and individual cells may display extensive echo overhangs on their downshear side

• In the mature stage, the updrafts within the new cells being triggered in the downshear direction by the cold pool remain strong and vertically erect through the mid-levels of the storm

• When the rear-inflow jet develops, it remains more elevated than in weakly sheared systems as it approaches the leading edge of the system. It descends to the surface and creates potentially severe winds

• A mature MCS in a stronger shear environment has a strong mesolow at mid-levels above the mesohigh with the cold pool and a mesohigh at the top of the anvil

• In the later stage, the system may finally began to tilt upshear, and the leading line convection begins to weaken

• The rear-inflow jet continues descending to the surface behind the leading edge of the cold pool, and the gust front begins to outrun the precipitation

• Hail, strong winds, and tornadoes are possible with any strong cell in the line during the early and mature stages (especially with any supercell), but enhanced system-scale winds associated with the rear-inflow jet and bowing line segments are most likely during the mature stage

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• Significant 3D mesoscale flow features can evolve at the ends of a squall line or at breaks within the line. The most prominent are a set of mid-level mesoscale vortices referred to as "line-end" or "bookend" vortices

• The effects of line-end vortices are most significant for MCSs less than 200 km (110 nm) in length

• Line-end vortices develop between 2-4 hours into the system's evolution, just behind the zone of most active convection

• The vortex on the northern end of the system has cyclonic rotation, while the vortex to the south has an anticyclonic sense (in the northern hemisphere)

• Initially, the cyclonic and anti-cyclonic line-end vortices are often of similar strength and promote the formation of a bow in the leading line convective cells

• Eventually, the northern vortex becomes dominant, and the entire system becomes asymmetric, with most of the stratiform precipitation residing on the northern end (in the northern hemisphere)

• The dominant cyclonic vortex can last beyond the lifetime of the original MCS and is then referred to as a mesoscale convective vortex (MCV)

• Typical line-end vortices extend from 1 to 6 km AGL in the vertical, and the region of most significant circulation ranges from 15 to 25 km (8 to 14 nm) in diameter, although evidence of circulation may be observed over 50 km (27 nm)

• Line-end vortices often act to enhance the strength of the rear-inflow jet

• Line-end vortices tend to remain closer to the leading-edge convection in stronger shear environments

• Small-scale bow-shaped systems within the larger MCS are more likely in stronger shear. Each bowed segment will follow its own symmetric-to-asymmetric evolution. These smaller-scale systems are referred to as bow echoes

• An MCS composed of multiple bow echoes is referred to as a line echo wave pattern (LEWP)

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• When the environmental vertical wind shear is both strong and deep, a squall line may be composed primarily of supercells

• In such systems, supercells initially may be spread along the entire extent of the line

• Later, the circulations of these supercells are often disrupted as cells along the line interact with each other

• Both the disruption of storms due to cell interaction and the ultimate survival of supercells are strongly dependent on the shape of the hodograph

• Even for the strongest, most favorable shear profiles, supercells in the interior of a squall line usually give way to linear convective segments (such as bow echoes) as the cold pool begins to dominate the system-scale circulation

• The southern end of a mature squall line and breaks in the leading line convection remain favorable spots for supercells and should be monitored closely

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• Squall line motion is a result of both the advection of individual cells within the line and discrete propagation due to the triggering of new cells

• In squall lines > 200 km (110 nm) long, individual cells may move at an angle to the line, but the net motion of the line is constrained to be perpendicular to its initial orientation

• For squall lines < 100 km (55 nm) long, the line may reorient itself perpendicular to the direction of the mean low-level shear vector and then propagate in that direction

• For most squall lines, the speed of system propagation tends to be controlled by the speed of the cold pool propagation

• Squall line motion can also be affected by variations in the environmental conditions along the line (CAPE and shear variability)

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• Bow echoes are typically 20-120 km (10-65 nm) long bow-shaped systems of convective cells that are noted for producing long swaths of damaging surface winds

• Bow echoes may occur as either isolated convective systems or as part of much larger convective systems such as squall lines

• When multiple bow echoes are observed within a squall line, the radar signature is referred to as a line echo wave pattern or LEWP

• The line-end vortices associated with bow echoes are often referred to as bookend vortices

• A weak echo notch behind the core of the bow, referred to as a rear-inflow notch (RIN), often signifies the location of a strong rear-inflow jet

• Bow echoes tend to propagate in the direction of the mean low-level vertical wind shear vector at a speed controlled by the cold pool propagation

• A vertical cross section in the core of the bow echo reveals a strong, vertically erect updraft at the leading edge of the system; a strong, elevated rear-inflow jet impinging to just behind the updraft region before descending rapidly to the surface; and a system-scale updraft that turns rapidly rearward, feeding into the stratiform precipitation region

• A bow echo follows the full squall line evolution, but on a smaller scale. Bow echoes often generate intense winds when the close proximity of the line-end vortices acts to strengthen the rear-inflow jet, leading to widespread, potentially damaging winds at the surface

• If the cumulative impact of the severe wind from one or more bow echoes covers a wide enough and long enough path, the event is referred to as a derecho

• Progressive derechos are characterized by a single bow-shaped system that propagates north of and parallel to a weak east-west oriented stationary boundary

• Serial derechos are composed of a series of bow-echo features along a squall line, usually located within the warm sector of a synoptic-scale cyclone

• Severe bow echoes are most often observed in environments with moderate-to-strong low-level shear and very high CAPE

• Bow echo and supercell environments overlap, with bow echoes often characterizing the later stages of a supercell event

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• To examine MCS dynamics, we must use the full vertical momentum equation when considering the vertical motions with individual cells that make up an MCS, but we can use the hydrostatic equation to look at the larger mesoscale circulations within the MCS

• MCSs typically have lifetimes greater than 4 hours long, therefore, Coriolis affects system evolution

• The horizontal vorticity equation, which states that horizontal vorticity can only be generated in the presence of horizontal buoyancy gradients, is useful for examining the physical processes that determine MCS structure and evolution

• A rising warm bubble of air can be explained by the horizontal vorticity couplet induced at its edges due to the buoyancy gradient across the bubble. The vorticity couplet acts together to accelerate the bubble upward and induce weak downward motion at the sides. (This action is noticeable along the edges of a billowing towering cumulus.)

• Likewise, the spreading of a surface cold pool can be explained by the horizontal vorticity across buoyancy gradient at its edges. This gradient creates diverging flow at the surface, upward motion at the leading edge, and return flow above the cold pool

• When a warm pool is placed over a cold pool at the surface, similar to the vertical structure of a mature MCS, mid-level convergence results
  • In the pressure perspective, air accelerates toward the mesolow at mid levels
  • In the horizontal vorticity perspective, buoyancy gradients at the edges of the warm and cold pools generate vertically stacked horizontal vorticity of opposite signs, producing strong mid-level convergence

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• The system-generated cold pool and the ambient low-level shear strongly modulate the tendency to generate new cells in multiple cell systems, including MCSs

• The deepest updrafts occur when the horizontal vorticity generated along the cold pool’s leading edge is nearly equal in magnitude to, and has rotation of opposite sense to, the horizontal vorticity associated with the low-level vertical wind shear

• When the low-level wind shear is weak and is associated with weaker horizontal vorticity than the cold pool, the updraft at the leading edge of the cold pool is tilted upshear and is not as deep and strong as when they are in balance

• When the low-level wind shear is stronger and is associated with stronger horizontal vorticity than the cold pool, the updraft at the leading edge of the cold pool is tilted downshear and is not as deep and strong as when they are in balance

• This cold pool/low-level shear relationship can be quantified as a ratio of the speed of the cold pool, c, over the value of the line-normal low-level vertical wind shear, Du.

• A c/Du, of 1 represents the optimal state for deep lifting by the cold pool. Values less than 1 signify that the ambient shear is too strong relative to the cold pool and values greater than 1 signify that the cold pool is too strong for the ambient shear. This balance is significant for anticipating the strength and longevity of an MCS

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• In the presence of vertical wind shear, the additive influence of the horizontal vorticity created on the sides of a buoyant developing updraft and the vorticity associated with the shear causes the early cells in an MCS to lean downshear

• With time and continued precipitation, the cold pool circulation within the MCS eventually becomes strong enough to balance the horizontal vorticity associated with the ambient shear. It is usually during this stage that the most intense and erect convective cells are observed along the squall line

• The cold pool continues to strengthen, often to the point that its circulation overwhelms the vorticity associated with the vertical wind shear. At this point the leading-line cells begin to tilt upshear and advect rearward over the cold pool

• The rearward advecting cells in an MCS produce an expanding region of lighter precipitation behind the stronger, leading-line convection. This is the characteristic trailing stratiform precipitation region associated with mature MCSs

• The time for a typical midlatitude MCS with a strong cold pool to evolve from downshear to upshear tilt is 2-6 hours for a 0-3 km shear of 10 m/s, and 4-8 hours for a 0-3 km shear of 20 m/s or greater

• The strength, and therefore the speed, of the system cold pool depends on such factors as the low-level moisture field (due its influence on low-level evaporation), the amount of buoyancy in the environment, and the source region of the system downdraft

• Even though the cold pool depth is typically 1-1.5 km, the low-level shear depth that is used to calculate Du is generally taken as 0-2.5 or 3 km AGL, since deeper shear also has an influence

• For a squall line, the only component of low-level shear that contributes to the c/Du balance is the component perpendicular to squall line orientation

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• During the mature stage of an MCS, the convective cells spread rearward transporting warm air aloft. The surface cold pool also extends rearward due to the rearward expanding rainfield

• The juxtaposition of the warm air aloft over a cold pool produces lower pressure at mid levels, leading to mid-level convergence. The flow that converges in from the rear of the system at mid levels is known as the rear-inflow jet

• The formation of the RIJ can also be explained by the horizontal buoyancy gradients at with the back edge of the system, which generate a vertically stacked horizontal vorticity couplet that induces the rear-inflow jet

• The strength of the RIJ is directly related to the strength of those buoyancy gradients, i.e., the relative warmth of the FTR current and the relative coolness of the cold pool

• The RIJ strength is also affected by the strength of the vertical wind shear. Stronger shear produces enhanced lifting at the leading edge of the system, which leads to a stronger FTR current and enhanced warm pool

• In weak shear, lower CAPE environments, the warm pool aloft tends to be weaker than the cold pool. In this case, the RIJ descends further back in the system

• In stronger shear/higher CAPE environments, the warm pool aloft tends to be comparable to the cold pool. This keeps the RIJ elevated until much closer to the leading line convection. This is usually the case with severe bow echoes

• Storm-relative RIJ strengths vary from a few m/s for weak systems, to 10-15 m/s for moderately strong systems, to 25 to 30 m/s for the most severe systems, such as bow echoes

• In general, RIJ strength increases for increasing CAPE and increasing vertical wind shear

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• With time, MCSs tend to develop vortex pairs with opposite sense rotation at the ends of the convective line. These are referred to as "line-end'' or "bookend" vortices

• Recent modeling studies have shown that northern cyclonic and southern anticyclonic rotation centers at the ends of squall lines (north/south oriented lines in the northern hemisphere) are primarily created when the system updraft tilts the easterly shear generated at the system's cold pool/updraft interface

• When the ambient vertical wind shear is weak, the leading line system updraft tilts strongly rearward above the cold pool, and the line-end vortices occur farther behind the leading-line convection

• When the ambient vertical wind shear is strong, the leading-line system updraft is more erect, and the line-end vortices remain closer to the leading edge of the system

• When the line-end vortices are widely spaced along a long squall line, they are unlikely to contribute significantly to the strength of the RIJ

• However, if the line-end vortices are 60 km (32 nm) apart or less, they can enhance the RIJ by 10-15 m/s or more. Such significant contributions to the strength of the RIJ are important to the development of severe bow echoes

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• Coriolis forcing causes converging flow to take on cyclonic rotation, while diverging flow becomes anticyclonic

• After about 3-4 h at midlatitudes, Coriolis begins to affect MCS system-scale flow. At the surface and in the anvil region of the MCS the diverging outflow will become anticyclonic, while at mid levels the air converging from all sides toward the mesolow takes on cyclonic rotation

• Over time, the impact of mid-level convergence in the presence of Coriolis forcing acts to weaken the southern anticyclonic line-end vortex, but strengthen the northern cyclonic vortex

• The strengthening of the cyclonic vortex at the northern line-end over time is what produces the often observed symmetric-to-asymmetric system evolution that occurs with most long-lived MCSs

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• Even though they are often more severe, bow echoes are associated with the same basic structural features and physical processes as other MCSs

• Because of their close proximity in these smaller scale isolated bow echoes, the line-end vortices act together to intensify the RIJ, and are often referred to as bookend vortices

• The development of the intense, elevated RIJ of severe bow echoes, generally requires both large CAPE and strong low-level vertical wind shear

• Severe, long-lived bow echoes often remain in the downshear-tilted to upright stage for several hours, allowing the cold pool to become exceptionally strong before the system begins to tilt upshear

• When a severe bow echo does tilt upshear (in a large CAPE environment), it produces an exceptionally warm FTR ascending current. The presence of the strong warm pool over the strong cold pool contributes to the creation of an exceptionally strong elevated RIJ

• When the ambient shear is strong enough to support supercells, the bookend vortices with the bow echo may first originate through the downward tilting of ambient shear within the supercellular downdrafts. But as the cold-pool strengthens, the horizontal vorticity that it generates eventually becomes the predominate source of vorticity at the line-ends

• Because they are typically long-lived and Coriolis has time to act upon them, severe bow echoes tend to evolve through a symmetric to asymmetric evolution like larger MCSs

• Strong surface winds in bow echoes result from both the exceptionally strong, deep surface cold pool as well as the downward transport of momentum by the RIJ as it descends just behind the leading edge of the system

• Because they both require strong vertical wind shear, supercells and severe bow echoes often occur in close proximity to one another, or evolve from one of these structures to the other during their lifetime

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• Both synoptic-scale and mesoscale features can significantly impact MCS structure and evolution

• Boundaries acting as convective triggers can greatly increase squall line longevity beyond what would be expected based on internal dynamics alone

• Favorable shear and buoyancy conditions conducive to severe MCSs do not develop in isolation, but often occur in association with identifiable synoptic patterns

• Mid-Altitude Radial Convergence (MARC) is a signature observed via Doppler velocity imagery of an MCS that can serve as a precursor to the initial onset of damaging winds

• MARC velocity differentials of >= 25 m/s (>=50 kts) along the forward flank or leading edge of an organized bowing MCS indicate the likely onset of damaging winds

• MARC may be observed at great distances from a radar and provide 10-30 minutes advance warning to the onset of severe winds with an MCS

• The MARC signature and associated convergence values will be greatly underestimated if the MCS has a significant component parallel to the radar beam

• Estimating the strength of an MCS cold pool can help a meteorologist estimate its speed of propagation, the stage of system evolution, and the potential system longevity (when the cold pool strength is compared to the strength of the low-level ambient shear)

• The relative strength of an MCS cold pool can be most easily calculated by measuring the change of surface pressure as the cold pool passes

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