squall lines

Mesoscale M. D. Eastin

Squall Lines


Squall Lines

Definitions

• Mesoscale Convective Systems• Squall Lines

Environmental Characteristics

Structure and Conceptual Model

• Three General Types• Classic 2-D Structure

2-D Evolution

3-D Evolution

Bow Echoes

Forecasting


Mesoscale Convective System (MCS):

• Any ensemble of thunderstorms producing a contiguous precipitation area >100 km2

• Coriolis force plays a role in their evolution• Result from either: (1) Widespread, strong forcing along an air-mass boundary

(2) Upscale growth of multi-cell convective storms

• Common examples include:

• Squall Lines / Bow Echoes / Line-Echo Wave Patterns (LEWP)• Mesoscale Convective Complexes (MCCs)

Definitions

Developing MCC over NebraskaBow Echoes / LEWP over IndianaSquall Line over Missouri


Squall Lines:

• Loosely defined as a quasi-linear collection of ordinary cells with finite length that contains a stratiform rain region either behind, parallel to, or ahead of the convective line.• There is no strict length definition (100 – 2000 km)

• Long lived (2-15 hours)• Tend to occur at night• Primarily quasi-2D (linear) but contain 3-D structure• Can produce weak tornadoes, large hail, localized flash flooding, and severe straight-line winds

Definitions


Basic Characteristics:

• A linear forcing mechanism is required to organize the early convection:

• Cold / warm front or dryline• Topographic features• Linear outflow from prior convection• Enhanced upper-level lift (jet streaks)

• Mid-level dry layer is needed to sustain the persistent gust front outflow that helps initiate new convective cells along the line

• Some CAPE is required (> 500 J/kg), but severe weather usually develops in more unstable environments (> 2500 J/kg)

• Moderate deep-layer shear (> 10 m/s below 6 km) is required to maintain updraft/downdraft separation

• Strong low-level shear (> 15 m/s below 3 km) is optimal → severe weather is common when the shear is large

Environment


The Importance of Low-Level Shear:

• For a given CAPE, the strength and longevity of a squall line increases with increasing strength of the low-level shear It is the vector component of low-level shear perpendicular to the line that is most critical for squall line evolution

For mid-latitude squall line environments we can classify the 0-3 km AGL vertical shear strengths as:

Weak = <10 m/s Moderate = 10-18 m/s Strong = >18 m/s

Environment


Three General Mature Structures:

Structure and Conceptual Models

From Parker and Johnson (2000)


Trailing Stratiform (TS) Squall Lines:

• Strong convection along leading edge with stratiform precipitation trailing behind the line

• Account for ~70% of all squall lines

• Average values: Duration = 12.2 hrs Line Motion = 13.0 m/s

CAPE = 1605 J/kg LI = -5.4 K

• Strong low and mid-level cross-line flow• Moderate upper-level along-line flow



LineMotion

AlongLine

Cross Line

Along LineFlow (m/s)

Cross LineFlow (m/s)

StratiformPrecipitation


Leading Stratiform (LS) Squall Lines:

• Strong convection along trailing edge with stratiform precipitation leading the line


• Average values: Duration = 6.5 hrsLine Motion = 7.1 m/s

CAPE = 1009 J/kgLI = -3.5 K

• Moderate low and upper-level flow (cross and along line)• Weak mid-level flow



LineMotion

AlongLine

CrossLine





Parallel Stratiform (PS) Squall Lines:

• Strong convection along the up-wind segment with stratiform precipitation located downwind


• Mean values: Duration = 6.3 hrsSpeed = 11.4 m/s

CAPE = 813 J/kgLI = -2.2 K

• Strong low-level cross-line flow• Strong mid and upper-level along-line flow



LineMotion

AlongLine

CrossLine





Trailing Stratiform Squall Line Structure:

• Numerous observational studies have identified common structural characteristics


Adapted from Houze et al. (1989)



Ascending Front-To-Rear (FTR) Flow:

• Results from forced ascent along gust front and buoyancy forces • Strong updraft, heavy precipitation, and strong latent heating along leading edge in association with developing convection• Weak updraft, stratiform precipitation, and less latent heating in rear in association with decaying convection


From Houze et al. (1989)



Convective Downdrafts and Gust Front:

• Mid-level downdrafts maintained by evaporation and water loading, as well as the near-surface meso-high and meso-lows (more on these later) • Gust front helps initiate new convection via forced ascent of low-level inflow





Descending Rear-to-Front (RTF) Flow:

• Results from a combination of dynamic and buoyancy forces associated with the environmental vertical shear, gust front, and ascending front-to-rear flow, as well as evaporational cooling and mesoscale pressure gradients (more on these later)• Helps keep the leading-edge convection “upright”• Can contribute to the gust front if it descends to the surface





Mid-Level Meso-Low:

• Result from warm buoyant air (and latent heat release in the clouds) located above the cold air associated with the gust front (and evaporative cooling) beneath cloud base• Hydrostatic effect of warm air above cold air (…recall the hypsometric equation)



Warm Warm

Warm

WarmWarm

Cold Cold ColdCold



Surface Pre-Squall Low:

• Results from a combination of warm air aloft in the spreading anvil cloud and adiabatic heating associated with descending mid-level flow in response to the leading edge convection• Hydrostatic effect of heating at multiple levels



Warm Warm

Warm

WarmWarm

Cold Cold ColdColdWarm



Surface Meso-High:

• Results from the continuous “pooling” of cold air near the surface by negatively buoyant downdrafts driven by water loading and evaporational cooling• Hydrostatic effect of the surface cold pool



Warm Warm

Warm

WarmWarm

Cold Cold ColdCold




Warm Warm

Warm

WarmWarm

Cold Cold ColdCold


Surface Wake Low:

• Results from a combination of warm air aloft in the spreading anvil cloud and adiabatic heating associated with the descending rear inflow• Hydrostatic effect of heating at multiple levels• Often marks the edge of the trailing stratiform precipitation and surface cold pool

Warm



Weak-Moderate Shear

• New cells develop on downshear side of initial cold pool and are advected upshear• Well defined stratiform rain region forms• Cold pool and meso-high intensify with the help of the descending rear inflow Gust front surges outward, well ahead of the leading line of convection → systems decays


Shear


Classic Squall Line Structure and Evolution:

Moderate-Strong Shear

• Initial development is the same• Cold pool and meso-high intensify Strong low-level inflow prevents outward surge of the gust front and enhances forced ascent• Leading edge convection intensifies• Long-lived squall line with less stratiform rain• Some cells exhibit a “bow” structure


Shear


Important Physical Processes:

Buoyancy

• Buoyancy (or temperature) gradients produce local circulations, mesoscale pressure anomalies, and air flow accelerations

Vertical Shear

• Interaction between the cold pool and the low-level vertical shear generate leading edge convection that tilts either upshear, vertically, or downshear depending on the relative strengths of the cold pool and low-level shear

2-D Evolution

Upshear Downshear

x

B

t

z

u


Initial Development of Ascending FTR flow:

A. Initial updraft tilts downshear due to the shear in the ambient flow (no cold pool)

B. As the convection produces precipitation, a surface cold pool is generated The horizontal vorticity associated with the cold pool begins to balance the low-level shear in the environment Therefore, the updraft becomes upright and the convective cells are quite strong

C. As the cold pool gets stronger, it’s horizontal vorticity becomes larger than that in the low-level environmental shear Therefore, the updraft tilts upshear, creating the front-to-rear flow

2-D Evolution

Upshear Downshear

A B C


Development of Descending RTF Flow:

D. Since the updraft is associated with warm, positively buoyant air and the cold pool with negatively buoyant air, the mid-level meso-low is created beneath the warm ascending front-to-rear updraft and the meso-high is created within the cold pool

E. The meso-low creates a horizontal pressure gradient that accelerates air at mid-levels from the rear of the system toward the leading edge

This air flow is often called the Rear-Inflow Jet (RIJ)

2-D Evolution

Upshear Downshear

D

E



• If the updraft contains very warm, positively buoyant air, the meso-low will be very strong and generate a very strong rear-inflow jet

• Will occur when CAPE is large (> 2000 J/kg) and/or the Lifted Index is large

• The rear-inflow jet will, therefore, be weaker when CAPE and/or the Lifted Index are smaller

2-D Evolution



• If the buoyancy gradient associated with the warm air in the ascending FTR flow is less than the buoyancy gradient on the back edge of the cold pool, then the RIJ will descend to the ground well behind the leading edge of the system

• Often occurs for weak-moderate environmental shear• The RIJ enhances the surface gust front

• When the buoyancy gradient associated with the warm air in the ascending FTR flow is similar in magnitude with the gradient on the back edge of the cold pool, the RIJ will tend to remain elevated and descend only when it reaches the leading edge

• Often occurs for strong environmental shear• The RIJ helps keep convection more upright• Often associated with strong bow echo formation

2-D Evolution


2-D EvolutionAnimation:


3-D EvolutionTransition to 3-D Structure:

• Later in the evolution of a squall line, it often evolves into a non-linear, 3-D structure.• Numerical simulations have shown that when squall lines have finite length (as they all do), larger-scale circulations form on the ends of the squall line…

Book-End Vortices

• Range in size from 10 – 200 km in scale• Located at mid-levels within the stratiform region behind the leading edge• Also called “line-end” vortices


3-D EvolutionDevelopment of Book-End Vortices:

Mechanism #1

• Recall how mid-level rotation was produced in supercell storms by an updraft tilting the horizontal vorticity associated with the environmental vertical shear…

• Same process, but for a mesoscale downdraft

• The RIJ is often descending• Thus, the descent helps to generate opposite circulations at each end of the squall line


3-D EvolutionDevelopment of Book-End Vortices:

Mechanism #2

• Recall how the cold pool generates horizontal vorticity along its leading edge due to the strong horizontal buoyancy gradient…

• The ascending FTR inflow can tilt the horizontal vorticity into the vertical, and generate opposite circulations at each end of the squall line


3-D EvolutionEvolution of Book-End Vortices and the RIJ:

• Once book-end vortices develop, their circulation can, in turn, enhance the RIJ by 30-50%• A positive feedback loop develops whereby the RIJ helps generate the line-end vortices which then enhance the RIJ, allowing the RIJ to intensify the vortices...• This feedback loop is believed to produce bow echoes

• With time, planetary vorticity (i.e. Coriolis force) enhances the northern (+) vortex and weakens the southern (-) vortex• This creates an asymmetric structure, which is often observed


Observed Case:

3-D Evolution

Example of a Squall Line with a Line-end Vortex Observed by WSR-88D Radar

From Atkins et al. (2004)


Bow EchoesDefinition and Basic Characteristics:

• A bow-shaped line of convective cells that is often associated with multiple downbursts, swaths of damaging straight line winds (or “derechos”), and weak tornadoes

• Key structural features include an intense rear inflow jet impinging on the core of the bow, with book-end (or line-end) vortices on both sides of the rear-inflow jet, behind the ends of the bowed convective segment

• Bow echoes have been observed with scales between 20 and 200 km, and often have lifetimes between 3 and 6 hours

• At early stages in their evolution, both cyclonic and anticyclonic book-end vortices tend to be of similar strength, but later in the evolution, the northern cyclonic vortex often dominates, giving the convective system a comma-shaped appearance

RIJ


Bow EchoesConceptual Model of Evolution:

Downburts andWind DamageAt Bow Apex

Vortices atLine Ends

RIJ

Adapted from Fuijta (1978)


Bow EchoesObserved Case:

• Dual-Doppler radar observations of a bow echo from the recent Bow Echo and MCV Experiment (BAMEX)

From Davis et al. (2004)


Bow EchoesObserved Case:

• Notice how the dual-Doppler analysis nicely captures the:

• Rear-Inflow Jet (RIJ)• Strong leading-edge updraft• Evidence of strong-downdraft near the surface

From Davis et al. (2004)


DerechosDefinition and Development:

Derecho – A widespread convectively induced straight-line wind storm. Specifically, a family of downburst clusters that produce surface wind gusts greater than 26 m/s over a concentrated area of at least 400 km2.

• Strong RIJ converges with FTR flow at mid-levels• RIJ is forced to descend and is further enhanced by evaporational cooling and water loading• Produces a family of downbursts at the surface



Example of Extensive Damage:


Derechos

Produced $171 million in property andcrop damage across Iowa and Illinois


TornadoesCommon Locations:

A. At the bow apexB. South of apex along gust frontC. Within the comma head, behind the the leading edge convection

Basic Characteristics:

• Generally weak (EF0-EF2)• Very hard to detect (rarely exhibit TVS)• Lifetime of 5-10 minutes

Development:

• Not well understood!

• Believed to result from stretching localized regions of vertical vorticity that develop along the gust front (i.e. the non-supercell mechanism)

• Can also occur at the intersection point between a squall line (or bow echo) and a pre-existing boundary (a front)

A

B

C


Tornadoes

Gust Front

MescocycloneCouplets

Example of Gust Front Mesovortices in WSR-88D Data


Tornadoes



ForecastingEnvironmental Factors:

• Weisman (1993) • Series of numerical simulations

• Conditions favorable for squall line, bow echo, and strong rear inflow jet development include:

Large CAPE (> 2000 J/kg) Strong low-level shear (> 20 m/s below 3 km)

Dry mid-levels Moist low-levels Linear forcing for the initial ascent up to the LFC

Squall-Line Organization Rear-Inflow Jet Magnitude


ForecastingSquall Line Motion:

• Individual cells within the squall tend to move in the direction of the 0-6 km mean wind• The overall propagation of the squall line tends to be controlled by the speed and direction of the system cold pool → new cells are constantly triggered along its leading edge • Cold pool speeds is can be on order of ~20 m/s (~40 kts)

Simple Guidance: Squall lines move at 40% of the 500mb wind speed, in the same direction


Onset of Downbursts and Derechos:

• Examine the Doppler radial velocities and look for evidence of a Mid-Altitude Radial Convergence (MARC) zone near the apex of “bowing” squall lines segments

• Provide small lead-time forecast for the onset of and downbursts and damaging straight-line winds

Forecasting


Squall Lines

Summary

Definitions

• Mesoscale Convective System• Squall Line

Environmental Characteristics


• Three General Types (structure, basic flow patterns)• Classic 2-D Structure (basic flow patterns)

2-D Evolution (physical processes)

3-D Evolution (physical processes)

Bow Echoes (definition, structure, physical processes)

Forecasting


ReferencesAtkins, N.T., J.M. Arnott, R.W. Przybylinski, R.A. Wolf, and B.D. Ketcham, 2004: Vortex structure and evolution within bow

echoes. Part I: Single-Doppler and damage analysis of the 29 June 1998 derecho. Mon. Wea. Rev., 132, 2224-2242.

Atkins, N.T., C.S. Bouchard, R.W. Przybylinski, R.J. Trapp, and G. Schmocker, 2005: Damaging surface wind mechanisms within the 10 June 2003 Saint Louis bow echo during BAMEX. Mon. Wea. Rev., 133, 2275-2296.

Bluestein, H. B., and M. H. Jain, 1985: Formation of mesoscale lines of precipitation: Severe squall lines in Oklahoma during spring. J. Atmos. Sci., 42, 1711-1732.

Davis, C. A., and Coauthors, 2004: the Bow Echo and MCV Experiment: Observations and opportunities. Bull. Amer. Meteor. Soc., 85, 1075-1093.

Fovell, R. G., and Y. Ogura, 1988: Numerical simulation of a mid-latitude squall line in two dimensions. J. Atmos. Sci., 45, 3846-3879.

Fujita, T. T., 1978: Manual of Downburst identification for Project NIMROD. Satellite and Mesometeorology Research Paper No. 156, Department of Geophysical Sciences, University of Chicago, 104 pp.

Houze, R. A. Jr., 1993: Cloud Dynamics, Academic Press, New York, 573 pp.

Houze, R. A., Jr., M. I. Biggerstaff, S. A. Rutledge, and B. F. Smull, 1989: Interpretation of Doppler weather radar displays of mid-latitude mesoscale convective systems. Bull. Amer. Meteor. Soc., 70, 608–619

Houze, R. A., Jr., B. F. Smull, and P. Dodge, 1990: Mesoscale organization of springtime rainstorms in Oklahoma. Mon. Wea. Rev., 118, 613-654.


ReferencesJohns, R. H., and W. D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Wea. Forecasting, 2, 32-49.

Parker, M. D., and R. H. Johnson, 2000: Organizational modes of mid-latitude mesoscale convective systems. Mon. Wea. Rev., 128, 3413-3436.

Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory for strong long-lived squall lines. J. Atmos. Sci., 45, 463- 485.

Wakimoto, R.M., H.V. Murphy, A. Nester, D.P. Jorgensen, and N.T. Atkins, 2006: High winds generated by bow echoes. Part I: Overview of the Omaha bow echo 5 July 2003 storm during BAMEX. Mon. Wea. Rev., 134, 2793-2812.

Wakimoto, R.M., H.V. Murphy, C.A. Davis, and N.T. Atkins, 2006: High winds generated by bow echoes. Part II: The relationship between the mesovortices and damaging straight-line winds. Mon. Wea. Rev., 134, 2813-2829.

Wheatley, D.M., R.J. Trapp, and N.T. Atkins, 2006: Radar and damage analysis of severe bow echoes observed during BAMEX. Mon. Wea. Rev., 134, 791-806.

Weisman, M. L., 1992: The role of convectively generated rear-inflow jets int eh evolution of long-lived meso-convective systems. J. Atmos. Sci., 49, 1827-1847.

Weisman, M. L., 1993: The genesis of severe long–lived bow echoes. J. Atmos. Sci., 50, 645-669.

Weisman, M. L. , and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms. Mesoscale Meteorology and Forecasting, Ed: Peter S. Ray, American Meteorological Society, Boston, 331-358.

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