Squall Lines
Mesoscale
M. D. Eastin
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
M. D. Eastin
Definitions
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)
Squall Line over Missouri
Mesoscale
Bow Echoes / LEWP over Indiana
Developing MCC over Nebraska
M. D. Eastin
Definitions
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
Mesoscale
M. D. Eastin
Environment
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
Mesoscale
M. D. Eastin
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
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Three General Mature Structures:
From Parker and
Johnson (2000)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform (TS) Squall Lines:
• Strong convection along leading edge with stratiform
precipitation trailing behind the line
Stratiform
Precipitation
• 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
Along
Line
• Strong low and mid-level cross-line flow
• Moderate upper-level along-line flow
Line
Motion
Cross Line
Along Line
Flow (m/s)
Cross Line
Flow (m/s)
From Parker and
Johnson (2000)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Leading Stratiform (LS) Squall Lines:
Line
Motion
• Strong convection along trailing edge with stratiform
precipitation leading the line
• Account for ~15% of all squall lines
• Average values:
Stratiform
Precipitation
Duration = 6.5 hrs
Line Motion = 7.1 m/s
CAPE = 1009 J/kg
LI = -3.5 K
Along
Line
Cross
Line
• Moderate low and upper-level flow (cross and along line)
• Weak mid-level flow
Along Line
Flow (m/s)
Cross Line
Flow (m/s)
From Parker and
Johnson (2000)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Parallel Stratiform (PS) Squall Lines:
Stratiform
Precipitation
• Strong convection along the up-wind segment with
stratiform precipitation located downwind
Along
Line
• Account for ~15% of all squall lines
• Mean values:
Duration = 6.3 hrs
Speed = 11.4 m/s
CAPE = 813 J/kg
LI = -2.2 K
Line
Motion
Cross
Line
• Strong low-level cross-line flow
• Strong mid and upper-level along-line flow
Along Line
Flow (m/s)
Cross Line
Flow (m/s)
From Parker and
Johnson (2000)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
• Numerous observational studies have identified common structural characteristics
Adapted from Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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
From Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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
From Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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
Warm
Cold
Warm
Cold
Cold
Cold
From Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Model
Trailing Stratiform Squall Line Structure:
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
Warm
Warm
Warm
Cold
Cold
Cold
Cold
From Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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
Warm
Cold
Warm
Cold
Cold
Cold
From Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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
Warm
Warm
Warm
Warm
Warm
Cold
Cold
Cold
Cold
From Houze et al. (1989)
Mesoscale
M. D. Eastin
Structure and Conceptual Models
Trailing Stratiform Squall Line Structure:
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
Mesoscale
M. D. Eastin
Structure and Conceptual Models
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
Mesoscale
M. D. Eastin
2-D Evolution
Important Physical Processes:
Buoyancy
• Buoyancy (or temperature) gradients produce
local circulations, mesoscale pressure anomalies,
and air flow accelerations

B
 
t
x
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
 
Mesoscale
Upshear
Downshear
u
z
M. D. Eastin
2-D Evolution
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
A
B
Upshear
Mesoscale
C
Downshear
M. D. Eastin
2-D Evolution
Development of Descending RTF Flow:
D
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
Upshear
Downshear
E
This air flow is often called the Rear-Inflow Jet (RIJ)
Mesoscale
M. D. Eastin
2-D Evolution
Development of Descending RTF Flow:
• 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
Mesoscale
M. D. Eastin
2-D Evolution
Development of Descending RTF Flow:
• 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
Mesoscale
M. D. Eastin
2-D Evolution
Animation:
Mesoscale
M. D. Eastin
3-D Evolution
Transition 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
Mesoscale
M. D. Eastin
3-D Evolution
Development 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
Mesoscale
M. D. Eastin
3-D Evolution
Development 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
Mesoscale
M. D. Eastin
3-D Evolution
Evolution 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
Mesoscale
M. D. Eastin
3-D Evolution
Observed Case:
Example of a Squall Line with a Line-end Vortex Observed by WSR-88D Radar
From Atkins et al. (2004)
Mesoscale
M. D. Eastin
Bow Echoes
Definition 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
RIJ
• 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
Mesoscale
M. D. Eastin
Bow Echoes
Conceptual Model of Evolution:
RIJ
Downburts and
Wind Damage
At Bow Apex
Vortices at
Line Ends
Adapted from Fuijta (1978)
Mesoscale
M. D. Eastin
Bow Echoes
Observed Case:
• Dual-Doppler radar observations of a
bow echo from the recent Bow Echo
and MCV Experiment (BAMEX)
From Davis et al. (2004)
Mesoscale
M. D. Eastin
Bow Echoes
Observed 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)
Mesoscale
M. D. Eastin
Derechos
Definition 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
From Atkins et al. (2005)
Mesoscale
M. D. Eastin
Derechos
Example of Extensive Damage:
Produced $171 million in property and
crop damage across Iowa and Illinois
From Atkins et al. (2005)
Mesoscale
M. D. Eastin
Tornadoes
Common Locations:
A. At the bow apex
B. South of apex along gust front
C. 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)
Mesoscale
C
A
B
M. D. Eastin
Tornadoes
Example of Gust Front Mesovortices in WSR-88D Data
Mescocyclone
Couplets
Gust
Front
Mesoscale
M. D. Eastin
Tornadoes
From Atkins et al. (2005)
Mesoscale
M. D. Eastin
Forecasting
Environmental Factors:
• Weisman (1993)
• Series of numerical
simulations
Squall-Line Organization
Rear-Inflow Jet Magnitude
• 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
Mesoscale
M. D. Eastin
Forecasting
Squall 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
Mesoscale
M. D. Eastin
Forecasting
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
Mesoscale
M. D. Eastin
Squall Lines
Summary
Definitions
• Mesoscale Convective System
• Squall Line
Environmental Characteristics
Structure and Conceptual Model
• 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
Mesoscale
M. D. Eastin
References
Atkins, 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.
Mesoscale
M. D. Eastin
References
Johns, 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.
Mesoscale
M. D. Eastin