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Squall Lines

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Squall Lines Mesoscale M. D. Eastin * * * * * * * * * * * * * * * * * * * * * * * * * * Mesoscale M. D. Eastin Bow Echoes Observed Case: Dual-Doppler radar ... – PowerPoint PPT presentation

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Title: Squall Lines


1
Squall Lines
2
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

3
Definitions
  • Mesoscale Convective System (MCS)
  • Any ensemble of thunderstorms producing a
    contiguous precipitation area gt100 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)

Developing MCC over Nebraska
Bow Echoes / LEWP over Indiana
Squall Line over Missouri
4
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

5
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 (gt 500 J/kg), but
  • severe weather usually develops in more
  • unstable environments (gt 2500 J/kg)
  • Moderate deep-layer shear (gt 10 m/s

6
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 lt10 m/s
  • Moderate 10-18 m/s
  • Strong gt18 m/s

7
Structure and Conceptual Models
Three General Mature Structures
From Parker and Johnson (2000)
8
Structure and Conceptual Models
  • 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

Stratiform Precipitation
Line Motion
Along Line
Cross Line
Along Line Flow (m/s)
Cross Line Flow (m/s)
From Parker and Johnson (2000)
9
Structure and Conceptual Models
  • Leading Stratiform (LS) Squall Lines
  • Strong convection along trailing edge with
    stratiform
  • precipitation leading the line
  • Account for 15 of all squall lines
  • Average values Duration 6.5 hrs
  • Line Motion 7.1 m/s
  • CAPE 1009 J/kg
  • LI -3.5 K
  • Moderate low and upper-level flow (cross and
    along line)
  • Weak mid-level flow

Line Motion
Stratiform Precipitation
Along Line
Cross Line
Along Line Flow (m/s)
Cross Line Flow (m/s)
From Parker and Johnson (2000)
10
Structure and Conceptual Models
  • Parallel Stratiform (PS) Squall Lines
  • Strong convection along the up-wind segment with
  • stratiform precipitation located downwind
  • 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
  • Strong low-level cross-line flow
  • Strong mid and upper-level along-line flow

Stratiform Precipitation
Along Line
Line Motion
Cross Line
Along Line Flow (m/s)
Cross Line Flow (m/s)
From Parker and Johnson (2000)
11
Structure and Conceptual Models
  • Trailing Stratiform Squall Line Structure
  • Numerous observational studies have identified
    common structural characteristics

Adapted from Houze et al. (1989)
12
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)
13
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)
14
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)
15
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
Warm
Cold
Cold
Cold
Cold
From Houze et al. (1989)
16
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)
17
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
Warm
Cold
Cold
Cold
Cold
From Houze et al. (1989)
18
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)
19
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

20
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

21
2-D Evolution
  • 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

Upshear
Downshear
22
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, its 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
C
Upshear
Downshear
23
2-D Evolution
  • 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)

D
Upshear
Downshear
E
24
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 (gt 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

25
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

26
2-D Evolution
Animation
27
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

28
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

29
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

30
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

31
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)
32
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
  • 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

RIJ
33
Bow Echoes
Conceptual Model of Evolution
Adapted from Fuijta (1978)
34
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)
35
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)
36
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)
37
Derechos
Example of Extensive Damage
Produced 171 million in property and crop damage
across Iowa and Illinois
From Atkins et al. (2005)
38
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

C
A
B
39
Tornadoes
Example of Gust Front Mesovortices in WSR-88D Data
Mescocyclone Couplets
40
Tornadoes
From Atkins et al. (2005)
41
Forecasting
  • Environmental Factors
  • Weisman (1993)
  • Series of numerical
  • simulations
  • Conditions favorable
  • for squall line, bow
  • echo, and strong rear
  • inflow jet development
  • include
  • Large CAPE
  • (gt 2000 J/kg)
  • Strong low-level shear
  • (gt 20 m/s below 3 km)
  • Dry mid-levels

Squall-Line Organization
Rear-Inflow Jet Magnitude
42
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

43
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

44
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)

45
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, 608619 Houze, R.
A., Jr., B. F. Smull, and P. Dodge, 1990
Mesoscale organization of springtime rainstorms
in Oklahoma. Mon. Wea. Rev., 118, 613-654.
46
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 longlived 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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