Bow Echoes

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Bow Echoes

• bow echoA bow-shaped line of convective cells
  that is often associated with swaths of damaging
  straight-line winds and small 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 h.
• 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.
• From the Glossary of Meteorology online

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Conceptual Model of Bow Echo Structural Evolution
Figure from Fujita (1978)
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Conceptual model of bow echo evolution

• A line of storms initially form
• RIJ and book-end vortices develop 3-4 hours after
  initiation.
• Planetary vertical vorticity enhanced the
  northern BEV, weakens the southern one produces
  asymmetric structure by 5-6 hrs.
• Notice the two primary flows
• Upshear tilted front to rear flow
• The RIJ

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• Example of Single-Doppler Radar Observations of a
  bow echo
• Notice
• Well-defined bowing segment in dBZ field,
• asymmetric structure of stratiform precip.
• Well-defined RIJ
• Multi cellular structure
• Well-defined northern BEV

From Atkins et al. (2004 - MWR)
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Dual-Doppler Radar observations of bow echoes
only very recently from the Bow Echo and MCV
Experiment (BAMEX)
From Davis et al. (2004) BAMS
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• Notice that the dual-Doppler analysis appears to
  capture the RIJ
• Strong updraft at leading edge of system

From Davis et al. (2004) BAMS
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Derechos

• derechoA widespread convectively induced
  straight-line windstorm. Specifically, the term
  is defined as any family of downburst clusters
  produced by an extratropical mesoscale convective
  system. Derechos may or may not be accompanied by
  tornadoes. Such events were first recognized in
  the Corn Belt region of the United States, but
  have since been observed in many other areas of
  the midlatitudes.           Johns, R. H., and W.
  O. Hirt, 1987 Derechos Widespread convectively
  induced windstorms. Wea. Forecasting, 2, 3249.
  (from Glossary of Meteorology online)
• There are two types of derechos
• Serial often a line of small bow echoes that
  form in the warm sector of a strongly forced mid
  latitude wave
• Often the small-scale bow echoes form a line echo
  wave pattern (LEWP)

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Derechos

• Progressive often a single bow echo system that
  propagates along a stationary boundary more
  common during the warm season (see Johns and
  Hirt, 87 WAF)

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Derechos
The event below produced 151 m in property
damage and 20 m in crop damage in IA and IL.
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Derecho Climatology
Figures from Ashley and Mote 2005
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Bow echo evolution physical processes

• Initially, the updrafts within the convective
  line tilt upshear due to the strong low-mid level
  environmental shear (c/Du lt 1).
• As the convective storms produce a cold pool, the
  environmental shear at low levels is balanced by
  the baroclinic horizontal vorticity within the
  cold pool c/Du 1
• The convection is quite strong at this point
• As the cold pool gets stronger, the baroclinic
  horizontal vorticity within the cold pool is
  stronger than that in the environmental shear
  (c/Du gt 1)
• The updraft then tilts upshear. The places warm
  buoyant air over cold air in the cold pool.
  Lower pressure is generated under the warm
  buoyant air.
• A horizontal pressure gradient is then produced,
  creating the RIJ

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RIJ Formation
Low-level shear vector
Updraft initially tilts down shear
As cold pool develops, updraft is upright as cold
pool (C) balances environmental shear (Du)
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RIJ Formation
From the vertical momentum equation

• As the cold pool continues to strengthen, the
  updraft tilts upshear and over the cold pool
  since C gt Du
• generates an area of low pressure beneath it at
  mid levels

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Bow echo evolution physical processes

• The stronger RIJs tend to form in environments
  where there is a lot of CAPE and strong low-level
  wind shear.
• In these environments, the RIJ tends to remain
  elevated until it reaches the leading edge of the
  convective system.
• When the CAPE and low-level shear are weaker, the
  RIJ will descend to the ground further to the
  rear of the convective system.

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Bow echo environments

• Based on the figure to the right, what
  environments produce bow echoes?
• This is based entirely on idealized modeling
  studies.

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Bow echo environments

• Here are some observational results for
  derecho-producing environments.

From Evans and Doswell (2001 WAF)
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Bow echo Damage
Both figures from Fujita (78)
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Bow Echo during BAMEX on 9 June 2003
Southeastern NE An example of RIJ Producing
straight-line wind damage swath
-5 5 15 25 dBZ 35 45
55 -30 -20
-10 0 m/s 10 20 30
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Bow echo Damage
from Fujita (78)
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Bow echo Damage
from Fujita (78)
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Bow Echo Tornadoes

• Within BEV
• Pfost and Gerard 97
• Howieson and Tipton 88

• North/At Bow Apex
• Burgess and Smull 90
• Przybylinski 95
• Przybylinski et al. 96
• Spoden et al. 98
• Dewald et al. 98
• Funk et al. 99
• Wolf 02
• Atkins et al. 04, 05

T
T

• South of Bow Apex
• Atkins et al. 04

T
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Bow Echo Tornadoes, cont.

• Tornadoes often form at intersection point of bow
  echo/ squall line and preexisting boundary
• Bow echo tornado genesis mechanism(s) not well
  understood!!!

From Przybylinski et al. (2000)
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Trapp et al. 2005
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• BAMEX
• Based near St. Louis, MO from 20 May 6 July,
  2003
• Objectives
• Bow Echoes Understand and improve prediction of
  the mesoscale and cell-scale processes that
  produce severe winds
• MCVs understand MCV formation within MCSs, the
  role of MCVs in initiating and modulating
  convection onto MCV intensity, and to improve the
  overall predictability of the vortex-convection
  coupled system

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10 June 2003 Bow Echo Event During BAMEX
From Atkins et al. (2005)
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Mesovortices and Bow Echo Tornadoes

• Mesovortices
• Form at the leading edge of the convective system
  on the large reflectivity gradient
• Observed at low levels (0-3 km AGL)
• Lifetime of an hour or less
• 1-20 in horizontal scale (meso-g) often hard to
  detect!!
• Often tornadic (commonly F0-F2, though up to F4
  has been documented
• Genesis mechanism(s) is(are) still under
  investigation

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EXAMPLE OF MESOVORTICES IN 88-D DATA
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• Just recently, its been shown that mesovortices
  within bow echoes are also capable of producing
  long, narrow swaths of straight-line wind damage.
• This damage has been shown to be located NORTH of
  the descending RIJ and not collocated with it.

From Atkins et al. (2005)
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From Atkins et al. (2005)
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Mesovortex Genesis
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Mesovortex Genesis

• Wakimoto et al. 2006b
• Airborne Doppler radar observations
• Mechanically-forced downdrafts tilt horizontal
  crosswise vorticity
• Wheatley and Trapp 2008
• Numerical simulation of a cool season event
• Shearing Instability

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• Quasi-Idealized WRF Simulation of the 10 June
  Saint Louis Bow Echo Event
• Research Objective
• Examine the mesovortex genesis mechanism(s)
• Experimental Design (Quasi-idealized simulation)
• Sounding 18 UTC Springfield, MO, 2003
• CAPE 2558 J/kg
• Wind vector difference of 18 m/s over lowest 2.5
  km (moderate low-level shear)
• Domain Dx Dy 500 m, Dz varies from 160-600m)
• Lin ice microphysics
• Three thermal bubbles oriented N-S initiated
  convection
• Boundaries open N-S, W-E, free slip surface
• Results to be shown are from Atkins and St.
  Laurent 2009b, MWR

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Cyclonic Mesovortices Anticyclonic Mesovortices
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Following Rotunno and Klemp (85) Using Bjerknes
First Circulation Theorem, the change in
circulation around a material surface is given by
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Mesovortex Genesis Cyclonic Only
Inflow Downdraft/updraft Vortex lines mesovortex
Mechanism is similar to low-level
mesocyclogenesis within supercells discussed by
Rotunno, Klemp, Davies-Jones and others
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Cyclonic Mesovortices Anticyclonic Mesovortices
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All fields at Z 0.2 km Streamlines U (ms-1) W
(ms-1) Qr (g kg-1)
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Mesovortex Genesis - Couplets

• Same mechanism used to explain the genesis of
• Bow echo line-end vortices (Weisman and Davis,
  1989)
• Low-level mesocyclones in supercells (Markowski
  et al. 2008)

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Cyclonic Mesovortices Anticyclonic Mesovortices
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• Summary
• Genesis of mesovortices
• Cellular Stage
• Cyclonic only tilting of baroclinically
  generated streamwise vorticity acquired by
  convective-scale downdraft parcels

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• Summary
• Genesis of mesovortices
• Early Bow Echo Stage
• Couplets - convective-scale downdraft creates
  bulge in gust front
• vortex couplet forms as updraft tilts
  baroclinically generated vortex lines.

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• Summary
• Genesis of mesovortices
• Mature Bow Echo Stage
• Cyclonic only Same as Cellular Stage except
  that downdraft parcels are descending within the
  rear inflow

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Mesovortex Damaging Potential

• Damage swaths will be found on southern flank of
  mesovortex formed near descending RIJ
• Due to linear combination of mesovortex and RIJ
  flows

RIJ mesovortex
From Atkins and St. Laurent (2009a, MWR)
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dBZ 0.5 degrees
KLSX Vr
From Atkins et al. (2005)
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Are Nocturnal Bow Echoes Less Damaging?
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From Jorgensen et al. 05

• Non damaging bow echo at time of aircraft
  observation!
• 25 ms-1 RIJ!
• Time of observation 0040 am

Low level stable layer sounding launched 95 km
SSW and 1.6 hrs prior to radar observations
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KENX data on 27 July 2008, 0601 UTC Bow Echo
over western MA there were NO damage reports
with this event dBZ
VE
80 knot RIJ
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RUC Sounding at 0500 UTC western MA
Did this low-level stable layer contribute to the
lack of near-surface damaging winds?
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Do low-level stable layers inhibit damaging
surface winds in bow echoes?
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Idealized Bow Echo Simulations with WRF

• Single sounding initialization 2400 J kg-1, U
  increases by 20 ms-1 over lowest 2.5 km i.e.,
  large CAPE and strong low-level shear)
• Domain 350 x 370 x 17.5 km Dx Dy 750 m,
  Dz160-700m
• Four thermal bubbles every 40 km in the north
  south direction initiated convection
• Six hour simulations
• Open lateral boundary conditions, free slip
  surface, rigid top with Rayleigh damping layer
• Kessler microphysics
• Turbulence 1.5 TKE closure
• f10-4 s-1

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Potential Temperature
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The RIJ does not weaken significantly when a
stable layer is present
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Low-level stable layer reduces number and
strength of mesovortices How?
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Mesovortex Genesis

• Not well understood
• State of our understanding is from recent
  modeling results

From Trapp and Weisman (2003) and Weisman and
Trapp (2003)
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Mesovortex Genesis Updrafts tilt horizontal
streamwise vorticity
downdraft
Horizontal vorticity
cold
warm
From Atkins and St. Laurent (2009b, MWR)
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How do low-level stable layers weaken
mesovortices?
downdraft
Horizontal vorticity
cold
warm
From Atkins and St. Laurent (2009b, MWR)
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Strong stable layers will inhibit mesovortex
formation
downdraft
Horizontal vorticity
cold
warm
From Atkins and St. Laurent (2009b, MWR)
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T 4.5 hours, Z 0.2 km Buoyancy
(ms-2) Horizontal vorticity vectors Vertical
vorticity
mesovortex
Control Run
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Control Run
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T 4.5 hours, Z 0.2 km Buoyancy
(ms-2) Horizontal vorticity vectors Vertical
vorticity
505 m deep Stable layer Dq 7K
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505 m deep Stable layer Dq 7K
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This is also possible
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• Conclusions
• Stable layers decrease surface winds within bow
  echoes
• Stronger and deeper stable layers are more
  effective
• RIJ strength decreases only a little, can not
  explain the large decrease in near-surface winds
• Mesovortices progressively become less numerous
  and weaker as stable layer becomes stronger
• Their source of vorticity is weakened in the
  presence of a stable layer
• Some nocturnal bow echoes may not pose as
  significant of a damaging threat as they would
  during daytime hours.
• Acknowledgments Research results supported by
  NSF under grant ATM-0630445 and Vermont EPSCoR
  EPS-0236976.