Bow Echoes
By Matthieu Desorcy
What is a Bow Echo?
• Bowing pattern on radar
• Severe straight-line winds at the surface
o Cause about 1/3 severe wind reports in US
• Have a specific structure with
rear-inflow jet reaching the surface
• Originate from various storm modes
• Typically need >2,000J/kg CAPE
• Move quickly and accelerate
http://www.crh.noaa.gov/dmx/?n=preparesvrintro
Structure
Note:
• Strong rear inflow
jet reaching the
surface
• Mid-level low
pressure
• Surface high
pressure (by cold
pool)
• Most lift near
leading edge of
cold pool =
strongest updrafts
L
H
H
Cross section taken from case-study of June 10, 2003 bow
echo during near NE/IA border during BAMEX (Bow Echo and
Mesoscale Convective Vortex Experiment)
Initial Stages and Types
• BE = classic bow echo
• BEC = bow echo complex
(bow echo with other storm
type embedded, ie.
supercell)
• CBE = cell bow echo (small
scale bow echo)
• SLBE = squall line bow echo,
also known as line echo wave
pattern. Part of elongated
linear convective system.
Klimowski et al. 2004
Types (cont.)
Klimowski et al. 2004
Kilmowski et al. Data and
Summary
• Used 273 scenarios between 1996-2003 in US
• More squall line bow echoes as you head further east
• Thunderstorm mergers likely to play a role in BE development consolidating cold pools, but need more data
• Also suggests that when initial cell that merges with another, the
speed and direction is maintained by BE
• Average life span higher further south in the plains
• CAPE and LLJ could be the reasoning for this – easier to get
widespread high value CAPEs in the region and a strong lowlevel jet
Mesovortices
• Play a role in the production of straight-line winds
and even tornadoes in bow echo events
o Straight-line winds enhanced on south side of mesovortex because
horizontal pressure gradient
• Most often observed near the apex of the bow
• Depends on the low level wind shear and strength
(temperature contrast) of cold pool
• Few to tens of kilometres in size
• Observed while rear inflow jets are strengthening
• Causes still under research
Mesovortices (cont.)
Note:
• The location of the
mesovortice, just
north of the apex
of the bow
• The damaging part
of the mesovortex
(strongest straightline winds) is on the
south side as it
wraps around the
descending rear
inflow jet
Nolan Atkins & St. Laurent 2009
Mesovortice Theories
Theory 1:
o Horizontal vorticity from downdraft gets tilted, stretched by
updraft along gust front creating the vortice
Theory 2:
o Along the apex of bow echo stronger updrafts are produced
(by either stronger downdrafts prior, or rear inflow jet)
o Horizontal vorticity just behind gust front is tilted by the stronger
updraft, which produces a cyclonic vortice just north of apex
and anticyclonic one south of apex
• Anticyclonic one not observed as much
Mesovortice Theory 2
Nolan Atkins & St. Laurent 2009
Other Bow Echo
Characteristics
• Can have bookend vortices during mature stage –
also caused by tilting of horizontal vorticity by
updraft
o Larger in scale than mesovortices
• Bow echoes can become derechos if they have
path >400km and meet wind criteria
• Shelf clouds will typically be visible along gust front
of bow echo
http://en.wikipedia.org/wiki/Bow_echo#mediaviewer/File:Bow_ec
ho_diagram.svg
Bow Echo Summary
• Produce strong winds, sometimes tornadoes
• Rear inflow jet creates the strong winds
• Bow echoes transition from other types of storms,
some cases include cold pool consolidation
• Can acquire characteristics that enhance tornado
threat and winds locally (mesovortices)
Questions?
References:
•
Atkins, N. T & St. Laurent, M. (2009). Bow Echo Mesovortices. Part I:
Processes That Influence Their Damaging Potential. Monthly Weather
Review, 137, 1498-1513. doi:10.1175/2008MWR2649.1
•
Atkins, N.T & St. Laurent, M. (2009, May). Bow Echo Mesovortices. Part
II: Their Genesis. Monthly Weather Review, 137, 1514-1532.
doi:10.1175/2008MWR2650.1
•
Davis, C et al. (2014, August). The Bow Echo and MCV Experiment.
BAMS, 1075-1093. doi:10.1175/BAMS-85-8-1075
•
Klimowski, B. A et al. (2004, February). Radar Observations of the Early
Evolution of Bow Echoes. Weather and Forecasting (AMS), 19, 727734.