Mesoscale Convective Systems 2
Weather Systems – Fall 2015
Outline:
Bow Echoes, Derechos and Mesoscale Convective Vortices
Previously …
 We looked at the effects that cold pools and vertical wind
shear can have on the tilt, intensity, and structure of
squall lines
 We also examined the various organizational structures
of observed MCSs
 Today:
 Bow echoes
 Mesoscale convective vortices
Convection as a function of shear and CAPE
Bulk Richardson No.
Rasmussen and Wilhelmson, 1983
From last time: rear-inflow jet
 Real squall lines are not simply 2D; and the cold pool and rear
inflow jet tend to be strongest near the center of the line
 This can cause the line to “bow”, creating what’s called a “bow
echo”
Bow echoes
Wakimoto et al. 2006
Adapted from Fujita (1978)
Bow echoes
 Bow echoes are often
associated with severe winds at
the surface
 This can happen both from the
descending RIJ, as well as from
smaller-scale mesovortices
 They typically have “bookend
vortices” – a cyclonic vortex on
the north end of the bow, and
anticyclonic on the south end
Bookend vortices
Klemp (1987)
 Updraft tilts ambient horizontal vorticity into the vertical and
stretches it; downdrafts tilt those vortex lines downward, creating the
bookend vortices
 The northern one tends to become dominant, because of Coriolis
effects
3 June 2003 Omaha Bow Echo
Observed during BAMEX field program
Produced damaging winds exceeding F1 intensity
Wakimoto et al. 2006
3 June 2003 Omaha Bow Echo
Extensive region of large CAPE
Significant warm moist advection
Presence of an upper-level shortwave
Wakimoto et al. 2006
Smaller-scale “mesovortices”
Red & purple = damage path
Black dashed = vorticity
Wakimoto et al. (2006)
8 May 2009
Radar reflectivity
(Springfield, MO, 1235 UTC)
Storm relative velocity
Derecho
 Sometimes a bow echo (or series
of bow echo) causes a derecho:
a widespread severe windstorm
 Although a derecho can produce
destruction similar to that of
tornadoes, the damage typically
is directed in one direction along
a relatively straight swath
 By definition, if the wind damage
swath extends more than 240
miles (about 400 kilometers) and
includes wind gusts of at least 58
mph (93 km/h) or greater along
most of its length, then the event
may be classified as a derecho
June 29, 2012
Source: http://earthsky.org/earth/videos-and-imagesviolent-us-storm-of-june-29-2012
Derecho
8 May 2009 “superderecho”:
http://www.spc.noaa.gov/misc/AbtDerechos/casepages/may8200
9page.htm
Derecho
29 June 2012: Radar
and Images
http://vielmetti.typepa
d.com/vacuum/2012/0
6/derecho-of-june-292012.html
Sterling, VA 00 UTC 30 June sounding
Mesoscale convective vortices (MCVs)
Sometimes, the northern bookend vortex is observed to
grow into a mesoscale convective vortex (MCV), which
lasts long after the parent MCS decays
Source: http://lukemweather.blogspot.com/2011/07/mesoscale-convective-vortex-mcv.html
PV thinking for MCSs
 We can consider MCSs in terms of potential vorticity
 However, for mesoscale motions, the traditional
balance constraints (QG, SG, etc.) don’t apply
 Need to use different balance condition: often nonlinear balance is used
 Otherwise, “PV thinking” progresses similarly…
PV
 The prognostic equation for potential vorticity q can be written
as:
 This says that if we neglect diabatic heating and friction, PV is
conserved
 Also note that the RHS is written as the divergence of a vector
PV
 If we take the volume integral between two isentropic
surfaces, and use the divergence theorem, we obtain the
following:
d
dt
òò ò rq dx dy dz = 0
V
 This states that PV between isentropic surfaces cannot
be created or destroyed (unless the isentropic surface
intersects the surface)
 We can think of the PV between isentropes as a
“substance” – the amount of PV in that volume doesn’t
change
 This is true even with diabatic and frictional effects
included!
PV
However, diabatic heating/cooling can move mass across
isentropes
θ+2Δθ
θ
Constant amount of “PV
substance”
θ-2Δθ
PV
However, diabatic heating/cooling can move mass across
isentropes
θ+2Δθ
.
θ
Constant amount of “PV
substance”
θ
θ-2Δθ
PV
However, diabatic heating/cooling can move mass across
isentropes
θ+2Δθ
More mass
W
Less mass
θ
θ-2Δθ
PV
However, diabatic heating/cooling can move mass across
isentropes
θ+2Δθ
W
Constant amount of “PV
substance”
θ
θ-2Δθ
PV
However, diabatic heating/cooling can move mass across
isentropes
θ+2Δθ
W
Constant amount of “PV
substance”
θ
θ-2Δθ
Same amount of “PV substance” between isentropes, but now
there’s less mass there – the PV is more concentrated
PV
When diabatic heating increases with height, PV will increase; when it
decreases with height, PV will decrease
θ+2Δθ
PV diluted
W
PV concentrated
θ
θ-2Δθ
PV in a mature MCS
Can approximate this effect as:
dq

 (  f )
dt
z
- PV
- PV
+ PV
+ PV
- PV
Relevance of MCVs
 Some MCVs last several days
 An MCV in shear creates lifting (and destabilization) on the
downshear side
 This lifting in some cases leads to the development of a new MCS
From Trier et al. (2000), after
Raymond and Jiang (1990)
 Consider the case of a strong low-level jet approaching the MCV: the
downshear side is the same side with the high θe (unstable) air
 Parcels are lifted above their LFC; a new MCS ensues—these
situations often lead to heavy rain/flash flood events
Fritsch et
al. (1994)
Typical MCV tracks
MCV movelment closely
approximated by the
mid-tropospheric flow
Trier et al. (2000)
Will convection develop near the MCV
on the 2nd (or 3rd) diurnal cycle?
There was less ambient
shear and more CAPE in
MCV cases that led to a
secondary MCS than in cases
where no secondary
convection occurred
Trier et al. (2000)
Precipitation episodes
 Carbone et al. (2002)
identified “episodes” of
precipitation in the US
that last several days
 They suggest that
identifying the
mechanisms
responsible can
improve prediction of
warm-season
precipitation