Mesoscale Convective Systems 1
Weather Systems – Fall 2015
Outline:
definitions and dynamics
Definition
 Isolated T-storms are generally classified as one of three types:
ordinary cells, multi-cell, or supercells
 However, groups of storms often join into larger systems,
generally referred to as mesoscale convective systems (MCSs)
 From the AMS glossary of meteorology, an MCS produces a
contiguous precipitation area greater than 100 km in horizontal
scale in at least one direction
Definition
There are many subclassifications of MCSs, including:
(Markowski and Richardson)
Why are they important?
 In the Midwest, MCSs provide 30-70% of the warm
season rainfall – crucial for agriculture
 MCSs cause a number of weather hazards, including
damaging hail, winds, lightning, and flash flooding
 They contribute significantly to the hydrologic cycle and
the general circulation
Mesoscale convective complexes
 One type of MCS that has been studied in detail is
the mesoscale convective complex (MCC)
 MCCs are large MCSs with a circular cloud shield
 Maddox (1980) first defined MCCs – the exact
definition is somewhat arbitrary, but their
characteristics when observed by IR satellite
include:
 Cold cloud shield > 100,000 km2 in area
 Large interior cloud shield with temperature < -52°C
 Ratio of minor axis to major axis ≥ 0.7 (i.e., nearly circular)
MCC examples
Global distribution of MCCs
Laing and Fritsch (1997)
 MCCs are mostly continental
 They tend to occur in the lee of elevated terrain
 MCCs worldwide are predominantly nocturnal
Fritsch and Forbes (2001)
 MCCs in the US usually
occur when a low-level
jet transports warm,
moist (high θe) air
northward, which
destabilizes the
environment
Temp (dashed), θe (thick black), height (thin black),
wind vectors
 The high-θe air glides
upward over a baroclinic
zone, which provides the
necessary lifting
 MCCs/MCSs typically
occur under relatively
weak synoptic forcing
 Conditions leading to
MCCs in other parts of
the world are very similar
to these (Laing and
Fritsch 2000)
Laing and Fritsch (2000)
Cold pool dynamics
 Many MCSs (especially squall lines) develop in the
warm sector, without frontal lifting – what maintains
these MCSs?
 Evaporative cooling creates a pool of cold outflow at
the surface
 The wind shear affects how this cold outflow will
spread out
 A prominent (and controversial) theory that describes
the interactions between cold pools and shear was
developed by Rotunno, Klemp, and Weisman
(1988), and is known as “RKW theory”
RKW theory
 Consider a 2-D (x,z) framework, so that the x-axis is
perpendicular to the squall line
u
w
 0,
0
z
x
Horizontal vorticity of this
sense is negative:
Buoyancy and horizontal vorticity
 First, start with the simple situation of a buoyant bubble:
B
0
x
B
0
x
The horizontal vorticity equation describes both the upward motion in
the center and the downward motion on the sides of the updraft
Cold pool
 Now, consider a pool of cold air at the surface
 It’s possible to describe all of these motions using the hydrostatic
and momentum equations
 But the vorticity equation nicely captures the flow using just one
equation
B
0
x
B
0
x
Lifting occurs on the edges of the
spreading cold pool, with return flow
and sinking motion in the middle
MCS-like structure
 Now we add a warm pool aloft
B
0
x
B
0
x
B
0
x
B
0
x
There is convergence at midlevels,
above the cold pool and below the
warm pool
Add a sprinkle of shear…
 Recall that vertical wind shear is associated with horizontal
vorticity of its own
+
RKW theory
 RKW theory: there exists an optimal state where the
(positive) horizontal vorticity from the shear exactly
balances the (negative) horizontal vorticity from the cold
pool
c
1
u
Where c is the strength of the cold pool and Δu is the shear over the depth
of the cold pool
 In this situation, the strength and longevity of the squall
line will be maximized
c  u
c  u
c  u
No shear
 In the case with no shear, updraft is initially upright
 When the cold pool forms, its vorticity dominates and
the updraft tilts over
Strong low-level shear
 With strong low-level shear, updraft initially leans
downshear
 When the cold pool forms, its vorticity balances
the ambient vorticity and the system is upright
(and can be long-lived)
Strong low-level shear
 In time, as more downdrafts contribute to the strength of
the cold pool, it becomes stronger and starts to
overwhelm the shear – the line tilts back over the cold
pool
Rear-inflow jet
 At this mature stage, the latent heating owing to condensation
leads to the warm-pool-above-cold-pool structure we saw
before: there is low pressure and convergence at midlevels
 This leads to the commonly observed rear-inflow jet
Rear-inflow jet
 The magnitude of the
midlevel warming
affects the strength of
the rear inflow
 If the warming aloft is
relatively weak, the RIJ
descends and causes
the line to tilt over
further
 This tends to weaken the
system, but the
descending RIJ can
produce severe winds at
the surface
 If the warming aloft is
strong, the RIJ remains
elevated, the line
remains upright, and the
system
strengthens/persists
Summary
 The essence of RKW theory is that there exists an
optimal state for long-lived squall lines—it is when
the horizontal vorticity of the shear and the cold pool
balance each other
 It does not say that severe squall lines can only
occur when conditions are optimal – they happen in
a variety of conditions (and are not simply 2dimensional)
Recent work
 Weisman and Rotunno (2004) revisited RKW theory
by considering a wider range of shears and came to
generally the same conclusions
 Bryan et al. (2006) used multiple models to confirm
the general findings
Issues/problems/controversy
 The theory neglects other sources of vorticity within the larger
MCS, or external lifting mechanisms
 Recent observational studies (Coniglio et al. 2004 and others)
suggest that the interaction between low-level shear and cold
pool are not as important as the shear over a deeper layer
 They also find that the c/Δu relationship is not very helpful in
explaining observed squall-line structure
 Coniglio et al. (2012) study on the 8 May 2009 “superderecho”:
“If cold pool–shear interactions were critical to producing such
a strong system, then the extension of the line-normal shear
above 3 km also appeared to be critical. It is suggested that
RKW theory be applied with much caution, and that examining
the shear above 3 km is important, if one wishes to explain the
formation and maintenance of intense long-lived convective
systems, particularly complex nocturnal systems like the one
that occurred on 8 May 2009.”
Organization of Linear MCSs
• The most common mode
of organization is the
“leading line, trailing
stratiform” squall line, but
other modes exist as
well
• Unique 3D flow fields for
each; mid- and upperlevel flow determine
where stratiform precip is
• Systems often evolve
between categories
58%
19%
19%
From Parker and Johnson (2000)
Trailing Stratiform
• TS MCSs are most
common for several
reasons:
− Convective lines tend
to become oriented
perpendicular to the
low-level shear
− Storm-relative flow in a
squall line is generally
front-to-rear, so
hydrometeors are
advected rearward
Johnson and Hamilton (1988)
Vertical structure
Houze et al. (1989)
 Main features: front-to-rear and rear-to-front flow
 Convective downdrafts lead to mesohigh
 Descending rear inflow and rear inflow notch lead to wake low
 adiabatic warming with descent, offset by evaporation
 feeds into cold pool
Trailing Stratiform
 Often, these MCSs transition from a “symmetric” to
“asymmetric” structure
Loehrer and Johnson (1995)
What causes this transition?
SOUTHERLY FLOW,
TRANSPORTING
HYDROMETEORS TO
NORTHERN END OF LINE;
CONTRIBUTES TO
ASYMMETRY
Hilgendorf & Johnson (1998)
Trailing Stratiform Example
Leading Stratiform
Pettet and
Johnson
(2003)
 These can be either “rear-fed”, which is mostly just a mirror
image of a TS system, or “front-fed”
 The image above is a cross-section of a rear-fed LS MCS
Front-fed LS
 And here’s a front-fed
LS MCS
 But shouldn’t inflow
from the east, which
passes through the
evaporatively cooled
air, be stable and cause
the system to dissipate?
Parker and Johnson (2004)
Front-fed LS
 But shouldn’t inflow from
the east, which passes
through the evaporatively
cooled air, be stable and
cause the system to
dissipate?
 Not if the evaporative
cooling increases with
height!
Parker and Johnson (2004)
Leading Stratiform Example
Storm et al. (2003)
Parallel stratiform
Parker (2007b)
 In PS MCSs, there is both along-line and across-line vertical shear,
which favors the advection of hydrometeors parallel to the line
 Eventually, however, almost all PS systems embark on the
“seemingly inexorable march toward TS structure” (Parker 2007b)
Parallel Stratiform Example
Parker (2007a)
Evolution among the archetypes
An MCS typically won’t
stay in the same category
for its full lifetime; it’s
common for them to
evolve from one to
another (and another…)