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amount of wind shear is impor tant for separating the updrafts from the downdrafts of

a thunderstorm, thus permitting the storm to be long lived. More speciﬁcally,

convection produces cold downdraft air that pools under neath the storm. A gust

front, marking the boundary between the cool downdraft air and the warm environ-

mental air, can lift the warm air, sparking new convective storms. In the absence of

shear, the downdraft air will spread uniformly in all directions. New cells formed

along the gust front will be quickly undercut by the expanding gust front from the

parent cell, thus limiting further convective development. As the shear increases,

new convective cells will move downshear away from the parent cell faster than the

evolving gu st front, allowing continual redevelopment of new cells. This effect is

particularly important in long-lived multicells and squall lines.

Types of Convective Storms

Ultimately, the role of convection in the atmosphere is to take an unstably stratiﬁed

sounding and make it more stable by lifting the warm, moist low-level air and

bringing cooler, drier air down in the downdrafts. This removes the conditional

instability present in the pre-convective atmosphere. It is the interplay between the

release of this instability within the storm itself and the environment that produces

the panoply of storm types that we see.

Although many of the factors that affect storm structur e and evolution are under-

stood, there is still much to be discovered about the relative roles of the large-scale

environment and the internal dynamics of the storm itself. This is an important issue

because it relates to the limits of predictability of convective storms. If the environ-

ment of the storm is a strong factor in storm evolution, then storms are potentially

more predictable since the large-scale data in the storm environment is usually well

observed. But if the internal dynamics of the storm are the most important factor,

then it may be a long time, if ever, until we have measurements within the storm that

could be useful for understanding, let alone predicting, the storm evolution. For the

purposes of this section, we consi der the effect of the environment on the type of

convective storm.

At least three environmental factors affect the type of convective storm that

forms: wind shear, instability, and synoptic setting (e.g., fronts, drylines, jet

streams). The type and direction of wind shear that the convection initiates is

important for the morphology (or mode) of convection that results. The amount

of instability affects the strength of the convective updrafts. The ﬂow p attern in

which the storms develop may also play a role in the mode and strength of the

resulting convection. In the following section, the types of convective storms and

their characteristics are summari zed. More detailed discussion of the individual

storm types can be found in later sections of this chapte r.

The individual cumulus clouds that constitute so-called pulse or air-mass thun-

derstorms are typically a few kilometers in diameter with updrafts on the order of

10 m=s or more. CAPE is usually less than 1000 J=kg and the deep-layer wind shear

is weak (less than 10 m=s over 10 km). These storms typically last 30 to 50 min and

produce short-lived localized showers with few, if any, reports of severe weather

(tornadoes, hail, or damaging winds). Typically, large areas tend to erupt in convec-

586 SEVERE THUNDERSTORMS AND TORNADOES

tive cloud about the same time when the surface temperature reaches the convective

temperature, the temperature required to eliminate any low-level CIN. While useful

for these situations, the use of the convective temperature for other types of storms is

not recommended.

With moderately strong low-level and deep-layer wind shear and moderate to

high CAPE, supercells may form. The essence of a supercell thunderstorm is a

single, nearly steady, rotating updraft. High winds, ﬂooding rains, large hail, and

potentially long-lived violent tornadoes can occur with supercells. If the wind shear

is constant in direction with height (called a straight-line hodograph), maturing

supercells will tend to split into left-moving and right-moving cells. If the wind

shear curves counterclockwise with height (in the Northern Hemisphere), right-

moving cells will tend to dominate with counterclockwise rotation. On the other

hand, if the wind shear curves clockw ise with height, left-moving cells will tend to

dominate with clockwise rotation in the Northern Hemisphere. Right-moving cells

tend to be more common. More will be said about supercells later.

Sometimes, individual convective clouds will join together as gust fronts caused

by downdrafts from the parent cells combine to lift unstable environmental air to its

LFC, forming secondary convection along the periphery of the parent cells. These

storms are called multicells and range in organization from poorly organized clusters

of individual convective elements to highly organized linear structures including

bow echoes and larger-scale squall lines. Individual storms withi n a multicell usually

move to the left (in the Northern Hemisphere) of the mean motion of the multicell

itself. Because the warm moist air tends to be ingested on the right side (or equator-

ward side, in the Northern Hemisphere) of the multicell, new cells form preferen-

tially on this side, forcing older cells toward the left of the multicell storm. Because

multicells propagate away from the original convection, a certain amount of low-

level shear is needed for gust-front propagation. If storm motion is very slow, local

areas may be affected by prolonged periods of rain, making ﬂooding a strong

possibility.

Sometimes multicellular convection will organize in lines. Typically this occurs

because linear features exist at the surface (such as a cold front) that organizes the

convection. Because of the linear forcing, a common location for development of

squall lines and bow echoes is along fronts. Sometimes these lines move ahead of

the front that spawned them; at other times, forcing above the surface will produce

squall lines in the warm sectors of extratropical cyclones. In some cases, often when

the vertical wind shear is weak, mesoscale convective complexes form.

4 MESOSCALE CON VECTIVE SYSTEMS, MESOSCALE

CONVECTIVE COMPLEXES, SQUALL LINES, AND BOW ECHOES

Mesoscale Convective Systems

While isolated thunderstorms are important producers of severe weather, it is just

as common for thunderstorms to merge and interact, forming a more complex

precipitation system. These more complex thunderstorm systems are called mesos-

4 MESOSCALE CONVECTIVE SYSTEMS 587

cale convective systems (MCSs)—cloud and precipitation systems that are

comprised of a group of thunderstorms that have a contiguous precipitation area

of 100 km in at least one direction (Fig. 6). MCSs are particularly important

because they are observed worldwide, over both the Tropics and midlatitudes, and

over land and water. MCSs also have been shown to produce approximately half of

the warm season rainfall in the central United States, indicating that they are

important components of the hydrologic cycle. Further, in the form of squall lines

and bow echoes, such systems likely account for most, if not all, of the widespread

convective windstorms that occur around the world.

Mesoscale Convective Complexes

Observations indicate that MCSs occur in a variety of shapes and sizes. The largest

1% of MCSs are called mesoscale convective complexes (MCCs). They are most

commonly identiﬁed from infrared satellite imagery as large regions of cold, nearly

circular, cloud tops. MCCs typically produce a contiguous cloud shield of

200,000 km

and last for over 15 h (Table 1). These cold cloud tops are the collective

Figure 6 (see color insert) Infrared satellite picture of a developing MCS over south-central

Kansas and northern Oklahoma at 0100 UTC May 28, 2001. A severe squall line was

developing underneath the cloud shield and subsequently moved southeasterly for the next

12 h to the Texas coast. [Image courtesy of University of Illinois WW2010 project (http:==

ww2010.atmos.uiuc.edu).] See ftp site for color image.

588

SEVERE THUNDERSTORMS AND TORNADOES

anvils from interacting thunderstorm cells constituting the MCC. MCCs tend to

initiate in the late afternoon, reach their maximum extent around midnight, and

terminate early in the morning. They tend to form in situations with large-scale

warm advection in ﬂow regimes with strong anticyclonic shear or anticyclonic

curvature at the jet-stream level. The processes by which individual cells join to

form an MCC are poorly understood. Some MCCs last for several days and can

inﬂuence large portions of a continent. In addition, when MCCs develop or move

over warm ocean waters, they have been obser ved to evolve into tropical storms.

Squall Lines and Bow Echoes

While MCCs represent only a small portion of all MCSs, commonly observed

subsets of MCSs include the squall-line and bow-echo complexes. Squall lines

consist of a well-deﬁned line of thunderstorms with an associated stratiform preci-

pitation region and are often identiﬁed from radar data. (Fig. 7). The stratiform

TABLE 1 Mesoscale Convective Complex (MCC) Criteria

Size A—Contiguous cold cloud shield with infrared (IR)

temperatures 241 K with area 100,000 km

B—Interior cold cloud region with IR temperatures 221 K

with area 50,000 km

Initiate Size deﬁnitions A and B are ﬁrst satisﬁed

Duration Size deﬁnitions A and B must be met for a period of at least 6 h

Maximum extent Contiguous cold cloud shield (IR temperatures 241 K)

reaches maximum size

Shape Eccentricity (minor axis=major axis) 0.7 at time of

maximum extent

Terminate Size deﬁnitions A and B no longer satisﬁed

Figure 7 Conceptual model of a squall line with a trailing stratiform area view in a vertical

cross section oriented perpendicular to the convective line (i.e., parallel to its motion).

(Adapted from Houze et al., 1989, American Meteorological Society.)

4 MESOSCALE CONVECTIVE SYSTEMS 589

precipitation region can be located either in front of, along, or behind the convective

line, although it is most common for the stratiform region to be behind the convec-

tive line. The convective and stratiform regions often have a symmetric radar depic-

tion early in their life cycle, evolving into a more asymmetric pattern with time (Fig.

8). The distinction between convective and stratiform precipitation is made by

comparing the typical vertical air velocities with the terminal fall velocities of ice

crystals and snow ( 1to3m=s). If the vertical air velocities are larger in magnitude

than the ice and snow fall velocities, then the precipitation is convective; otherwise

the precipitation is stratiform. On radar, the convective regions have much larger

values of reﬂectivity, indicating heavier preci pitation or hail. While the typical

vertical motions in stratiform precipitation regions are not large, these regions

provide 40% of the total precipitation reaching the ground for many squall

lines owing to their large areal extent.

Four typical phases have been identiﬁed in the life cycles of MCSs (Fig. 9). The

ﬁrst phase is the for mative stage in which the initial thunderstorms are developing

and act independently from each other. This is often the time during which the most

severe weather occurs. As the thunderstorms grow and merge, the MCS enters the

intensifying stage. During this stage the MCS is seen ﬁrst to have a contiguous

precipitation region using radar. The mature stage occurs as a large stratiform rain

region is produced, often from older convective cells that weaken and move to the

rear of the convective line. This stratiform rain region often grows in size until it is

Figure 8 Conceptual model of a midlevel horizontal cross section through (a) an approxi-

mately two-dimensional squall line, and (b) a squall line with a well-deﬁned mesoscale vortex

in the stratiform region. In each case, the midlevel storm-relative ﬂow is superimposed on the

low-level radar reﬂectivity. The stippling indicates regions of higher reﬂectivity. (Adapted

from Houze et al., 1989, American Meteorological Society.)

590

SEVERE THUNDERSTORMS AND TORNADOES

an order of magnitude larger in area than the convective region. Signiﬁcant lightning

can occur within the stratiform precipitation region, along with a potential for heavy

rainfall and ﬂooding. Positive cloud-to-ground lightning strikes appear to be more

common within the stratiform precipitation region than in the convective line. The

ﬁnal, dissipating stage is when the convective portion of the MCS weakens, with

fewer and fewer convective cells developing along the convective line. Eventually,

all that remains is a weakening region of stratiform precipitation.

One of the most important aspects of MCSs is that the combined effects of the

individual thunderstorms and the stratiform precipitation region produce dynamical

features that are unique to MCSs and inﬂuence both smaller and larger scales of

motion. Pools of cooler air are produced near the ground by the evaporation of

falling precipitation underneath the thunderstorm cells. In MCSs, these pools

merge and inﬂuence the development of new convective cells, since warm air

approaching these cold pools is lifted up and over the cooler surface air. Thus,

MCSs often move in different directions than the individual thunderstorms that

constitute the convective line. In the midlevels of the atmosphere, the latent heat

released from the change of phase from water vapor to liquid water within the

stratiform precipitation region often gives rise to the development of front-to-rear

ﬂow in the upper portion of the MCS (Fig. 7). A rear inﬂow jet also may develop

below the front-to-rear ﬂow region of the MCS and approaches the MCS from the

rear. Other circulations can develop that produce vortices within the stratiform

precipitation regions; these vortices can persist for several days after the initial

parent MCS has dissipated. Finally, the upper-level outﬂows from MCSs alter the

mass ﬁeld near the tropopause and can produce upper-level jet streaks that move

away from the MCS and can inﬂuence other weather systems downstream. On

occasion smaller scale features known as bow echoes (20–200 km in length) form

within squall lines or as individual MCSs (Fig. 9). In such situations the severe

weather threat, particularly in the form of damaging wind gusts, is enhanced and

may continue through much of the life cycle of the MCS. Widespread convection

windstorms known as derechos may occur with the longer-lived bow echo domi-

nated MCS events. In such cases it appears the cold pool is enhanced and often

Figure 9 Idealized morphology of an isolated bow echo associated with strong and

extensive downbursts. (after Fujita 1978).

4 MESOSCALE CONVECTIVE SYSTEMS 591

moving more rapidly than the mean tropospheric wind. This rapid movement

enhances low level storm relative ﬂow and the development of new cells along

the gust front. As was mentioned in the last paragraph, predictions of MCS devel-

opment is complicated. However, in those situations in which bow echo development

is dominant, the environmental air above the boundary layer typically displays low

relative humidity. This drier air likely enhances the development of the downdraft

and cold pool strength as well as the risk of damaging winds at the surface. Owing to

the myriad interactions that occur within MCSs, it is probably not surprising that the

prediction of MCS development and evolution has proven to be a challenging

problem.

5 SUPERCELLS

Deﬁnition

The idea that some severe thunde rstorms have a mark edly different character from

other types of thunderstorms owes its origins to the development of radar as an

observing tool. Radar gave meteorologists the ability to see the distribution and time

evolution of precipitatio n in thunderstorms. Keith Browning and Frank Ludlam, in

England, were pioneers in the interpretation of radar. They participated in ﬁe ld

observation campaigns in the United States during the early 1960s after having

observed a particularly severe and long-lived hailstorm that struck in and close to

the town of Wokingham, England. After observing a number of severe stor ms with

radars, Browning made use of the fact that the distribution of precipitation and its

changes with time provides indirect evidence of a thunderstorm’s up- and down-

drafts. A careful examination of that radar-depicted structure and evolution gave

meteorologists a detailed look at what was going on inside a thunderstorm.

Browning and Ludlam noticed that a few severe thunderstorms exhibited a parti-

cular set of characteristics: (1) they produced extreme severe weather events (torna-

does, giant hail, and violent wind gusts), (2) they tended to move to the right of the

winds, whereas most storms moved more or less with the winds, (3) they exhibited a

columnar region of reduced radar echo originally called a vault (later renamed the

bounded weak echo region, or BWER), and (4) they had extended lifetimes in

comparison to most thunderstorms, persisting for many hours on occasion. At

ﬁrst, these were referred to as severe, right-moving (or SR) storms. In an informal

report published in 1962, Browning and Ludlam ﬁrst used the term supercell to refer

to these storms. As more and more storms were subjected to scrutiny by radar, the

so-called hook echo became associated with supercells (Fig. 10).

The development in the early 1970s of Doppler radar, which can observe the

motion of precipitation echoes and, therefore, can be used to infer the airﬂow,

provided conclusive evidence of what Browning and his collaborators had surmised

from non-Doppler radar observations: Supercells are characterized by rotation on the

scale of the thunderstorm itself. Although from a formal viewpoint, the rotation can

be either cyclonic or anticyclonic, the SR storms that Browning called supercells are

592 SEVERE THUNDERSTORMS AND TORNADOES

characterized by cyclonic rotation. The center of that cyclonic rotation came to be

called the mesocyclone. It is the presence of this rotation that distinguishes super-

cells from other types of thunderstorms. Therefore, it is generally agreed that super-

cells are thunderstorms that have a deep, persistent mesocyclone. It is the

mesocyclone that is deemed responsible for the high probability of severe weather

and a supercell’s characteristic features.

Supercell Structure and Evolution

Figure 10 illustrates the primary features of a supercell thunderstorm during its

mature phase. The fact that supercells can persist for many hours suggests that

their structure evolves relatively slowly. However, every storm must have a begin-

ning and end, so no storm is ever truly steady. Some supercells are more nearly

Figure 10 Plan view of supercell showing hook echo, bounded weak echo region (BWER),

and forward ﬂank precipitation regions. (Courtesy of National Severe Storms Laboratory.)

5 SUPERCELLS 593

steady than others, but all exhibit a basic evolutionary pattern. Thunderstorms begin

as pure updrafts, creating towers of cloud called cumulus congestus, which then

develop precipitation aloft. The production of precipitation triggers the development

of downdrafts, both from the drag effect of having precipitation particles and from

the evaporation of some of those particles. Thus, whereas the thunderstorm is initi-

ally dominated by updrafts in its mature phase, the thunderstorm has both updrafts

and downdrafts. In dissipation, a thunderstorm’s updraft weakens and eventually

ceases, leaving only the weakening downdraft and precipitation.

Supercells also follow this evolution although the mature phase is prolonged.

During the early stages of a supercell, the updraft begins to rotate cyclonically,

typically several kilometers above the surface. Thus, the rotation is initially on the

same axis as the updraft. With the development of preci pitation and the descent of

downdrafts, the mesocyclone structure is transformed. Rather than being centered on

the updraft, the cyclonic rotation extends into the downdraft, such that the mesocy-

clone comes to be centered near the interface between updraft and downdraft. The

updraft changes its shape from being nearly circular as seen at a given horizontal

level, to become elongated and crescent-shaped, with the crescent aligned with the

low-level boundar y between updraft and downdraft (the so-called gust front). With

time, the downdraft and outﬂow at low levels goes through an evolution called

occlusion, whereby the gust front undercuts the updraft, which then dissipates.

Most supercells exhibit a cyclic behavior during their extended mature phase,

with new updrafts and mesocyclones forming on the leading edge of the outﬂow

boundary, even as the previous updraf t is in the process of dissipation. This process

can go on many times, so the mature phase of supercells undergoes a ﬂuctuation on a

time scale of 20 to 30 min or so (although the time between ‘‘pulsations’’ is by no

means constant). Eventually, of course, the storm can no longer be maintained and

the last of updrafts and mesocyclones in the series ﬁnishes its life cycle and the

storm dissipates.

Origins of Supercell Structure

Browning and his collaborators noticed right from the beginning that supercell

storms formed in environments having enhanced vertical wind shear. That is, the

winds typically changed both direction and speed with height. Poleward low-level

winds became more westerly and increased in speed rapidly in the vertical. Vertical

wind shear in the atmosphere is associated with regions of enhanced horizontal

temperature differences (or fronts) and also means that the airﬂow possesses a

property called vorticity or spin, which is created when regions of air move past

each other at different speeds and=or different directions. When this occurs in

association with vertical wind shear, this means the wind proﬁle has horizontal

vorticity. This is illustrated in Figure 11, where it can be seen that the change of

wind in the vertical implies a potential for rotation about a horizontal axis.

When thunderstorm updrafts develop in regions of strong vertical wind shear,

they cause the horizontal vorticity in their surroundings to be tilted upward into the

vertical, and it is this uptilted vorticity that gives rise to the rotation of the updraft

594 SEVERE THUNDERSTORMS AND TORNADOES

about a vertical axis during its development. This has been demonstrated in

computer simulation models and is clearly the source for mesocyclonic rotation in

thunderstorms.

Available evidence indicates, however, that the creation of rotation near the

surface is associated with a more complex process that involves a supercell’s down-

drafts. In some supercells, the mesocyclone aloft and that developing near the

surface can interact strongly, creating a deep column of intense rotation. Such

storms are the primary producers of long-lasting, strong tornadoes and often

result in families of tornadoes, with each tornado being a reﬂection of another

pulsation in a cyclic supercell. Other supercells fail to develop a strong interaction

between the low-level mesocyclone and the mesocyclone aloft. Such storms

can produce other forms of severe weather, notably giant hail, but tornadoes are

relatively infrequent and tend to be brief and weak if they do form.

Hazardous Weather Associated with Supercells

It appears that the development of a mesocyclone has a strong inﬂuence on the

storm, and that inﬂuence is such that the likelihood of severe weather in all forms

increases if a storm becom es supercellular. Supercel l storms constitute only a small

fraction of the total number of severe thunderstorms, but they account for a dispro-

portionate share of the most intense forms of hazardous weather.

Figure 11 Tilting of horizontal vorticity into the vertical via a thunderstorm updraft.

(Courtesy of National Severe Storms Laboratory.)

5 SUPERCELLS 595