Wallace J.M., Hobbs P.V. Atmospheric Science. An Introductory Survey

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354 Weather Systems

line or reasoning, it can be argued that the left side of

the updraft, which is the mirror image of the flow in

the right side, will acquire a clockwise rotation.

These counterrotating vortices at midlevels of the

updrafts induce negative pressure perturbations,

which intensify the upward pressure gradient at the

base of the updraft. That this reenforcement occurs,

not in the middle of the updraft, but along its right and

left flanks causes the updraft to widen and eventually

split into counterrotating right- and left-moving super-

cell storms, as shown in subsequent panels of Fig. 8.52.

A casual inspection of Fig. 8.52b might suggest that

the splitting of the storm is due to the spontaneous

formation of a strong downdraft along its axis of

symmetry. However, splitting is observed in numeri-

cal simulations of this phenomenon, even when the

microphysical processes that enhance the downdraft

are neglected. It is a consequence of the pressure

perturbations induced by counterrotating vortices in

the updraft. This storm-splitting process does not, in

and of itself, require veering or backing of the envi-

ronmental wind profile with height.

Nearly all intense supercell storms in the northern

hemisphere exhibit right-moving and counterclockwise

(i.e., cyclonically) rotating updrafts. Until the 1980’s it

was widely believed that this bias was due to the influ-

ence of the Coriolis force, as explained in Section 7.2.9.

However, it is now well established, on the basis of

numerical experiments, that the contribution of the

planetary vorticity to the spin of the updraft of a super-

cell storm is negligible. The prevalence of right-moving

storms is due to the prevalence of clockwise-turning

hodographs, like the example in Fig. 8.44b, in large-

scale settings conducive to the formation of supercell

storms. Hodographs with unidirectional wind shear, like

the example in Fig. 8.44a, are equally conducive to left-

and right-moving storms. Why veering of the wind with

height should favor counterclockwise-rotating storms

that move to the right of the steering flow in the north-

ern hemisphere (and vice versa in the southern hemi-

sphere) is beyond the scope of this text. Suffice it to say

that veering perturbs the pressure field in and around

convective updrafts in a manner that reinforces right-

moving storms and suppresses left-moving storms.

Figure 8.53 shows a composite hodograph formed

by averaging the hodographs of soundings made in

the vicinity of 62 tornadic supercell thunderstorms

over the central United States. The average move-

ment of the storms is toward the east–northeast

(ENE), well to the right of the vertically averaged

steering flow. Wind speed increases rapidly with

height, especially in the lower troposphere, and wind

direction veers continuously from SSE at the ground

to WSW in the upper troposphere, a change in direc-

tion of almost 90°. The lower portion of the hodo-

graph exhibits strong curvature in the same sense as

the idealized profile in Fig. 8.44b.

The prevalence of cyclonically rotating updrafts in

supercell storms is thus a consequence of the facts that

• thermodynamic conditions conducive to deep

convection occur more frequently under conditions

of warm advection than under conditions of cold

advection,

• warm advection occurs in association with

veering of the wind with height,

• veering of the wind with height is reflected in

anticyclonically curved hodographs that favor

cyclonically rotating updrafts.

(a)

(b)

(c)

(d)

Fig. 8.52 Schematic showing the splitting of a multicell storm

into right and left moving supercell storms. Bold black arrows

denote updrafts and downdrafts, shading denotes radar

echoes, the thin tube in the upper two panels represents a

cylinder of marked boundary layer air parcels, and thin circular

arrows denote the sense of the rotation of the parcels.

[Reprinted from Adv. Geophys., 24, R. A. Houze and P. V. Hobbs,

“Organization and Structure of Precipitating Cloud Systems,

p. 263, Copyright (1982), with permission from Elsevier.]

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8.3 Deep Convection 355

That warm advection occurs in association with veer-

ing of the wind with height is a consequence of the

thermal wind equation [Eq. (7.20)], in which the

Coriolis parameter f  2 sin



appears as a linear

factor. The veering of the wind with height in the

boundary layer due to frictional drag, as explained in

Section 7.2.5, also contributes to the prevalence of anti-

cyclonically curved hodographs. Hence, it can be said

that the Coriolis force is indirectly responsible for the

prevalence of cyclonically rotating supercell storms.

The structure of a typical supercell storm, as

revealed by radar imagery, is shown in Fig. 8.54. The

strong asymmetry of the patterns at the lower levels

reflects the prevalence of right-moving storms. The

bounded weak echo region (BWER) or echo free

vault at the 4- and 7-km levels corresponds to

the updraft, which is coincident with the axis of

rotation. The heaviest rain and hail tend to occur in

the downdraft wrapping around the northwest flank

of the axis of rotation in a cyclonic sense.

sfc

(m s

–1

)

850

700

500

300

200

(m s

–1

)

010 30

Storm

motion

Plume

0 10 20 30 40 km

max

BWER

–40°C

0°C

BWER

010

30 km

010

20 km

–40°C

0°C

Storm motion

BWER

Height (km)Height (km)

(a)

(b)

(c)

1 km

4 km

7 km

10 km

20 dBZ

Fig. 8.53 Composite hodograph formed by averaging the

hodographs of soundings made in the vicinity of 62 tornadic

supercell thunderstorms over the central United States. Labels

represent pressure levels in hPa. The red vector from the ori-

gin to point O indicates the average movement of the storms,

and blue arrows represent the winds at various levels as

viewed in a coordinate system moving with the storms. This

plot was the basis for constructing the idealized hodograph in

Fig. 8.44b. [Based on data in Mon. Wea. Rev., 104, 133–142

as adapted in Cloud Dynamics, R. A. Houze, p. 291, Copyright

(1993), with permission from Elsevier.]

Fig. 8.54 Schematic based on a composite of radar imagery for supercell storms over the Canadian prairies. Horizontal cross

sections at left and vertical cross sections at right. The reflectivity scale in dBZ is a logarithmic measure of rainfall intensity.

BWER denotes the bounded weak echo region that is the signature of the updraft and Z

max denotes the strongest echoes.

[Reprinted from Cloud Dynamics, R. A. Houze, p. 293, Copyright (1993), with permission from Elsevier. Adapted from

A. J. Chisholm and J. H. Renick, Preprints, International Cloud Physics Conference, London (1972).]

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356 Weather Systems

conditions, the rotation of the right-moving storm

formed by the splitting of a multicell storm can be

self-amplifying.

• The vertical shear of the environmental

air flowing into the storm strengthens the

rotation of the updraft and the associated

mesocyclone.

• The existence of the mesocyclone, in turn,

amplifies the updraft and strengthens the

gust front.

Distinctive features of the cloud configuration of a

right-moving supercell storm are illustrated schemat-

ically in Fig. 8.56, and examples of the associated

cloud formations are shown in Figs. 8.57–8.60.

8.3.3 Damaging Winds Associated

with Convective Storms

The severe weather that occurs in association with

convective storms is often accompanied by, and

sometimes dominated by, strong winds associated

with tornadoes, gust fronts, and downbursts.

a. Tornadoes

A tornado

is a rapidly rotating column of air, in

contact with the ground, either hanging from or posi-

tioned beneath the base of a deep convective cloud.

Tornadoes are rendered visible by the lowered lifting

condensation level and by the ground debris that

they raise, both of which are evident in the examples

shown in Figs. 8.61 and 8.62. The distinctive funnel

cloud that often appears in association with torna-

does is an indicator of the low atmospheric pressure

that prevails in the interior of the vortex. The value

of atmospheric pressure that defines the outline of

the cloud corresponds roughly to the pressure at the

lifting condensation level of the environmental air,

although it may be somewhat higher if appreciable

quantities of premoistened downdraft air are being

entrained into the updraft. Damaging winds associ-

ated with tornadoes extend outside the outline of

the funnel cloud and may be present even in the

absence of a well-defined funnel cloud. The airflow

within and around a tornado is more complex than

Flanking line

Light rain

Moderate - heavy rain

Small hail

Large hail

Center of mesocyclone

Anvil edge

Gust front

′

Fig. 8.55 Idealized structure of a right-moving supercell

storm. [Based on NOAA National Severe Storms Laboratory

publications. Reprinted from Cloud Dynamics, R. A. Houze,

p. 279, Copyright (1993), with permission from Elsevier.]

The structure of the storm at the Earth’s surface is

shown in Fig. 8.55. The mesocyclone marked by the

M symbol centered on the tornado and the configu-

ration of the gust front, depicted in cold front sym-

bols, resemble a miniature extratropical cyclone.

Most of the low level inflow into the rotating

updraft takes place along the stationary segment of

the gust front to the east of the mesocyclone.

Relative to the moving storm, the inflow is west-

ward, parallel to the front. Along section AA,

warm, moist boundary-layer is streaming northward

up and over the gust front, while evaporatively-

cooled downdraft air remains in place and, in some

cases, even advances southward below it. The verti-

cal shear of the air flowing through AA into the

updraft is thus enhanced by a factor of 3 or more

relative to that of the undisturbed environment due

to the presence of the gust front, and the rate of

vorticity generation by the tilting of the boundary

layer air into the updraft is correspondingly

increased. Hence, under favorable environmental

From the Spanish verb tornar, to twist.

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8.3 Deep Convection 357

Tornado

Large hail

Small hail

Warm air

Heavy rain

Light rain

Forward-flank

gust front

~10 km

Storm motion

Precipitation curtain

(hook echo)

Precipitation-free

cloud base

Rear-flank

gust front

Mammatus

Cold air

Back-sheared anvil

Flanking line towers

Anvil

Overshooting top

Mammatus

Virga

Wall cloud

Striations

12–13 km

Fig. 8.56 Structure of a typical tornadic supercell storm.

Motion of the warm air is relative to the ground. [Based on NOAA

National Severe Storms Laboratory publications and an unpublished manuscript by H. B. Bluestein. Reprinted from Cloud

Dynamics, R. A. Houze, p. 279, Copyright (1993), with permission from Elsevier.]

Supercell storms can be divided into three categories: classic supercells that resemble this schematic; low precipitation (LP) super-

cells in which the anatomy of the rotating updraft is often clearly visible; and high precipitation (HP) supercells in which heavy rain (often

accompanied by hail) wraps around the trailing side of the mesocyclone, obscuring any embedded tornadoes. In contrast to most classic

supercells, HP storms develop strong rotation along the forward (eastern) flank of the gust front.

would be suggested by the compact smooth shape

of the funnel cloud. Depending on the ambient light-

ing, funnel clouds may be dark or light in color.

Tornadoes come in a variety of sizes and shapes and

a single tornado may have multiple vortices.

The radial profile of tangential wind speed and

vertical velocity in tornadoes are not well known,

but for assumed profiles, the pressure at the center

of a tornado can be estimated on the basis of meas-

urements of the maximum tangential wind speed,

assuming that the vortex is in cyclostrophic balance,

as demonstrated in the following exercise.

Exercise 8.6 The circulation around many torna-

does resembles a Rankine vortex, which is character-

ized by solid body rotation for radius r  r

and

irrotational flow with tangential velocity v inversely

proportional to radius for r  r

. Assuming that the

flow is in cyclostrophic balance, estimate the pressure

deficit



p (relative to the ambient air pressure) in a

tornado with a maximum velocity of 100 m s

1

.The

density of the air is 1.25 kg m

3

. Ignore the variation

of air density with pressure.

Solution: The flow is in cyclostrophic balance, so that

Hence, if we ignore the variation of density

Substituting v  v

(rr

) for r  r

and v  v

r)

for r  r

, where v

is the tangential velocity at radius

, we obtain

















p 





rdr 









p 









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358 Weather Systems

Fig. 8.59 Looking north toward the approaching forward

flank of a strong gust front marked by a distinctive arcus cloud

with strong counterclockwise rotation about a horizontal axis

pointing into the page. [Courtesy of Kathryn Piotrowski.]

Fig. 8.60 Thunderstorms over the midwestern United States,

just before sunset July 10, 1994, as revealed in satellite imagery.

The low sun angle accentuates the overshooting cloud tops.

[NASA-GSFC GOES Project.]

Fig. 8.58 Shelf cloud at the base of the updraft of a supercell

storm looking southward along the forward flank of the gust

front in the direction of the mesocyclone. The gust front is

advancing rapidly toward the left (E), propelled by rain-

cooled downdraft air. When viewed from a distance looking

from left to right, rising cloud motion often can be seen in the

leading (outer) part of the shelf cloud, while the underside

often appears turbulent, and wind-torn, as in this image.

[Courtesy of Brian Morganti.]

Substituting v

 100 m s

1



 1.25 kg m

3

, then



p  1.25  10

Pa  125 hPa. This estimated value is

consistent with the observation that funnel clouds

often extend all the way down to the ground.

Pressure deficits in excess of 40 hPa have been docu-

mented at the Earth’s surface, and it is conceivable

that even larger ones would be observed if it were

possible to obtain measurements in the middle of

tornadoes. The lifting of dense objects by tornadoes

is suggestive of the existence of upward pressure

gradient forces many times the gravitational acceler-

ation



(i.e., pressure decreasing with height at a rate

far in excess of the hydrostatic value of 1 hPa per

8 meters). ■

Fig. 8.57 Wall cloud developing along the base of a bell-shaped

rotating updraft at the center of the mesocyclone of a supercell

storm. The view is looking SW. Observers located to the south

of the storm at this time observed a tornado that rapidly

became enshrouded in rain. [Courtesy of Brian Morganti.]

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8.3 Deep Convection 359

Fig. 8.61 Supercell tornadoes come in many different sizes, shapes and colors. (Top left) Near Sharon, Kansas, May 29, 2004,

looking east. The whirling orange cloud is ground debris illuminated by the setting sun. The thinner, light gray funnel cloud

extending downward from cloud base is condensed water vapor. [Photo courtesy of Kathryn Piotrowski.] (Top middle) Tornado

over Attica, Kansas from the same supercell storm, looking north. Much of the ground debris is from bales of hay. (Top right)

Rope-like tornado near Mulvane, Kansas, June 12, 2004, looking east. (Lower left) “Stove pipe” tornado with a smooth white

condensation funnel near Big Spring, Nebraska, June 10, 2004, looking NW. Towers of power lines, faintly visible just above the

horizon, provide a sense of scale. (Lower right) Wedge tornado (informal term for a large tornado with a condensation funnel

that appears at least as wide at the ground as the distance from the ground to cloud base) over Argonia, Kansas, May 29, 2004,

looking NNW. [Photos courtesy of Eric Nguyen.]

Fig. 8.62 Tornado over western Nebraska, June 10, 2004, in three stages of development. In the left panel the condensation

funnel is just beginning to emerge from the wall cloud, but the circulation at the ground is already raising dust. The condensation

funnel is more fully developed in subsequent panels. The wall cloud is clearly visible in the center and right-hand images, together

with a tail cloud entering from the right and wrapping around the near flank of the wall cloud. [Courtesy of Kathryn Piotrowski.]

P732951-Ch08.qxd 9/12/05 7:47 PM Page 359

360 Weather Systems

To generate a tornado the vorticity in the rotating

updraft needs to reach values on the order of 10

2

1

two orders of magnitude stronger than the Earth’s

rotation. Then the rotating air column needs to be

stretched in the vertical (or more generally, in the

direction parallel to its axis of rotation) to further

enhance the vorticity.

The effect of the stretching in concentrating the

vorticity can be understood as follows. As a closed

ring of air converges toward the axis of rotation,

its circulation C (or angular momentum) tends to

be conserved, as explained in Section 7.2.9. Hence,

v increases in inverse proportion to r. Since, from

(7.3), the average vorticity within the area enclosed

by the loop is equal to the circulation divided by the

area enclosed by the loop, vorticity must increase in

inverse proportion to the area A of the loop, i.e.,

Making use of (7.2) and the continuity equation (7.39),

this expression can be rewritten as

(8.6)

Invoking (7.33), and bearing in mind that the vertical

stretching that leads to the formation of tornadoes

takes place within the lowest 1–2 km above the

ground, where density is constant to within 10–20%,

we can write

Integrating and taking the antilog, we obtain

(8.7)

The stretching that ultimately leads to tornado

formation is due to the upward acceleration of the

air at the base of the updraft. In a typical supercell

storm the rate of ascent w increases from near zero

at the ground to 3m s

1

at 1 km above the ground

so that



w



z 3  10

3

1

. Hence, the e-folding

time T for the amplification of the vorticity is 300 s.

Exercise 8.7 Given an ambient vorticity of

2

1

and vertical stretching that leads to the





where T  (







1

ln  





d



ln 









concentration of the vorticity with an e-folding

time of 5 min, how long would it take to generate

an axially symmetric tornado with a maximum

wind speed v  100 m s

1

at a radius r

of 200 m.

Assume that the region of the tornado inside the

radius of peak wind speed is characterized by solid

body rotation.

Solution: From Exercise 7.1, the vorticity of the

fully developed tornado is equal to 2



, where  is the

rotation rate vr. Hence

To attain this value, the vorticity must amplify by a

factor of 100 relative to the ambient value.Taking the

natural log of (8.7)

and substituting







 100 and T  5 min, we obtain

t  23 min. ■

Rather than being spread uniformly (as would be

the case for solid body rotation) most of the vorticity

and the ascent within the interior of the tornado vor-

tex tend to become concentrated within a narrow

ring, just inside the radius of strongest winds, as

shown in Fig. 8.63. Under certain conditions this ring

of extremely high vorticity can break down into mul-

tiple vortices, whose signatures are clearly evident in

aerial photos of debris.

The rotation in a tornadic supercell storm builds

up gradually over a period of several hours, but the

development of the tornado itself takes place much

more quickly. Typical tornado lifetimes are on the

order of tens of minutes, during which time they

move with the storm. Most tornadoes eventually

become surrounded by cooler, less buoyant down-

draft air as the flanking line or rear flank gust front

(Figs. 8.55 and 8.56) wraps around the mesocyclone,

reminiscent of the way in which the cold air wraps

around an occluding extratropical cyclone. As the

mature tornado and its associated mesocyclone

weaken and die, a new mesocyclone may form along

the gust front, setting the stage for the formation of a

second tornado. Over its lifetime a single supercell

storm may spawn a family of tornadoes.





2

 2 

100 m s

1

200 m

 1 s

1

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8.3 Deep Convection 361

Non-supercell tornadoes (Fig. 8.64) form when a

patch of boundary layer air with circulation about

a vertical axis comes into vertical alignment with a

vigorous convective-scale updraft. The source of the

vorticity may be a gust front, a convergence line, or

wind shear induced by flow around a topographical

Fig. 8.63 Schematic of the flow in the interior of a tornado

during various stages of development. [Based on diagrams in

articles in Thunderstorm Morphology and Dynamics, University of

Oklahoma Press, Norman (1986) 197–236 and in Proceedings

of the Symposium on Tornadoes, Institute for Disaster Research,

Texas Tech University, Lubbock (1976) 107–143, as adapted

by Cloud Dynamics, R. A. Houze, p. 308, Copyright (1993),

with permission from Elsevier.]

Fig. 8.64 Non-supercell tornado that developed in thunder-

David Blanchard.]

Fig. 8.65 When sufficient low level rotation is present, dust

devils can form beneath strong updrafts in dry convection.

This was one of numerous dust devils observed over a recently

plowed field late on an August morning near Corvallis, OR.

feature. Over a matter of 15 min or so, the vertical

stretching under a 3-m s

1

updraft can amplify an

initial vorticity perturbation by two orders of magni-

tude. Waterspouts and dust devils (Fig. 8.65) are

smaller, less dangerous vortices that form in a similar

manner.

b. Downbursts

Downward penetrating downdrafts known as down-

bursts (or, in their most concentrated form, as

microbursts, Fig. 8.66) pose major hazards to aircraft

and can sometimes produce damaging winds at the

Earth’s surface. In contrast to the surface wind pat-

tern in a tornado, the wind in a microburst does not

circulate: it is almost purely divergent. The remark-

able strength of the winds associated with

microbursts is due not to the concentration of vortic-

ity, but to the negative buoyancy of the downdraft

P732951-Ch08.qxd 9/12/05 7:47 PM Page 361

362 Weather Systems

air, which enables it to penetrate to within a few hun-

dred meters of the Earth’s surface before it begins to

decelerate. Modeling results indicate that this nega-

tive buoyancy is due mainly to evaporative cooling

and, in some cases, the melting of hail. In contrast to

tornadoes, which form beneath echo-free vaults in

the three-dimensional pattern of radar reflectively,

microbursts tend to coincide with shafts of heavy

precipitation. Vertical accelerations in microbursts

are on the order of 0.01



, which is indicative of a

temperature depression of several degrees C relative

to the surrounding air, and downward wind speeds of

several meters per second extending down to just a

few hundred meters above the ground.

The aviation hazard posed by microbursts is illus-

trated by Fig. 8.67. The danger lies not so much in the

loss of altitude due to the downdraft itself, but in the

sudden loss of lift that occurs in response to the rapid

change in the horizontal velocity when the plane

passes through the downdraft, during which time

the wind speed relative to the plane may drop by

10 m s

1

or more in a matter of 20 s.

c. Gust fronts and derechos

The outflow from downdrafts in convective storms

forms a pool of cool, dense air, which displaces

the warmer, more buoyant environmental air as it

spreads out at the Earth’s surface. The advance of

the downdraft air tends to be concentrated in the

so-called gust front on the forward side of the storm

relative to the direction of propagation. In coordinates

moving with the storm, the gust front is more or less

stationary, with warm environmental air approaching

it and riding up over it as shown in Fig. 8.68. Gust

fronts, sharp cold fronts, and incursions of salt water

advancing into estuaries on the incoming tide are

examples of a class of fluid flows referred to as density

currents, which have been extensively investigated

in laboratory and numerical modeling simulations. In

the laboratory, and sometimes in nature, the interface

between the denser and the lighter fluids is rendered

visible by suspended particles, as in Fig. 8.69.

A distinctive feature of the gust front (Fig. 8.68)

is the bulbous head, with its overturning circulation in

which surface wind speeds exceed the rate of advance

of the front itself. At the leading edge of the head

is a pressure maximum at the Earth’s surface, with

an amplitude on the order of 1 hPa and a horizontal

scale of just a few kilometers.

An observer at a

fixed point at the Earth’s surface would experience

a pressure surge coincident with the passage of the gust

front. In accordance with the horizontal equation of

motion, the observer would note a rapid increase in

wind speed (toward the right) as the front approaches,

peak winds coincident with the passage of the front,

and a more gradual decline in wind speed thereafter.

The lifting of the warm, moist environmental air above

Midair microburst Surface microburst

Touchdown

High winds High winds

Fig. 8.66 Conceptual model of a downburst. [From

T. T. Fujita,

The Downburst–Microburst and Macroburst.

Reports of Projects NIMROD and JAWS, SMRP, University

of Chicago, Chicago (1985).]

Tetsuya Theodore Fujita (1920–1998). Professor at the University of Chicago, noted for his innovative research on tropical cyclones,

tornadoes, and downbursts. Coined the term microburst and, based on evidence from aerial photographs of tornado debris, developed a

scale for classifying the intensity of tornadoes.

The pressure distribution at the earth’s surface in the vicinity of the gust front is made up of a hydrostatic contribution from the

excess weight of the cool downdraft air in the head and a dynamical contribution induced by the abrupt deceleration of the downdraft air

as it approaches the gust front.

Increasing

tailwind

Increasing

headwind

OutflowOutflow

(1–3 km)

Strong downdraft

Fig. 8.67 Idealized depiction of an aircraft taking off

facing into a microburst. In passing from 2 to 4, the aircraft

loses its headwind and begins to experience a tailwind, caus-

ing a loss of lift, and consequently a sudden loss of altitude.

[From J. Clim. Appl. Meteorol., 25 (1986), p. 1399.]

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8.3 Deep Convection 363

the head is sometimes revealed by a laminar arcus

cloud, which moves with the gust front (Fig. 8.59).

When an arcus cloud becomes detached from the deep

convective cloud that spawned the gust front, it is

referred to as a roll cloud.

Surface winds observed in association with gust

fronts are especially strong when the leading edges of

the downdrafts from a series of strong thunderstorms

coalesce to form an unbroken line of high winds. These

intense, long-lived wind-storms, known as derechos,

tend to develop along forward-curving segments of

lines of convection whose signatures in radar imagery

are referred to as bow echoes (Fig. 8.70).

8.3.4 Mesoscale Convective Systems

Like living organisms, mesoscale convective systems

have an identity that transcends the cells (i.e., the

individual single-, multi- and supercell convective

storms) of which they are made up: in particular, they

exhibit larger spatial dimensions and longer lifetimes.

Another distinguishing characteristic of mesoscale

convective systems is the coexistence of convective

and stratiform precipitation. The convective elements

within mesoscale convective systems exhibit varying

degrees and types of organization, depending on the

large-scale environment in which they form. The most

distinctive forms of mesoscale organization are squall

From the Spanish word for straight ahead.

Fig. 8.68 Schematic cross section through a gust front, in coordinates moving with the front. [From J. Atmos. Sci., 44 (1987)

p. 1181.]

Fig. 8.69 The nose of a gust front from a thunderstorm

cell near Denver, Colorado, as revealed by a cloud of blowing

erated sand and dust storms are common in the Middle East

and sub-Saharan Africa, where they are referred to as haboobs.

Fig. 8.70 Schematic showing the radar echo of a thunder-

storm (a) evolving into a bow echo (b,c) and finally into a

comma echo (d) in the northern hemisphere as it moves

eastward. Arrows indicate surface winds relative to the moving

system. The regions of cyclonic and anticyclonic rotation at the

ends of the echo are favored sites for tornado development and

the axis of strongest winds is indicated by the dashed line.

[Adapted from J. Atmos. Sci., 38 (1981) p. 1528.]

Body

Gust front

Backflow

Cold air

Turbulent wake

Arc

cloud

Head

Warm

air

5 km

Height

Nose

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