Houze Robert A., Jr. Cloud Dynamics

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9.2 The Squall Line with Trailing Stratiform Precipitation 349

\ @

............

,I"-STRATIFORM

--\'--..

t -,,--

...J

! CONVECTIVE

g 12

~ W

0 2 4 6

5 SEPTEMBER

Figure

9.U

Total rain integrated over the convective (circled points) and stratiform regions of a

squall-line mesoscale convective system located over the eastern tropical Atlantic Ocean. The data

were obtained by three shipborne radars. The symbols

e, +, and x indicate different methods used for

combining the information from the three radars. (From Houze, 1977. Reproduced with permission

from the American Meteorological Society.)

L '

Region of heavy

stratiformrain

cell

Mature

cell

~'-c;;:

Region of trailing Regionof .

stratiform rain heavy convecllve

showers

Figure 9.13 Conceptual model of the kinematic, microphysical, and radar-echo structure of a

convective line with trailing stratiform precipitation viewed in a vertical cross section oriented

perpendicular to the convective line (and generally parallel to its motion). Intermediate and strong

radar reflectivity is indicated by medium and dark shading. The location of line AB is shown in Fig.

9.14. (From Houze et al., 1989. Reproduced with permission from the American Meteorological

Society.)

convective line with trailing stratiform precipitation. This structure was shown by

examples in Figs. 9.7,9.8, and 9.lOa and b and in idealized form in Fig. 9.9. In this

section, we examine this type of mesoscale convective system structure in more

detail. Since this forin is so sharply defined, it is more amenable to study and has

been examined intensively in a variety of ways. Thus, at the time of this writing,

more can be said about its dynamics than is possible for other forms of mesoscale

convective system organization.

appears though, that many aspects of leading-

line/trailing stratiform organization apply also to other forms of mesoscale organi-

zation.

A conceptual model based on many observational studies of the kinematic

structure of a convective line with trailing stratiform structure is shown in Fig.

9.13.

presents a cross section oriented perpendicular to the leading line of

350 9 Mesoscale Convective Systems

convection. The heavy, black line indicates the boundary of the precipitation, as

seen by radar. The light, scalloped line indicates the horizontal and vertical extent

of the cloud, as determined from visual observation, satellite imagery, or ra-

diosonde data. The intermediate and dark shading indicate regions of enhanced

radar reflectivity. Vertically oriented cores of high reflectivity mark the showers

of heavy rain in the leading convective region of the storm. The trailing region of

stratiform rain shown in Fig. 9.13 is characterized by a marked radar bright band

(Sec. 6.1.2).

Streamlines of the airflow in Fig. 9.13 indicate a general trend of upward motion

beginning in the boundary layer near the gust front, extending up through the

convective region, and sloping more gently into the trailing stratiform cloud at

middle to upper levels. There is, at the same time, a general trend of downward

motion in a current of rear inflow, which runs under the base of the trailing

stratiform cloud and enters the stratiform region

just

above the

O°C

level. This

descending current subsides to the level of the radar bright band, passes through

the melting level, and finally enters the back of the convective region at low levels,

where it reinforces convergence and overturning at the leading gust front.

Superimposed on the general upflow within the convective region are intense,

localized updrafts and downdrafts, associated with the intense rain showers lo-

cated there. New convective cells tend to form on or

just

ahead of the leading edge

of the region of heavy convective showers. The first radar echo appears aloft,

evidently associated with a strong, convective updraft. This developing cell is

followed by a mature cell, which has a deep, strong reflectivity core and is associ-

ated with heavy surface rainfall. The mature cell contains an intense narrow

updraft that can penetrate above the top of the broad cirriform cloud shield. This

updraft is often followed by a convective-scale downdraft at middle to upper

levels. Following the mature cell is an older cell. Though in a weakening stage, the

older cell is also characterized by an updraft core, which is, in turn, followed by

another mid- to upper-level convective-scale downdraft. Older cells are advected

rearward over a layer of dense, subsiding, storm-relative rear inflow. In the heavy

rain from the mature and older cells, low-level convective-scale downdrafts

spread out in the boundary layer behind the gust front and toward the rear of the

system. They are of the type usually associated with precipitation drag and evapo-

ration in isolated thunderstorms and lines of thunderstorms (Sees. 8.2, 8.3, 8.9,

8.10, and 8.11). The arrangement of cells in the convective region, in order of their

stage of development, is similar to that of the multicell thunderstorm (cf. Fig. 8.7)

and in the line of thunderstorms discussed in Sec. 8.11 (Figs. 8.57 and 8.58). The

multicellular nature of the convective region will be examined in more detail in

Sec. 9.2.2.3.

The schematic cross section in Fig. 9.13 represents the mature stage in the life

of the mesoscale convective system (which undergoes the type of life cycle sche-

matized in Figs. 6.11 and 9.11). During the formative and intensifying stages, the

stratiform rain region is not present, and the cells tend to be more intense. The

numerical-model simulations of thunderstorm-line development discussed in Sec.

8.11 represent these earlier stages of the storm. Those calculations suggest that

the cells may regenerate somewhat periodically for the first few hours of the

9.2 The Squall Line with Trailing Stratiform Precipitation

351

storm's existence. As the low-level cold pool associated with precipitation fallout

strengthens, a slantwise circulation develops, upon which the convective cells are

superimposed as they continue to form. As the older cells are advected rearward

in the sloping flow, the structure of the mature stage of the system indicated in

Fig. 9.13 evolves. This structure persists for

5-10

h and is accompanied by large

amounts of stratiform rain falling from the trailing region.

The forward overhang of cloud and precipitation shown in Fig. 9.13 is not

always seen. Its presence depends on the wind shear normal to the line in the

environment. When there is strong relative flow toward the line at upper levels,

there is no upper-level overhang. The example in Fig. 9.8 is of this latter type.

The strength of the rear inflow entering the stratiform region of the mature

storm varies considerably from case to case, and it is associated with a variety of

horizontal circulation patterns. Figure 9.14 shows two possible horizontal configu-

rations with which the vertical section in Fig. 9.13 could be associated. The rear

inflow could be one branch of a midlevel mesoscale vortex in the trailing strati-

form rain region of an asymmetric squall line (Fig. 9.14b), or it could be associated

with a windshift line that is more or less uniform in the along-line direction of a

symmetric-type system (Fig. 9.14a).

Above the rear inflow, within the stratiform cloud, the layer of upward-sloping

front-to-rear flow emanating from the upper portions of the convective line ad-

vects rearward ice particles falling from the convective cells (asterisk in Fig. 9.13).

The trajectories of these particles (which are similar to the sequence of ice particle

positions 3, 2, and 1 in Fig. 6.11f) lead to the bright band. A stratiform precipita-

tion region is thus produced in which the particles formed in the region of deep

convection, by vigorous riming growth in the convective updrafts, exit the con-

vective drafts and grow thereafter by vapor deposition as they slowly fall through

the gently ascending front-to-rear flow. These ice particles also collect smaller ice

particles and aggregate with each other to form large snow aggregates as they

approach the

DOC

level. When they fall through this level, they melt and produce

the radar bright band and the region of heavier stratiform rain directly below.

The precipitation processes in a stratiform region of the type shown in Fig. 9.13

were shown schematically in Fig. 6.12, when we discussed nimbostratus of the

type that occurs in the vicinity of deep convection (Sec. 6.3). We noted that in

storms of the type shown in Fig. 9.13, only the location of the region of heavier

stratiform rain in the trailing region is explained by the fallout trajectories of the

ice particles produced in the convective updrafts. The correct amount of strati-

form rain is calculated only when the growth of the particles by vapor deposition,

which occurs as they pass through the region of mesoscale ascent, is taken into

account. The ascending front-to-rear flow in the stratiform region is thus both

qualitatively and quantitatively important since its horizontal component spreads

the particles out into the region of the bright band and its vertical component

supplies the moisture for the growth by vapor deposition of the ice particles. The

precipitation processes in the stratiform region are discussed further in Sec.

9.2.3.1.

The pressure field associated with the squall line with trailing stratiform precip-

itation is marked by several characteristic lows and highs (indicated by

Land

H in

B-~+-

Figure 9.14 Two possible midlevel horizontal flow fields with which the vertical section between

A and B in Fig. 9.13 could be associated. (a) Rear inflow is associated with a windshift line that is more

or less uniform in the along-line direction of a symmetric-type system. (b) Rear inflow is one branch of

a midlevel mesoscale vortex in the trailing-stratiform rain region of an asymmetric squall line. The flow

fields represented by streamlines are superimposed on schematic low-level radar reflectivity fields

indicated by contours and shading. Black areas are the regions of highest reflectivity. Gray areas are of

intermediate intensity. (Adapted from Houze

et al., 1989. Reproduced with permission from the

American Meteorological Society.)

200

+------'--....L..--.&...----L

-'-_---l

........

100

II:

:I:

·100

++ +

({

...

200

100

·100

HORIZONTAL

DISTANCE

(km)

Figure 9.15 Horizontal map of the low-altitude radar reflectivity (dBZ) and cloud-to-ground

lightning strike locations in a squall line with trailing stratiform region observed over Kansas at 0351

GMT

June 1985. Positive and negative flashes during a 30-min period centered on this time are

indicated by the appropriate sign. Other aspects of this storm are illustrated in Figs. 9.18,9.31, 9.34,

9.40, 9.41, 9.43, 9.44, 9.45, 9.48, 9.53, and 9.54. (From Rutledge and MacGorman, 1988. Reprinted

with permission from the American Meteorological Society.)

9.2 The Squall Line with Trailing Stratiform Precipitation 353

Fig. 9.13). A wake low (L

occurs at the surface at the back edge

ofthe

stratiform

rain, in association with warming by unsaturated descent. A high (HI) occurs

below the convective region; it is a hydrostatic gust-front high of the type dis-

cussed in Sec. 8.9. A separate, dynamically produced high is also found at the

gust-front leading edge but is not shown here. Also, a surface low (L

z),

associated

with warming by compensating downward displacement at mid- to upper levels is

often noted ahead of the convective line at the surface. In the midtroposphere, a

small, apparently hydrostatic low (L

)

is located below the primary sloping buoy-

ant convective updraft. Farther to the rear, in the vicinity of the melting layer or

just above, is another low (L

)

that is larger in scale and evidently associated with

the positively buoyant air in the stratiform cloud above. A high (Hz) occurs at

upper levels atop the entire mesoscale cloud system in association with the pre-

dominantly buoyant air in the convective region and the ascending front-to-rear

flow. Some of these pressure maxima and minima will be further elaborated on in

later discussions (Sees. 9.2.2.7 and 9.2.3.2).

Like smaller thunderstorms, mesoscale convective systems are electrically ac-

tive. A horizontal map

the CG lightning strikes for an actual squall line with

trailing stratiform precipitation is shown in Fig. 9.15. The leading line of high radar

reflectivity is characterized by a preponderance of negative strikes, while the

trailing region

stratiform precipitation is characterized by positive strikes. This

behavior is similar to that of the multicell thunderstorm (Fig. 8.6), where the

active convective regions are characterized by negative strikes and the dissipating

cell with a bright band is characterized by positive strikes.

The vertical distribution

charge in a squall line with trailing stratiform precip-

itation may be as shown in Fig. 9.16. The leading anvil is postulated to have a

structure similar to the supercell anvil (Fig. 8.14). The part of the stratiform region

immediately behind the active convective zone was found by balloon measure-

ments in one case to have the vertical layering indicated in Fig. 9.16. A region of

positive charge is found at the

-lOoC

level and has been hypothesized to be

associated with the fallout of graupel from the upper levels of the convective

region.F' This temperature regime is on the warm side of the charge-reversal

temperature (Sec. 8.1). Therefore, the graupel would be expected to become

positively charged as a result of collisions with smaller ice particles at these levels.

The distinction between the leading convective region and the trailing strati-

form region in Fig. 9.13 is not a sharp one in terms of air motions and thermody-

namics. The ascending front-to-rear current of buoyant air and the descending

rear-to-front current of negatively buoyant air flow continuously across the entire

storm. However, the distinction in terms of the precipitation process is clear. As

noted in Sec. 6.1.1, the strength of the vertical air motion as compared to the fall

speeds

the precipitation particles determines whether the mechanism is convec-

tive or stratiform. The more intense, localized, and transient vertical air motions

in the leading zone make the precipitation processes there tend to be of the

convective type, while the predominance of gentler, more widespread, and

253 See Rutledge et al. (1990) and Schuur et al. (1991).

354 9 Mesoscale Convective Systems

Figure 9.16 Schematic of the electrical structure of a convective line with trailing stratiform

structure observed over Oklahoma. The main rear-to-front and front-to-rear circulation components

are indicated by double streamlines; they are separated at the leading edge of the storm by the gust

front (frontal symbol). Solid contours indicate areas of radar reflectivity at two intensity levels (both

>35 dBZ). Light shading indicates regions of negative charge. The distribution of charge in the trailing

anvil was deduced from measurements taken on the indicated balloon ascent. The distribution of

charge in the leading anvil is assumed to be like that in other thunderstorm anvils (e.g., Fig. 8.14). The

dashed contour (adapted from the model results in Fig. 6.14b) indicates a local maximum in graupel

mixing ratio, with particles falling along the indicated trajectory; it is introduced to explain the layer of

positive charge at the

-10°C

level. The distribution of charge in the convective region (large + and -

signs) is assumed to be like that in a mature thunderstorm cell in a single-cell or multicell storm (Figs.

8.3 and 8.6). (From Schuur

et al., 1991. Reprinted with permission from the American Meteorological

Society.)

longer-lasting vertical air motions to the rear comprise a zone favoring the strati-

form precipitation mechanism. To facilitate the more detailed examination of the

storm features indicated qualitatively in Fig. 9.13, we will divide the discussion in

the remainder of this section into parts corresponding to the storm's two different

precipitation regimes. In Sec. 9.2.2, we will consider details of the convective

region. Then in Sec. 9.2.3, we will examine the stratiform region.

9.2.2 The Convective Region

In Figs. 9.13 and 9.14, we have seen the general structure of the squall line with

trailing stratiform precipitation in a qualitative and idealized form. We now de-

scribe this type of storm more quantitatively.

is possible to proceed beyond the

conceptual model because the circulations in several of these storms have been

documented in special field experiments (utilizing Doppler radar, aircraft, sound-

ing, and surface instrumentation) and because they have been successfully simu-

lated in numerical models of the type discussed in Sec. 7.5.3. In this subsection,

we will concentrate on the structure and dynamics of the convective region. We

will begin by looking at the kinematic and thermodynamic structure of the convec-

tive cells. Then we will consider some aspects of the life cycles of individual cells

and their interaction with the stratosphere. Finally, we will examine some of the

factors relevant to the maintenance of the convective region. In Sec. 9.2.3 we will

examine aspects of the stratiform region.

9.2 The Squall Line with Trailing Strlltiform Precipitation 355

9.2.2.1 Observed Airflow

A typical example of the airflow pattern inferred by multiple Doppler radar

synthesis (Sec. 4.4.6) in the convective region of a mesoscale convective system is

shown in Fig. 9.17. The main current of slantwise upward flow toward the rear of

the system is evident. Superimposed on this upward flow are sharp bumps corre-

sponding to each convective cell within this cross section through the convective

region. Each bump in the flow is seen to be associated with a reflectivity maxi-

mum, with each successive peak reaching a higher altitude in a stair-step fashion.

The second stair step corresponds to the strongest reflectivity cell. Another exam-

ple, for which isotachs of vertical velocity are shown, is in Fig. 9.18. Again the

updraft peaks are found at successively higher altitudes. In this case, strong

downdraft cells are also seen in the convective region. There are two types of

these downdrafts. Low-level downdrafts are associated with the heavy rain

showers. These evidently are the precipitation-driven downdrafts typically associ-

ated with the rainshafts and hailshafts of thunderstorms (Sees. 8.2, 8.3, 8.9, and

8.10). They feed the pool of cold air that accumulates below the convective region

and whose leading edge is the gust front forming the leading edge of the mesoscale

system (Fig. 9.13). In addition to the low-level downdrafts are upper-level down-

drafts that tend to occur on either side of the updraft cores. They are apparently

forced by the downward pressure perturbation force found on either side of a

buoyant element (Fig. 7.1).

can be shown by application of the appropriate

boundary condition to (7.8) that this downward force becomes intensified when

the buoyant element approaches a rigid upper boundary.

9.2.2.2 Thermodynamic Structure

The thermodynamic structure of mesoscale convective systems is much more

difficult to document than the airflow, which can be derived from Doppler radar

observation. Some data on the thermodynamic structure have been obtained dur-

ing aircraft missions. The most significant thermodynamic feature of the squall

line revealed by aircraft data is the existence of a small, low-pressure center,

about 10 km wide, located under the upward- and rearward-sloping channel of

buoyant air in the convective region in Fig. 9.19. The low-pressure area was found

by averaging measurements from several flights through squall lines.

is appar-

ently centered beneath the strongest cell in the convective region. Thus, it is

associated with the buoyancy component of the perturbation pressure

(p~),

which

is required to be a minimum at the base of a buoyant column as illustrated by the

lines of force in Fig. 7.1. The aircraft-measured pressure perturbation minimum is

roughly in hydrostatic balance with the buoyancy perturbation, also measured by

aircraft in the column of air above. From the aircraft data alone, it is not apparent

whether there is a dynamic contribution to this pressure perturbation minimum in

addition to the buoyancy contribution. We will see below that model simulations

indicate that the pressure perturbation minimum is largely hydrostatic but that in

strong squall-line circulations there is also a significant contribution from the

dynamic pressure perturbation

(pi» associated with the characteristic vortex in

the gust-front head [Sec. 8.9, Eq. (8.44), Fig. 8.42].

15-r-------..,.."..-----..,..,.......--

....

40·

20 30

DISTANCE

(kin)

20ms·

lc10'~'40~~$2

o 10

-e

Figure 9.17 Airflow pattern inferred by multiple Doppler radar synthesis in the convective region

of a tropical squall-line system observed by dual-Doppler radar in Ivory Coast, west Africa, on

23 June

1981. System is moving from right to left. Vertical arrow indicates scale of airflow vectors. Horizontal

arrow C shows velocity of individual convective cells. Airflow vectors are computed relative to the

cells. Contours show radar reflectivity (dBZ). This storm is also represented in Fig.

9.21. (From Roux,

1988. Reprinted with permission from the American Meteorological Society.)

40 60

Vertical

Velocity

-1,

' \

I-

,-'

I I

" ,

-1

\ f

\/1

J I

:-1/"

(a)

dBZ

10~~

I-

15/

f2S

iB1r25~/,\

(b) -60

-40 -20

DIS

TAN

C E (krn)

Figure 9.18 Isotachs of (a) vertical velocity (m

S-l)

and (b) radar reflectivity (dBZ) in the

convective region of a squall line with trailing stratiform precipitation. Other aspects of this storm are

illustrated in Figs.

9.15, 9.31, 9.34, 9.40, 9.41, 9.43, 9.44, 9.45, 9.48, 9.53, and 9.54. (From Houze,

1989. Reprinted with permission from the Royal Meteorological Society.)

9.2 The Squall Line with Trailing Stratiform Precipitation 357

-60

.....

-40

-40 -40

\ ,

'_I

O'-:--l.---.Jl.---.Jl.----.J_.::.J~~=___.:..I__....:~~:._...J

-48

-42

-36

-30

-24

-18

DISTANCE

(km)

Q 3

Figure 9.19 Composite pressure perturbation field (contour interval 20 Pal based on aircraft data

obtained in tropical squall lines moving faster than 7 m

S-I.

(Adapted from LeMone et al., 1984.

Reproduced with permission from the American Meteorological Society.)

Since aircraft flight data are so scarce, it has been necessary to derive the

thermodynamic fields in the squall line with trailing stratiform precipitation, as

well as their cloud and precipitation content, by more indirect means from data

that have better coverage but do not directly indicate the thermodynamic vari-

ables. A productive method has been to apply the retrieval method to Doppler

radar data (Sec. 4.4.7). According to this technique, the thermodynamic and

microphysical fields consistent with radar-observed reflectivity and velocity fields

can be estimated from the momentum, thermodynamic, and water-continuity

equations. Examples of water and thermodynamic fields derived by this method-

ology are shown in Fig. 9.20 for the tropical squall-line system whose radar

reflectivity structure was shown in Figs. 9.7 and 9.8. The first cross section (Fig.

9.20a) shows the Doppler radar-derived wind and the observed radar reflectivity

in the convective region (very similar to the case in Fig. 9.17), while Fig. 9.20b

shows isotachs of the vertical velocity component (similar to Fig. 9.18, except

downdrafts are not well represented in this analysis). The retrieved rainwater,

cloud water, and potential temperature perturbation fields are shown in Fig.

9.20c-e, respectively. The derived rainwater field in Fig. 9.20c is seen to be

similar to the reflectivity field in Fig. 9.20a, while the cloud water content in Fig.

9.20d is consistent with the vertical velocity field in Fig. 9.20b. Maxima of cloud

water are located in the regions of updraft peaks. Saturation deficits (negative

values) are found in the downdraft zones at low levels. The retrieved thermal field

in Fig. 9.20e shows cold air with temperature perturbation ranging from

O°C

- 5°C in the outflow pool. The

O°C

isotherm has been superimposed on the vertical

velocity pattern in Fig. 9.20b to illustrate how the updraft cells are triggered at the

leading edge of the cold pool and rise over it as the air above the cold pool flows

358 9 Mesoscale Convective Systems

6 12 18 24 30 36 42 48 54

DISTANCE (kin)

5.0

2.5

0+-¥~.....c;;..c:..--.--

.......

-r'C::;===1

o 6 12 18 24 30 36 42 48 54

DISTANCE (kin)

motlOn

10.0

7.5

5.0

2.5

-e

:[10.0

7.5

(d) ( 1 3

<;;_

!1:::

~Z~

~)~~ZJ

~25~~

c.QJ_

+----."'-'-.!fL---f::::"-..----T---''r--.--,--1

-l=-::.Lr-1C..

l'"

:"-

":::

::;==r=-

1.;

=;==r===;===;r==::::J

54 0 6 12 18 24 30 36 42 48 54

DISTANCE (kin)

10.0

7.5

5.0

2.5

-c

(e)

:[100

W 7.5

5.0

2.5

0 6 12

Figure 9.20 Airflow, water, and thermodynamic fields in the convective region of the west African

tropical squall line with trailing stratiform precipitation shown in Figs. 9.7, 9.8, and 9.47. Airflow was

determined by synthesis of dual-Doppler radar observations. The other fields were obtained from the

flow pattern by the retrieval method. (a) Airflow (vectors; 15 m S-I lengths indicated at upper left) and

reflectivity (dBZ). (b) Vertical air velocity (m S-l). (c) Precipitation water (rain below 6 km, graupel

above) mixing ratio (g kg-I). (d) Cloud water (positive) and saturation deficit (negative) mixing ratio (g

kg:"). (e) Temperature perturbation (K). (From Hauser

et al., 1988. Reprinted with permission from

the American Meteorological Society.)

rearward. The cold air below the updrafts is seen to be sinking. Figure 9.20e

shows that the air in the updraft zones has a positive temperature perturbation of

up to 3°C. In cases in which the updrafts are stronger than those indicated in Fig.

9.20b, the thermal perturbation can be expected to be somewhat

higher-perhaps

twice the values seen in this example.

Another example of retrieved thermodynamic fields in the convective region of

a tropical squall line system is shown in Fig. 9.21. The retrieved total buoyancy is

in Fig. 9.21a. The total buoyancy is defined here as in (2.50), so that it contains the

effect of condensed water as well as temperature, vapor, and pressure contribu-

tions.

is expressed here in temperature units by the quantity ()a defined in (4.47).

was not decomposed further into its various components in this case. Thus, the

condensate contributes negatively, counterbalancing some of the positive buoy-

ancy associated with positive potential temperature and water vapor anomalies.