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Structure and maintenance of squall lines: A historical overview. Robert Fovell UCLA Atmospheric and Oceanic Sciences rfovell@ucla.edu. Scope and Objectives. Historical overview “ Broken lines ” of “ ordinary cells ” having trailing stratiform precipitation

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Structure and maintenance of squall lines a historical overview

Structure and maintenance of squall lines: A historical overview

Robert Fovell

UCLA Atmospheric and Oceanic Sciences

rfovell@ucla.edu


Scope and objectives
Scope and Objectives

  • Historical overview

  • “Broken lines” of “ordinary cells” having trailing stratiform precipitation

  • Evolution of squall line conceptual models

  • Conceptual models of squall line evolution, structure and behavior


Definition of squall line
Definition of “squall line”

  • Glossary of Meteorology (2000):

    “a line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of thunderstorms.”


Newton and newton 1959
Newton and Newton (1959)

  • “[A] squall line generally consist[s] of a large number of thunderstorm cells” with lifetime ~30 min

  • “[C]ontinuous formation of new cells is necessary” created via “successive triggering… by lifting of unstable air over a [rain-produced] ‘pseudo-cold front’”


Characteristics
Characteristics

  • Long-lived

  • Unsteady and multicellular

  • Evaporationally-produced subcloud cold pools

  • Cold pool is principal propagation mechanism



A modern conceptual model e g houze et al 1989

A modern conceptual model(e.g., Houze et al. 1989)




Squall line vertical x section2
Squall line vertical x-section

Storm-relative flow in storm and far-field;

note non-constant shear and upshear tilt


Squall line vertical x section3
Squall line vertical x-section

Radar echo envelope


Squall line vertical x section4
Squall line vertical x-section

Principal echo features; implied multicellularity


Squall line vertical x section5
Squall line vertical x-section

Principal pressure perturbations


Conceptual model of a trailing stratiform ts squall line
Conceptual model of a “trailing stratiform” (TS) squall line

Houze et al. (1989)



An isolated ordinary cell
An isolated “ordinary cell”

Ludlam (1963)


Thunderstorm life cycle
Thunderstorm life cycle

  • The Thunderstorm Project (Braham’s reminiscence)

    • Aug. 1940: DC-3 crash killed Minnesota senator during storm

    • 1944: Civil Aeronautics Board called for study of storm air motions, after another DC-3 lost lift

    • Jan. 1945: HR 164 authorized Weather Bureau to study thunderstorm causes, characteristics (didn’t become law)

    • End of WWII provided the planes and personnel

    • Project based in Orlando in 1946, Ohio in 1947 (based on storm frequency and military base proximity)



T storms not always isolated
T-storms not always isolated

Horizontal cross-section

U = updraft

D = downdraft

T-storm Project



Early models of squall circulation1
Early models of squall circulation

Newton (1963)

“[T]he downdraft is drawn as continuous from

cloud top to base for the sake of discussion, though

there are inadequate observations to verify whether

this is typical.”


Early models of squall circulation2
Early models of squall circulation

Newton (1966)

“[N]o appreciable portion of the updraft air

is likely to descend again to the lower troposphere.”


Zipser s 1977 model
Zipser’s (1977) model

(reversed for midlatitude context)


Zipser s 1977 model1
Zipser’s (1977) model

Transience permits this in 2D (e.g., Rotunno et al. 1988;

Fovell and Ogura 1988)


Zipser s 1977 model2
Zipser’s (1977) model

Inflow layer overturns in “crossover zone”


Layer lifting
Layer lifting

“Moist absolutely unstable layer” (MAUL)

Bryan and Fritsch (2000)


Pressure perturbations in and near squall lines
Pressure perturbations in and near squall lines

LeMone et al. (1984)

Both buoyancy and dynamic pressure contribute,

dominated by former (Fovell and Ogura 1988)



Mesohigh and wake low
Mesohigh and wake low

Fujita (1955)


Mesohigh and wake low1
Mesohigh and wake low

Fujita (1955)

Johnson and Hamilton (1988)


Pre squall low
Pre-squall low

Pre-squall low ascribed to subsidence warming.

Hoxit et al. (1977)


Rear inflow current
Rear inflow current

Pandya and Durran (1996)


Rear inflow current1
Rear inflow current

Colored field: temperature perturbation;

Contoured field: horizontal velocity perturbation



Rear inflow current3
Rear inflow current

Pandya and Durran (1996)


Rear inflow current4
Rear inflow current

Pandya and Durran (1996)


The multicell storm
The multicell storm

Four cells at a single time

Or a single cell at four times

Browning et al. (1976)


The multicell storm1
The multicell storm

Unsteadiness represents

episodic entrainment owing

to local buoyancy-induced

circulations.

Browning et al. (1976)

Fovell and Tan (1998)


Structure and maintenance of squall lines a historical overview

Life cycle of a tropical

squall line

Leary and Houze (1979)


The severe squall line environment
The severe squall line environment

From 10 years of severe spring Oklahoma storms

Bluestein and Jain (1985)



The severe squall line environment2
The severe squall line environment

Similar in tropical squall lines (below 4 km);

e.g., Barnes and Sieckman (1984)



Some questions leading to very incomplete answers
Some questions maintenance(leading to very incomplete answers)

  • How are pre-frontal squall lines initiated?

  • Is a squall line self-maintaining?

  • Why does the storm updraft airflow lean upshear?

  • What determines how strong a storm can be?


Cold pool and vertical shear
Cold pool and vertical shear maintenance

  • Cold pool and shear are irrelevant

  • Cold pool good, shear bad

  • Cold pool good, shear good

  • Cold pool bad, shear bad, but combination may be good


Tepper 1950
Tepper (1950) maintenance

“[S]quall lines are propagated pressure jump lines,

whose genesis, propagation and destruction

are independent of the precipitation

which they themselves produce.”

“Consequently in following a squall line

across the country, it is most important to

follow the progress of the pressure jump line,

And not… the line of convective activity.”


Tepper 19501
Tepper (1950) maintenance

(Figure augmented)


Newton 1950
Newton (1950) maintenance

“[T]he air above the warm-sector inversion,

if one is present, is usually relatively dry

and a great amount of lifting would be required…”

Cold pools are “insufficient to wholly explain

the maintenance of squall-line activity

since it is frequently observed that large rain-cooled

areas [persist] after squall-line activity dissipates”


On shear
On shear maintenance

“It is remarkable that in spite of the marked

vertical wind shears associated with squall-storms,

they are long-lived, often travelling long distances

at rather uniform speed”

Ludlam (1963)


A role of strong shear
A role of strong shear? maintenance

Newton and Newton (1959)


Upshear tilt
maintenanceUpshear” tilt

Ludlam (1963),

via Rotunno et al. (1988)



Hane 1973
Hane (1973) maintenance

2D model initialized with moderate shear


Hane 19731
Hane (1973) maintenance


Hane 19732
Hane (1973) maintenance


Hane 19733
Hane (1973) maintenance


Hane 19734
Hane (1973) maintenance


Hane 19735
Hane (1973) maintenance


Hane 19736
Hane (1973) maintenance


Hane 19737
Hane (1973) maintenance


Hane 19738
Hane (1973) maintenance


Hane 19739
Hane (1973) maintenance

“[T]he system, rather than reaching a quasi-steady state,

undergoes a series of developments…” owing to

the “adverse effects” of 2D


Hane 197310
Hane (1973) maintenance

[T]he squall line thunderstorm, once initiated, maintains itself”

…as long as it remains in a favorable environment.


Discussion of hane 1973
Discussion of Hane (1973) maintenance

  • Convection strong prior to cold pool development

  • Storm weaker, more intermittent after pool appearance

  • Upshear tilt


Thorpe et al 1983
Thorpe et al. (1983) maintenance

2D model


Thorpe et al 19831
Thorpe et al. (1983) maintenance

Steadiest storm, most precipitation

(amount and intensity)


Thorpe et al 19832
Thorpe et al. (1983) maintenance

This nearly steady storm “required strong low-level

shear to prevent the upstream gust front from propagating

rapidly away from the storm.”


Thorpe et al 19833
Thorpe et al. (1983) maintenance

This nearly steady storm “required strong low-level

shear to prevent the upstream gust front from propagating

rapidly away from the storm.”


Rkw theory
RKW theory maintenance

“Cold pool bad, shear bad,

But combination may be good.”

Rotunno et al. (1988)


My take on rkw theory cold pool not an unalloyed good lifting comes at a price
My take on RKW theory: maintenanceCold pool not an unalloyed good;lifting comes at a price



Cold pool experiment no cloud cloud model
Cold pool experiment maintenance(‘no-cloud cloud model’)

Control run

Deactivate evaporation

cooling


Clouds without cold pool lifting
Clouds without cold pool lifting maintenance

Crook and Moncrieff (1988)


Cell initiation by trapped gravity waves
Cell initiation by trapped gravity waves maintenance

Fovell et al. (2006)


Summary
Summary maintenance

  • Historical overview (incomplete)

  • Modern conceptual model of a TS squall line

  • Evolution of squall line conceptual models

  • Conceptual models of squall line evolution


Structure and maintenance of squall lines a historical overview
end maintenance



Other squall type configurations
Other squall-type configurations maintenance

Parker and Johnson (2000)