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Structure and maintenance of squall lines:
A historical overview
Robert FovellUCLA Atmospheric and Oceanic Sciences
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”
• 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)
• “[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
• Long-lived
• Unsteady and multicellular
• Evaporationally-produced subcloud cold pools
• Cold pool is principal propagation mechanism
Squall line vertical x-section
Storm-relative flow in storm and far-field;note non-constant shear and upshear tilt
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)
Early models of squall circulation
“[T]he downdraft is drawn as continuous fromcloud top to base for the sake of discussion, thoughthere are inadequate observations to verify whether
this is typical.”
Newton (1963)
Newton (1966)
Early models of squall circulation
“[N]o appreciable portion of the updraft airis likely to descend again to the lower troposphere.”
Zipser’s (1977) model
Transience permits this in 2D (e.g., Rotunno et al. 1988;Fovell and Ogura 1988)
Pressure perturbations in and near squall lines
LeMone et al. (1984)
Both buoyancy and dynamic pressure contribute,dominated by former (Fovell and Ogura 1988)
Rear inflow current
Colored field: temperature perturbation;Contoured field: horizontal velocity perturbation
The multicell storm
Browning et al. (1976)
Fovell and Tan (1998)
Unsteadiness representsepisodic entrainment owingto local buoyancy-inducedcirculations.
The severe squall line environment
From 10 years of severe spring Oklahoma stormsBluestein and Jain (1985)
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)
• 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 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)
“[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 lineacross the country, it is most important to
follow the progress of the pressure jump line,And not… the line of convective activity.”
Newton (1950)
“[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 explainthe maintenance of squall-line activity
since it is frequently observed that large rain-cooledareas [persist] after squall-line activity dissipates”
On shear
“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)
Hane (1973)
“[T]he system, rather than reaching a quasi-steady state,undergoes a series of developments…” owing to
the “adverse effects” of 2D
Hane (1973)
[T]he squall line thunderstorm, once initiated, maintains itself”…as long as it remains in a favorable environment.
Discussion of Hane (1973)
• Convection strong prior to cold pool development
• Storm weaker, more intermittent after pool appearance
• Upshear tilt
Thorpe et al. (1983)
This nearly steady storm “required strong low-levelshear to prevent the upstream gust front from propagating
rapidly away from the storm.”
Thorpe et al. (1983)
This nearly steady storm “required strong low-levelshear to prevent the upstream gust front from propagating
rapidly away from the storm.”
Summary
• Historical overview (incomplete)
• Modern conceptual model of a TS squall line
• Evolution of squall line conceptual models
• Conceptual models of squall line evolution