Structure and maintenance of squall lines: A historical overview

Robert Fovell UCLA Atmospheric and Oceanic Sciences [email protected] 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 8 July 2003, Lincoln, NE A modern conceptual model (e.g., Houze et al. 1989) Squall line vertical x-section Squall line vertical x-section Squall line vertical x-section

Storm-relative flow in storm and far-field; note non-constant shear and upshear tilt Squall line vertical x-section

Radar echo envelope Squall line vertical x-section

Principal echo features; implied multicellularity Squall line vertical x-section

Principal pressure perturbations Conceptual model of a “trailing stratiform” (TS) squall line

Houze et al. (1989) Evolution of squall line conceptual models An isolated “ordinary cell”

Ludlam (1963) 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) Stages of isolated t-storm

T-storm Project T-storms not always isolated Horizontal cross-section

U = updraft D = downdraft

T-storm Project 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 circulation

Newton (1966)

“[N]o appreciable portion of the updraft air is 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) Zipser’s (1977) model

Inflow layer overturns in “crossover zone” Layer lifting

“Moist absolutely unstable layer” (MAUL) Bryan and Fritsch (2000) Pressure perturbations in and near squall lines

LeMone et al. (1984) Both buoyancy and dynamic pressure contribute, dominated by former (Fovell and Ogura 1988) Pressure perturbations in and near squall lines

Fujita (1963) Mesohigh and wake low

Fujita (1955)

Johnson and Hamilton (1988) Pre-squall low

Pre-squall low ascribed to subsidence warming.

Hoxit et al. (1977) Rear inflow current

Pandya and Durran (1996) Rear inflow current

Colored field: temperature perturbation; Contoured field: horizontal velocity perturbation Rear inflow current Vertical heating profiles (surface to tropopause) height height

Q1+Q2 = “top heavy profile”

Fovell (2002) Rear inflow current

Pandya and Durran (1996) The multicell storm

Four cells at a single time Browning et al. (1976) Or a single cell at four times The multicell storm

Unsteadiness represents episodic entrainment owing to local buoyancy-induced circulations.

Browning et al. (1976)

Fovell and Tan (1998) Life cycle of a tropical squall line

Leary and Houze (1979) The severe squall line environment

From 10 years of severe spring Oklahoma storms Bluestein and Jain (1985) The severe squall line environment The severe squall line environment

Similar in tropical squall lines (below 4 km); e.g., Barnes and Sieckman (1984) Conceptual models of squall line evolution, initiation, and maintenance 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 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 (1950)

(Figure augmented) 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 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

“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?

Newton and Newton (1959) “Upshear” tilt

Ludlam (1963), via Rotunno et al. (1988) Early numerical experiments Hane (1973)

2D model initialized with moderate shear Hane (1973) Hane (1973) Hane (1973) Hane (1973) Hane (1973) Hane (1973) 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)

Steadiest storm, most precipitation (amount and intensity) Thorpe et al. (1983)

This nearly steady storm “required strong low-level shear to prevent the upstream gust front from propagating rapidly away from the storm.” RKW theory

“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 A garden-variety multicell storm Cold pool experiment (‘no-cloud cloud model’)

Control run

Deactivate evaporation cooling Thunderstorm initiation ahead of established squall lines

Fovell (2002), Fovell et al. (2006)

Two superimposed gravity waves • Low-frequency gravity waves (responding to persistent heating and cooling in convective line) • High-frequency gravity waves (intermittent thermal and mechanical disturbances) Vertical heating profiles height (surface to tropopause)

(these alter airflow on the storm’s front side too!) height

Fovell (2002) Fovell (2002) Water vapor perturbations (red > 0, blue < 0)

Fovell et al. (2006) Fovell et al. (2006) Steady heat source

Oscillating heat source

Contoured – vertical velocity Shaded – temperature perturbation

(made with DTDM) Fovell et al. (2006) Cell initiation by trapped gravity waves

Fovell et al. (2006) Summary

• Historical overview (incomplete) • Modern conceptual model of a TS squall line • Evolution of squall line conceptual models • Conceptual models of squall line evolution end