Squall Lines

1. Squall Lines M. D. Eastin

2. Squall Lines • Definitions • Mesoscale Convective Systems • Squall Lines • Environmental Characteristics • Structure and Conceptual Model • Three General Types • Classic 2-D Structure • 2-D Evolution • 3-D Evolution • Bow Echoes • Forecasting M. D. Eastin

3. Definitions • Mesoscale Convective System (MCS): • Any ensemble of thunderstorms producing a contiguous precipitation area >100 km2 • Coriolis force plays a role in their evolution • Result from either: (1) Widespread, strong forcing along an air-mass boundary • (2) Upscale growth of multi-cell convective storms • Common examples include: • Squall Lines / Bow Echoes / Line-Echo Wave Patterns (LEWP) • Mesoscale Convective Complexes (MCCs) Squall Line over Missouri Bow Echoes / LEWP over Indiana Developing MCC over Nebraska M. D. Eastin

4. Definitions • Squall Lines: • Loosely defined as aquasi-linear collection of • ordinary cells with finite length that contains a • stratiform rain region either behind, parallel to, • or ahead of the convective line. • There is no strict length definition (100 – 2000 km) • Long lived (2-15 hours) • Tend to occur at night • Primarily quasi-2D (linear) but contain 3-D structure • Can produce weak tornadoes, large hail, localized • flash flooding, and severe straight-line winds M. D. Eastin

5. Environment • Basic Characteristics: • A linear forcing mechanism is required to • organize the early convection: • Cold / warm front or dryline • Topographic features • Linear outflow from prior convection • Enhanced upper-level lift (jet streaks) • Mid-level dry layer is needed to sustain the • persistent gust front outflow that helps • initiate new convective cells along the line • Some CAPE is required (> 500 J/kg), but • severe weather usually develops in more • unstable environments (> 2500 J/kg) • Moderate deep-layer shear (> 10 m/s • below 6 km) is required to maintain • updraft/downdraft separation • Strong low-level shear (> 15 m/s below 3 km) • is optimal → severe weather is common • when the shear is large M. D. Eastin

6. Environment • The Importance of Low-Level Shear: • For a given CAPE, the strength and • longevity of a squall line increases with • increasing strength of the low-level shear • It is the vectorcomponent of low-level • shear perpendicular to the line that is • most critical for squall line evolution • For mid-latitude squall line environments • we can classify the 0-3 km AGL vertical • shear strengths as: • Weak = <10 m/s • Moderate = 10-18 m/s • Strong = >18 m/s M. D. Eastin

7. Structure and Conceptual Models Three General Mature Structures: From Parker and Johnson (2000) M. D. Eastin

8. Structure and Conceptual Models • Trailing Stratiform (TS) Squall Lines: • Strong convection along leading edge with stratiform • precipitation trailing behind the line • Account for ~70% of all squall lines • Average values: Duration = 12.2 hrs • Line Motion = 13.0 m/s • CAPE = 1605 J/kg • LI = -5.4 K • Strong low and mid-level cross-line flow • Moderate upper-level along-line flow Stratiform Precipitation Line Motion Along Line Cross Line Along Line Flow (m/s) Cross Line Flow (m/s) From Parker and Johnson (2000) M. D. Eastin

9. Structure and Conceptual Models • Leading Stratiform (LS) Squall Lines: • Strong convection along trailing edge with stratiform • precipitation leading the line • Account for ~15% of all squall lines • Average values: Duration = 6.5 hrs • Line Motion = 7.1 m/s • CAPE = 1009 J/kg • LI = -3.5 K • Moderate low and upper-level flow (cross and along line) • Weak mid-level flow Line Motion Stratiform Precipitation Along Line Cross Line Along Line Flow (m/s) Cross Line Flow (m/s) From Parker and Johnson (2000) M. D. Eastin

10. Structure and Conceptual Models • Parallel Stratiform (PS) Squall Lines: • Strong convection along the up-wind segment with • stratiform precipitation located downwind • Account for ~15% of all squall lines • Mean values: Duration = 6.3 hrs • Speed = 11.4 m/s • CAPE = 813 J/kg • LI = -2.2 K • Strong low-level cross-line flow • Strong mid and upper-level along-line flow Stratiform Precipitation Along Line Line Motion Cross Line Along Line Flow (m/s) Cross Line Flow (m/s) From Parker and Johnson (2000) M. D. Eastin

11. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Numerous observational studies have identified common structural characteristics Adapted from Houze et al. (1989) M. D. Eastin

12. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Ascending Front-To-Rear (FTR) Flow: • Results from forced ascent along gust front and buoyancy forces • Strong updraft, heavy precipitation, and strong latent heating along leading edge • in association with developing convection • Weak updraft, stratiform precipitation, and less latent heating in rear in association • with decaying convection From Houze et al. (1989) M. D. Eastin

13. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Convective Downdrafts and Gust Dront: • Mid-level downdrafts maintained by evaporation and water loading, as well as • the near-surface meso-high and meso-lows (more on these later) • Gust front helps initiate new convection via forced ascent of low-level inflow From Houze et al. (1989) M. D. Eastin

14. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Descending Rear-to-Front (RTF) Flow: • Results from a combination of dynamic and buoyancy forces associated with the • environmental vertical shear, gust front, and ascending front-to-rear flow, as well • as evaporational cooling and mesoscale pressure gradients (more on these later) • Helps keep the leading-edge convection “upright” • Can contribute to the gust front if it descends to the surface From Houze et al. (1989) M. D. Eastin

15. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Mid-Level Meso-Low: • Result from warm buoyant air (and latent heat release in the clouds) located above the • cold air associated with the gust front (and evaporative cooling) beneath cloud base • Hydrostatic effect of warm air above cold air (…recall the hypsometric equation) Warm Warm Warm Warm Warm Cold Cold Cold Cold From Houze et al. (1989) M. D. Eastin

16. Structure and Conceptual Model • Trailing Stratiform Squall Line Structure: • Surface Pre-Squall Low: • Results from a combination of warm air aloft in the spreading anvil cloud and • adiabatic heating associated with descending mid-level flow in response to • the leading edge convection • Hydrostatic effect of heating at multiple levels Warm Warm Warm Warm Warm Warm Cold Cold Cold Cold From Houze et al. (1989) M. D. Eastin

17. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Surface Meso-High: • Results from the continuous “pooling” of cold air near the surface by negatively • buoyant downdrafts driven by water loading and evaporational cooling • Hydrostatic effect of the surface cold pool Warm Warm Warm Warm Warm Cold Cold Cold Cold From Houze et al. (1989) M. D. Eastin

18. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Surface Wake Low: • Results from a combination of warm air aloft in the spreading anvil cloud and • adiabatic heating associated with the descending rear inflow • Hydrostatic effect of heating at multiple levels • Often marks the edge of the trailing stratiform precipitation and surface cold pool Warm Warm Warm Warm Warm Warm Cold Cold Cold Cold From Houze et al. (1989) M. D. Eastin

19. Structure and Conceptual Models • Trailing Stratiform Squall Line Structure: • Weak-Moderate Shear • New cells develop on downshear side of • initial cold pool and are advected upshear • Well defined stratiform rain region forms • Cold pool and meso-high intensify with the • help of the descending rear inflow • Gust front surges outward, well ahead of the • leading line of convection → systems decays Shear M. D. Eastin

20. Structure and Conceptual Models • Classic Squall Line Structure and Evolution: • Moderate-Strong Shear • Initial development is the same • Cold pool and meso-high intensify • Strong low-level inflow prevents outward surge • of the gust front and enhances forced ascent • Leading edge convection intensifies • Long-lived squall line with less stratiform rain • Some cells exhibit a “bow” structure Shear M. D. Eastin

21. 2-D Evolution • Important Physical Processes: • Buoyancy • Buoyancy (or temperature) gradients produce • local circulations, mesoscale pressure anomalies, • and air flow accelerations • Vertical Shear • Interaction between the cold pool and the • low-level vertical shear generate leading edge • convection that tilts either upshear, vertically, or • downshear depending on the relative strengths • of the cold pool and low-level shear Upshear Downshear M. D. Eastin

22. 2-D Evolution Initial Development of Ascending FTR flow: A. Initial updraft tilts downshear due to the shear in the ambient flow (no cold pool) B. As the convection produces precipitation, a surface cold pool is generated The horizontal vorticity associated with the cold pool begins to balance the low-level shear in the environment Therefore, the updraft becomes upright and the convective cells are quite strong C. As the cold pool gets stronger, it’s horizontal vorticity becomes larger than that in the low-level environmental shear Therefore, the updraft tilts upshear, creating the front-to-rear flow A B C Upshear Downshear M. D. Eastin

23. 2-D Evolution • Development of Descending RTF Flow: • D.Since the updraft is associated with warm, positively • buoyant air and the cold pool with negatively • buoyant air, the mid-level meso-low is created • beneath the warm ascending front-to-rear updraft • and the meso-high is created within the cold pool • E. The meso-low creates a horizontal pressure gradient • that accelerates air at mid-levels from the rear of • the system toward the leading edge • This air flow is often called the Rear-Inflow Jet (RIJ) D Upshear Downshear E M. D. Eastin

24. 2-D Evolution • Development of Descending RTF Flow: • If the updraft contains very warm, positively • buoyant air, the meso-low will be very strong • and generate a very strong rear-inflow jet • Will occur when CAPE is large (> 2000 J/kg) • and/or the Lifted Index is large • The rear-inflow jet will, therefore, be weaker • when CAPE and/or the Lifted Index are • smaller M. D. Eastin

25. 2-D Evolution • Development of Descending RTF Flow: • If the buoyancy gradient associated with the warm air • in the ascending FTR flow is less than the buoyancy • gradient on the back edge of the cold pool, then the • RIJ will descend to the ground well behind the leading • edge of the system • Often occurs for weak-moderate environmental shear • The RIJ enhances the surface gust front • When the buoyancy gradient associated with the warm • air in the ascending FTR flow is similar in magnitude • with the gradient on the back edge of the cold pool, • the RIJ will tend to remain elevated and descend • only when it reaches the leading edge • Often occurs for strong environmental shear • The RIJ helps keep convection more upright • Often associated with strong bow echo formation M. D. Eastin

26. 2-D Evolution Animation: M. D. Eastin

27. 3-D Evolution • Transition to 3-D Structure: • Later in the evolution of a squall line, it often evolves into a non-linear, 3-D structure. • Numerical simulations have shown that when squall lines have finite length (as they all do), • larger-scale circulations form on the ends of the squall line… • Book-End Vortices • Range in size from 10 – 200 km in scale • Located at mid-levels within the stratiform region behind the leading edge • Also called “line-end” vortices M. D. Eastin

28. 3-D Evolution • Development of Book-End Vortices: • Mechanism #1 • Recall how mid-level rotation was produced • in supercell storms by an updraft tilting the • horizontal vorticity associated with the • environmental vertical shear… • Same process, but for a mesoscale downdraft • The RIJ is often descending • Thus, the descent helps to generate opposite • circulations at each end of the squall line M. D. Eastin

29. 3-D Evolution • Development of Book-End Vortices: • Mechanism #2 • Recall how the cold pool generates horizontal • vorticity along its leading edge due to the strong • horizontal buoyancy gradient… • The ascending FTR inflow can tilt the horizontal • vorticity into the vertical, and generate opposite • circulations at each end of the squall line M. D. Eastin

30. 3-D Evolution • Evolution of Book-End Vortices and the RIJ: • Once book-end vortices develop, their circulation can, in turn, enhance the RIJ by 30-50% • A positive feedback loop develops whereby the RIJ helps generate the line-end vortices • which then enhance the RIJ, allowing the RIJ to intensify the vortices... • This feedback loop is believed to produce bow echoes • With time, planetary vorticity (i.e. Coriolis force) enhances the northern (+) vortex and • weakens the southern (-) vortex • This creates an asymmetric structure, which is often observed M. D. Eastin

31. 3-D Evolution Observed Case: Example of a Squall Line with a Line-end Vortex Observed by WSR-88D Radar From Atkins et al. (2004) M. D. Eastin

32. Bow Echoes • Definition and Basic Characteristics: • A bow-shaped line of convective cells that is often • associated with multiple downbursts, swaths of • damaging straight line winds (or “derechos”), and • weak tornadoes • Key structural features include an intense rear • inflow jet impinging on the core of the bow, with • book-end (or line-end) vortices on both sides of • the rear-inflow jet, behind the ends of the bowed • convective segment • Bow echoes have been observed with scales • between 20 and 200 km, and often have lifetimes • between 3 and 6 hours • At early stages in their evolution, both cyclonic • and anticyclonic book-end vortices tend to be • of similar strength, but later in the evolution, • the northern cyclonic vortex often dominates, • giving the convective system a comma-shaped • appearance RIJ M. D. Eastin

33. Bow Echoes Conceptual Model of Evolution: RIJ Downburts and Wind Damage At Bow Apex Vortices at Line Ends Adapted from Fuijta (1978) M. D. Eastin

34. Bow Echoes • Observed Case: • Dual-Doppler radar observations of a • bow echo from the recent Bow Echo • and MCV Experiment (BAMEX) From Davis et al. (2004) M. D. Eastin

35. Bow Echoes • Observed Case: • Notice how the dual-Doppler • analysis nicely captures the: • Rear-Inflow Jet (RIJ) • Strong leading-edge updraft • Evidence of strong-downdraft • near the surface From Davis et al. (2004) M. D. Eastin

36. Derechos • Definition and Development: • Derecho – A widespread convectively induced • straight-line wind storm. Specifically, a family of • downburst clusters that produce surface wind • gusts greater than 26 m/s over a concentrated • area of at least 400 km2. • Strong RIJ converges with FTR flow at mid-levels • RIJ is forced to descend and is further enhanced • by evaporational cooling and water loading • Produces a family of downbursts at the surface From Atkins et al. (2005) M. D. Eastin

37. Derechos Example of Extensive Damage: Produced $171 million in property and crop damage across Iowa and Illinois From Atkins et al. (2005) M. D. Eastin

38. Tornadoes • Common Locations: • A. At the bow apex • B. South of apex along gust front • C. Within the comma head, behind the • the leading edge convection • Basic Characteristics: • Generally weak (EF0-EF2) • Very hard to detect (rarely exhibit TVS) • Lifetime of 5-10 minutes • Development: • Not well understood! • Believed to result from stretching • localized regions of vertical vorticity • that develop along the gust front • (i.e. the non-supercell mechanism) • Can also occur at the intersection point • between a squall line (or bow echo) • and a pre-existing boundary (a front) C A B M. D. Eastin

39. Gust Front Tornadoes Example of Gust Front Mesovortices in WSR-88D Data Mescocyclone Couplets M. D. Eastin

40. Tornadoes From Atkins et al. (2005) M. D. Eastin

41. Forecasting • Environmental Factors: • Weisman (1993) • Series of numerical • simulations • Conditions favorable • for squall line, bow • echo, and strong rear • inflow jet development • include: • Large CAPE • (> 2000 J/kg) • Strong low-level shear • (> 20 m/s below 3 km) • Dry mid-levels • Moist low-levels • Linear forcing for • the initial ascent • up to the LFC Squall-Line Organization Rear-Inflow Jet Magnitude M. D. Eastin

42. Forecasting • Squall Line Motion: • Individual cells within the squall tend to move in the direction of the 0-6 km mean wind • The overall propagation of the squall line tends to be controlled by the speed and direction • of the system cold pool → new cells are constantly triggered along its leading edge • Cold pool speeds is can be on order of ~20 m/s (~40 kts) • Simple Guidance: Squall lines move at 40% of the 500mb wind speed, in the same direction M. D. Eastin

43. Forecasting • Onset of Downbursts and Derechos: • Examine the Doppler radial velocities and • look for evidence of a Mid-Altitude Radial • Convergence (MARC) zone near the • apex of “bowing” squall lines segments • Provide small lead-time forecast for the • onset of and downbursts and damaging • straight-line winds M. D. Eastin

44. Squall Lines • Summary • Definitions • Mesoscale Convective System • Squall Line • Environmental Characteristics • Structure and Conceptual Model • Three General Types (structure, basic flow patterns) • Classic 2-D Structure (basic flow patterns) • 2-D Evolution (physical processes) • 3-D Evolution (physical processes) • Bow Echoes (definition, structure, physical processes) • Forecasting M. D. Eastin

45. References Atkins, N.T., J.M. Arnott, R.W. Przybylinski, R.A. Wolf, and B.D. Ketcham, 2004: Vortex structure and evolution within bow echoes. Part I: Single-Doppler and damage analysis of the 29 June 1998 derecho. Mon. Wea. Rev., 132, 2224-2242. Atkins, N.T., C.S. Bouchard, R.W. Przybylinski, R.J. Trapp, and G. Schmocker, 2005: Damaging surface wind mechanisms within the 10 June 2003 Saint Louis bow echo during BAMEX. Mon. Wea. Rev., 133, 2275-2296.  Bluestein, H. B., and M. H. Jain, 1985: Formation of mesoscale lines of precipitation: Severe squall lines in Oklahoma during spring. J. Atmos. Sci., 42, 1711-1732. Davis, C. A., and Coauthors, 2004: the Bow Echo and MCV Experiment: Observations and opportunities. Bull. Amer. Meteor. Soc., 85, 1075-1093. Fovell, R. G., and Y. Ogura, 1988: Numerical simulation of a mid-latitude squall line in two dimensions. J. Atmos. Sci., 45, 3846-3879. Fujita, T. T., 1978: Manual of Downburst identification for Project NIMROD. Satellite and Mesometeorology Research Paper No. 156, Department of Geophysical Sciences, University of Chicago, 104 pp. Houze, R. A. Jr., 1993: Cloud Dynamics, Academic Press, New York, 573 pp. Houze, R. A., Jr., M. I. Biggerstaff, S. A. Rutledge, and B. F. Smull, 1989: Interpretation of Doppler weather radar displays of mid-latitude mesoscale convective systems. Bull. Amer. Meteor. Soc., 70, 608–619 Houze, R. A., Jr., B. F. Smull, and P. Dodge, 1990: Mesoscale organization of springtime rainstorms in Oklahoma. Mon. Wea. Rev., 118, 613-654. M. D. Eastin

46. References Johns, R. H., and W. D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Wea. Forecasting, 2, 32-49. Parker, M. D., and R. H. Johnson, 2000: Organizational modes of mid-latitude mesoscale convective systems. Mon. Wea. Rev., 128, 3413-3436. Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory for strong long-lived squall lines. J. Atmos. Sci., 45, 463- 485. Wakimoto, R.M., H.V. Murphy, A. Nester, D.P. Jorgensen, and N.T. Atkins, 2006: High winds generated by bow echoes.  Part I:  Overview of the Omaha bow echo 5 July 2003 storm during BAMEX.  Mon. Wea. Rev., 134, 2793-2812. Wakimoto, R.M., H.V. Murphy, C.A. Davis, and N.T. Atkins, 2006:  High winds generated by bow echoes.  Part II:  The relationship between the mesovortices and damaging straight-line winds.  Mon. Wea. Rev., 134, 2813-2829. Wheatley, D.M., R.J. Trapp, and N.T. Atkins, 2006:  Radar and damage analysis of severe bow echoes observed during BAMEX.  Mon. Wea. Rev., 134, 791-806. Weisman, M. L., 1992: The role of convectively generated rear-inflow jets int eh evolution of long-lived meso-convective systems. J. Atmos. Sci., 49, 1827-1847. Weisman, M. L., 1993: The genesis of severe long–lived bow echoes. J. Atmos. Sci., 50, 645-669. Weisman, M. L. , and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms. Mesoscale Meteorology and Forecasting, Ed: Peter S. Ray, American Meteorological Society, Boston, 331-358. M. D. Eastin