Deep Moist Convection (DMC) Part 2 – Modes of Isolated Organization

1. Deep Moist Convection (DMC)Part 2 – Modes of Isolated Organization AOS 453 – Spring 2014 4/3/2014

2. Structure Of DMC Lectures This Week • Tuesday • DMC (Convective/Convection) Initiation • Chapter 7 in MR09 • Isolated DMC Organization (Part 1) • Chapter 8 in MR09 • SPC Mesoanalysis Introduction • Thursday • Isolated DMC Organization (Part 2) • Chapter 8 in MR09 • Diagnostic Indices and Variables for Isolated DMC • SPC Mesoanalysis Continued

3. Tuesday Recap • What is DMC? • Troporpheric Overturning • Parcels lifted to LFC and freely convect throughout significant portion of troposphere • How does DMC get initiated? • Need to reach LFC by either: • Mechanically Lifting Parcels • Air mass boundaries (including density current/outflow boundaries) • Orographic Lifting • Decrease pre-existing CIN to make it easier to overcome (takes a long time) • Via large scale processes • Synoptic UVM • Differential moisture advection • Surface heating • Via manipulating lapse rate (lapse rate tendency) • DMC initiation is a complex problem • Driven largely by mesoscale processes, but can be helped by synoptic scale • Need to account for entrainment. • Drier mid-troposheric air will decrease buoyancy of plumes • Vertical wind shear will increase turbulent mixing across periphery of plumes increasing entrainment and decreasing the buoyancy of plumes

4. Tuesday Recap • Isolated DMC Organization (part 1) • The role of vertical wind shear • Vertical wind shear is vertical gradient of horizontal wind component • Often represented as wind vector difference between two layers • Can plot wind shear using a hodograph • Wind shear alone prevents initiation, but wind shear + buoyancy (CAPE) promotes organization • Bulk Richardson Number • Wind shears role in organization is twofold: 1.) Displaces precipitation away from updraft 2.) Induces a dynamic pressure gradient

5. Structure Of DMC Lectures This Week • Tuesday • DMC (Convective/Convection) Initiation • Chapter 7 in MR09 • Isolated DMC Organization (Part 1) • Chapter 8 in MR09 • SPC Mesoanalysis Introduction • Thursday • Isolated DMC Organization (Part 2) • Chapter 8 in MR09 • Diagnostic Indices and Variables for Isolated DMC • SPC Mesoanalysis Continued

6. Modes of Organization – And their relationship to wind shear

7. Modes of Organization – And their relationship to wind shear

8. Modes of Organization – And their relationship to wind shear

9. Modes of Organization – And their relationship to wind shear

10. Modes of Organization – And their relationship to wind shear

11. Modes of Organization – And their relationship to wind shear

12. Ordinary/Single Cell Convection

13. Single-Cell Convection(Ordinary Thunderstorms) • Think one updraft • Driven by buoyancy • No organized, initiated convection from its outflow • Gust front lifting in associated (weak shear) environments is weak and shallow (no dynamic PGF!) • Usually initiated in concert with several other single cells (usually there is a field of deep convection on satellite imagery) • COLLIDING outflow (not necessarily individual outflow) is often associated with new convection initiation • Not to be confused as “organized” or “multicell” convection. We’ll see the difference soon…

14. Ordinary/Single Cell Convection- 3 Stages In Life Cycle

15. Ordinary/Single Cell Convection- 3 Stages In Life Cycle

16. Ordinary/Single Cell Convection- 3 Stages In Life Cycle Ordinary/Single Cell Lifetime (τ)

17. Ordinary/Single Cell Convection- Like The Mayfly • Can come up with approximate quantitative relationship for lifecycle time, τ • Function of depth of convection: H • Speed of updraft : wup • Speed of precip (terminal velocity): vt

18. Ordinary/Single Cell Convection- Like The Mayfly If H = 10km If H = 10km wup = 5 m s-1 wup = 10 m s-1 vt = 5 m s-1 vt = 10 m s-1 τ ≈ 1 hrτ ≈ 30 min

22. Multicell Thunderstorms

23. Multicell Thunderstorms • ♬ Just a spoon-full of wind shear helps the thunderstorm live long…the thunderstorm live long…the thunderstorm live long ♬

24. Multicell Thunderstorms • 0-6km wind shear ~ 10-20 m s-1 • So there is environmental HORIZONTAL vorticity (prove it to yourself) • NOT just one individual updraft (so last slide’s little song number was a little deceiving) • Multiple ordinary/single cells going through life cycle one after the other • Driven by cold pool propagation • Organized initiation of new cells along leading edge outflow boundary • New cells initiate along the downshear outflow boundary or gust front • Can think of collective low-level outflow (cold pool) as a thunderstorm “machine” or “conveyor belt”

33. Why Does Wind Shear Matter? • How come the same organization doesn’t occur in ordinary/single cell convection? • Multicell convection is composed of a bunch of ordinary/single cells that have same life cycle, so why in ordinary/single cell case do we not see the same organization?

34. First Need To Discuss Baroclinically Generated Vorticity and Vortex Rings WARM / LESS DENSE WARM / LESS DENSE COLD / DENSE WARM / LESS DENSE WARM / LESS DENSE

35. First Need To Discuss Baroclinically Generated Vorticity and Vortex Rings WARM / LESS DENSE WARM / LESS DENSE COLD / DENSE WARM / LESS DENSE WARM / LESS DENSE

36. First Need To Discuss Baroclinically Generated Vorticity and Vortex Rings WARM / LESS DENSE WARM / LESS DENSE COLD / DENSE WARM / LESS DENSE WARM / LESS DENSE

37. Once It Hits The Ground…

38. Once It Hits The Ground…

39. It Spreads Radially Away From Center Of Maximum Pressure (directly beneath the downdraft) H

40. It Spreads Radially Away From Center Of Maximum Pressure (directly beneath the downdraft)

41. Who cares? • Interaction with environmental wind shear promotes enhanced vertical lifting along downshear outflow boundary! • Compared to ordinary/single cell case • Better/more-likely for lifting to lift air to LFC and initiate new DMC

43. Who cares about the wind shear? • Taller downshear outflow boundary • More mechanical lifting • Cooperative horizontal vorticity between baroclinically generated vortex ring and environmental shear

45. WHAT DIRECTION IS THE SHEAR VECTOR?