1. Convective: Part 2 Weather Systems – Fall 2017
Outline:
a. dynamics of rotating thunderstorms (supercells)
b. storm splitting – right vs. left movers

2. Last time… Identified the main classifications of convective storms
Looked at how vertical wind shear affects the mode of convection
Saw how we can use a hodograph to look at the shear

3. Questions for today…
What is helicity and how does it relate to convective storms?
Why do updrafts rotate?
Why can supercells maintain their intensity for so long?
What causes storm splitting?
Why do supercells move in the directions they do?

4. The hodograph A way to visualize the vertical wind shear
Using polar coordinates, a point is plotted at the tip of the wind vector at each level
The tangent line represents the shear vector

5. Storm-relative motionc
The storm motion vector can also be plotted on the hodograph
Then, the storm-relative winds can also be plotted

6. Storm-relative motionc
The area signed out underneath this is the storm-relative helicity
This is often calculated from 0-1 or 0-3 km

7. Storm Relative Environmental Helicity (SREH)
A wind profile that maintains a single direction and increases its speed with height generates a shear vector parallel to the wind direction.
This shear results in a horizontal vorticity whose axis (the vorticity vector) is perpendicular to the wind direction.
We refer to this as crosswise vorticity.

8. Storm Relative Environmental Helicity (SREH)
a wind profile whose speed remains constant, but whose direction changes with height generates a shear vector perpendicular to the mean wind.
the resulting vorticity vector is parallel to the mean wind. We refer to this as streamwise vorticity
in the real world, vorticity is rarely perfectly crosswise or streamwise. Thus, when we say streamwise vorticity we refer to the vector component of the vorticity that is oriented parallel to the mean flow.

9. Helicity - Tilting Legend: Pink area = updraft
Blue arrow = shear vector
Red arrow = vertical velocity gradient
Curved arrow = relative vorticity anomaly

10. Storm motion is “on the hodograph” Helicity = 0
Updraft will not rotate
Storm motion is “off the hodograph”
Helicity = large
Updraft likely to rotate
(Markowski and Richardson, adapted from Davies-Jones 1984)

11. Helicity

12. Helicity
updraft
vorticity

13. Helicity
To summarize the development of midlevel rotation within a thunderstorm environment:
Initially a vorticity couplet develops owing to the tilting of environmental horizontal vorticity. This is immediately advected by the storm-relative winds such that, in the case of streamwise vorticity, the rotation and the updraft become more in phase.

14. (Markowski and Richardson, adapted from Markowski et al. 2003)
The wind hodograph
Composite hodograph of > 400 proximity soundings near cyclonic supercells in the US
Note that the storm motion is completely “off the hodograph” – it lies to the right of the wind at all levels!
(Markowski and Richardson, adapted from Markowski et al. 2003)

15. The wind hodograph Wind profiler obs near Hallam, NE F4 tornado
(courtesy Matt Bunkers, NWS Rapid City)

16. The wind hodograph Jackson, MS 1200 UTC sounding, 27 April 2011
0—1-km SRH of 490 m2/s2 for right-moving storm (!!!)

17. Conceptual Model of Tornadic Thunderstorm
(Lemon and Doswell 1979)

19. Dallas/Fort Worth tornadic supercells from 2012

20. Six-state supercell, March 2006

21. Tuscaloosa/Birmingham tornadic supercell, 27 April 2011
(from Brian Tang, NCAR)

22. Supercell dynamics Use anelastic equations (no sound waves)
Also neglect friction and Coriolis (Coriolis unimportant on these scales)

23. Horizontal and vertical vorticity equations
Horizontal vorticity
Can get vertical vorticity by tilting and/or stretching
If no vertical vorticity initially, it must be produced by tilting
No direct baroclinic generation of vertical vorticity

24. vertical perturbation pressure gradient
Vorticity tilting
Consider an isolated updraft in unidirectional shear
Linearize (11.10), assuming no vertical vorticity initially:
vertical perturbation pressure gradient y

25. vertical perturbation pressure gradient
Nonlinear effects
This “spin” term dominates the nonlinear pressure perturbation
This says that pressure perturbations are related to the magnitude of vorticity: there will be low pressure where the magnitude of vorticity is great
Therefore, the strong rotation in the vortex pairs induces low pressure regardless of the direction of rotation
vertical perturbation pressure gradient

26. We have counter-rotating vortices on the flanks of the updraft
We’ve now tilted horizontal vorticity (which exists because of the wind shear) into the vertical
We have counter-rotating vortices on the flanks of the updraft
As the cloud grows, precipitation particles grow and lead to a downdraft
In weak shear, the outflow spreads out in all directions eventually cutting off the supply of warm moist air
In strong shear:
easterly storm-relative flow prevents the cold outflow from surging eastward ahead of the storm
Lifting pressure gradients reinforce new updraft growth on the southern and northern flanks of the storm

27. vertical perturbation pressure gradient
Storm Splitting
vertical perturbation pressure gradient y
The low pressure induced by the counter-rotating vortices leads to two separate enhanced updrafts – the storm splits!
A downdraft is not even required for the storm splitting process, though it can enhance it

28. After the split, one (cyclonic) supercell moves to the right, and one (anticyclonic) to the left of the wind
In unidirectional shear (straight hodograph), both storms now have storm-relative helicity, which enhances the updraft rotation

29. STORM SPLITTING

30. straight hodograph
A high perturbation pressure will exist where the shear vector (green arrows) points toward increasing vertical motion (and vice versa)
In the case of a clockwise-turning hodograph, the pressure perturbations (and helicity) favor the right- moving supercell
curved hodograph

31. straight hodograph
Enhanced upward motion on downshear side: keeps storms from tilting over but doesn’t have an effect on right or left side
curved hodograph
Now, we get enhanced upward motion on the right flank (relative to the shear) – the right mover is enhanced and left mover weakens
Right-mover is favored!

32. Straight vs. curved hodograph
Markowski and Richardson, adapted from Klemp (1987)

33. Straight hodograph
Markowski and Richardson

34. Curved hodograph
Markowski and Richardson

35. Summary – shear-induced pressure gradients
In an environment with vertical wind shear, updrafts tilt horizontal vorticity into the vertical, creating counter-rotating vortices
When the updraft grows, the linear effect keeps it from tilting over in the shear by enhancing upward motion on the downshear side
The nonlinear effect leads to low pressure at both of the vortices – both updrafts strengthen and the storm splits
In an environment with a straight hodograph, the rotation in both storms is enhanced by storm-relative helicity
In an environment with a clockwise-turning hodograph, the right- moving storm is favored:
Storm-relative helicity is greater, therefore greater rotation
Pressure perturbations favor development of new convection on right sides of updrafts

36. Straight vs. curved hodograph
From Davies-Jones et al. (2001); also Table 8.1 in MMM

37. Convective storm matrix
_Spreadsheet.xls
Interactive hodograph spreadsheet

38. Time lapse photography
© 2005, Kevin Tory & Wesley Terwey