1. Convective: Part 2 Weather Systems – Fall 2015 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 motion The storm motion vector can also be plotted on the hodograph Then, the storm-relative winds can also be plotted c

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

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

11. Helicity

13. 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. 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 m 2 /s 2 for right-moving storm (!!!)

17. Except… Weisman and Rotunno (2000) point out that supercells are also observed in environments with straight hodographs −The helicity theory would require fundamentally different dynamics for this to occur They point out that the helicity argument assumes a priori that a steady, propagating updraft exists −Better for after the fact explanation, rather than prediction Does the storm rotate because it propagates, or does it propagate because it’s rotating? Weisman and Rotunno argue that it is the latter To understand the development and maintenance of a rotating storm, it’s better to consider the effects of shear on an updraft −In this theory, the formulation of the dynamics of supercells with straight vs. curved hodographs are basically the same Weisman, Morris L., Richard Rotunno, 2000: The Use of Vertical Wind Shear versus Helicity in Interpreting Supercell Dynamics. J. Atmos. Sci., 57, 1452–1472.

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

20. Dallas/Fort Worth tornadic supercells from 2012

21. Six-state supercell, March 2006

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

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

24. Horizontal and vertical vorticity equations 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 Vertical vorticity Horizontal vorticity

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

26. 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

27. 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: 1.easterly storm-relative flow prevents the cold outflow from surging eastward ahead of the storm 2.Lifting pressure gradients reinforce new updraft growth on the southern and northern flanks of the storm

28. Storm Splitting y vertical perturbation pressure gradient 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

29. 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

30. STORM SPLITTING

31. A high perturbation pressure will exist where the shear vector 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 straight hodograph curved hodograph

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

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

34. Markowski and Richardson Straight hodograph

35. Markowski and Richardson Curved hodograph

36. 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

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

38. Convective storm matrix http://www.meted.ucar.edu/convectn/csmatrix http://www.crh.noaa.gov/images/unr/soo/scm/Hodograph _Spreadsheet.xls http://www.crh.noaa.gov/images/unr/soo/scm/Hodograph _Spreadsheet.xls Interactive hodograph spreadsheet

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