1. 1 Supercell Thunderstorms Adapted from Materials by Dr. Frank Gallagher III and Dr. Kelvin Droegemeier School of Meteorology University of Oklahoma Part II

2. 2 Supercell Updraft Rotation n In order for supercells to rotate, there must be some type of rotation already available in the environment. n We shall consider several different ways of creating vertical vorticity or rotation about a vertical axis:

3. 3 Convergence n Consider your sink. Initially there is some weak rotation, but as the water converges toward the drain, the speed of the rotation increases. This is similar to the way an ice skater speeds up when their arms are pulled in.

4. 4 Horizontal Temperature Differences n Air travelling along a frontal zone will develop a horizontal rotation.

5. 5 Recall the Cold-Air Outflow

6. 6 Vertical Wind Shear n Another method of creating horizontal rotation is by vertical wind shear. Fast Wind Slower Wind

7. 7 Vertical Wind Shear East North Up Westerly Winds Increase in Speed with height

8. 8 Development of Rotation East North Up

9. 9 Tilting n In order to create vertical rotation from horizontal rotation, we must tilt the horizontal rotation into the vertical.

10. 10 Development of Rotation East North Up Thunderstorm

11. 11 Development of Rotation East North Up Updraft - Stretch

12. 12 Tilting n In thunderstorms, this tilting is achieved by the updraft. Updraft

13. 13 Tilting n Viewed from above, we see a pair of counter-rotating vortices: “Positive Rotation” “Negative Rotation”

14. 14 Tilting Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie

15. 15 Supercell Rotation n In supercells, the updraft usually spins only in one direction (usually CCW). n Because of the environmental shear, the updraft is enhanced on the southern flank of the storm. n The CCW rotation is typically found on the southern flank and is favored if the storm moves toward the south (inflow along vortex lines) n The northern flank rotation is not favored and usually is weak.

16. 16 Importance of Storm-Relative Winds Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie Want to intensify the cyclonic vortex on the south side

17. 17 Importance of Storm-Relative Winds Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie Want to intensify the cyclonic vortex on the south side Storm-Relative Winds

18. 18 Importance of Storm-Relative Winds Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie Storm-Relative Winds

19. 19 Importance of Storm-Relative Winds Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie Storm-Relative Winds

20. 20 Importance of Storm-Relative Winds Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie Storm-Relative Winds

21. 21 Importance of Storm-Relative Winds Vortex Tube Updraft © 1990 *Aster Press -- From: Cotton, Storms Play Movie Storm-Relative Winds We obtain strong updraft rotation if the storm-relative winds are parallel to the horizontal vorticity – or perpendicular to the environmental shear vector – this is easily determined via a wind hodograph

22. 22 Vertical Wind Shear East North Up Westerly Winds Increase in Speed with height

23. 23 Vertical Wind Shear East North Up Shear = V(upper) – V(lower)

24. 24 Vertical Wind Shear East North Up Shear = V(upper) – V(lower)

25. 25 Vertical Wind Shear East Up Shear = V(upper) – V(lower) Shear Vector

26. 26 Development of Rotation East North Up Note that the vorticity vector points 90 deg to the left of the shear vector Shear Vector