Supercell Thunderstorms

1. Supercell Thunderstorms Part II Adapted from Materials by Dr. Frank Gallagher IIIand Dr. Kelvin DroegemeierSchool of MeteorologyUniversity of Oklahoma

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

3. Convergence • 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. Horizontal Temperature Differences • Air travelling along a frontal zone will develop a horizontal rotation.

5. Recall the Cold-Air Outflow

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

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

8. Development of Rotation Up North East

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

10. Development of Rotation Up Thunderstorm North East

11. Development of Rotation Up Updraft - Stretch North East

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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