Weather and Climate

1. Weather and Climate Lecture 13: Severe Weather and Hurricanes

2. Thunderstorms • A thunderstorm is simply a storm containing lightning and thunder. • Thunderstorms occur all over the world

3. Formation - Ingredients • Moisture (water vapor) in the lowest levels of the atmosphere • Cold, dry air aloft (2-3 miles above the surface) • Lifting mechanism

4. Air Mass (Single-Cell) Thunderstorms • Formed by uneven heating of the earth’s surface • Brief, but well-defined lifespan with three stages • Cumulus Stage • Mature Stage • Dissipating Stage

5. Three Stages

6. Mature Thunderstorms • The updraft can become so strong that it penetrates into a region of stable air, resulting in a overshooting top.

7. Overshooting Tops

8. Which is mature and which is dissipating?

9. Lifespan of a Thunderstorm • A single cell, consisting of an updraft and downdraft, lasts 20 minutes. • But we have all observed thunderstorms that appear to last longer than 20 minutes…. WHY?

10. Multicell Storms • Sometimes when the cold downdraft reaches the surface, it may force the warm, moist surface air upward • This rising air can condense and build a new thunderstorm Multicell Cluster Multicell Line (Squall Line)

11. Downdrafts

12. Gust Fronts

13. Gust Front

14. Downbursts

15. Straight-Line Winds • Associated with a cluster of severe thunderstorms • May exceed 90 knots • If wind damage extends 250 miles along storm path, it is called a derecho.

16. Tree Damage What does it look like from a downburst vs. a derecho?

17. What makes a thunderstorm a severe thunderstorm?

18. Severe Thunderstorm • Hail ¾” or greater • Winds in excess of 50 knots (57.5 mph) • Tornado

19. Severe Thunderstorms

20. Supercell Thunderstorms

21. Capping Inversion • An important component of creating a severe thunderstorm is the strength of the boundary layer cap. • Too strong nothing develops • Too weak numerous cumulus clouds • Just righthelps to funnel the air into a strong updraft

22. Supercell Thunderstorms

23. Squall Line

24. Squall Line

25. Dry Line

26. Tornadoes

27. Tornadoes

30. Tornadoes

31. Tornado Alley

32. Tornado Wind Speed As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side. Figure 15.31

33. Suction Vortices • Many violent tornadoes contain small whirls inside them.

34. Tornadoes vs. Mesocyclone • Development of mesocyclone • Stretching – gives rotating air a faster spin

35. Supercell View from East

36. Tornadoes • Vertical Wind Shear (pencil example) • Horizontal roll meets updraft and is tilted vertically

37. Rotating Clouds as Tornado Signal The first sign that a supercell may form a tornado is the sight of rotating clouds at the base of the storm, which may lower and form a wall cloud, shown in this picture. Figure 15.41

38. Tornadoes

39. Position

40. Fujita Scale

41. Tornadoes and Corn

42. Modeling Thunderstorms

43. Modeling Thunderstorms

48. Lightning

49. Cloud to Ground

50. Cloud to Cloud