1. Outline for Lecture 22 Thunderstorm Review Lightning and Thunder Tornadoes Tornado Forecasting The Development and Occurrence of Tornadoes 4/10/2003

2. Thunderstorm development cumulus stage: strong updrafts act to build the storm. mature stage: heavy precipitation and cool downdrafts in part of the storm. dissipating stage: warm updrafts disappear, rain stops, cloud dissipates

3. Well-developed supercell lower stratosphere updrafts downdrafts

4. Downbursts (microbursts) Strong localized downdrafts below thunderstorms are called downbursts.

5. Microbursts—small downbursts (<4km wide) The colder air is, the more dense, the more dense the faster it will fall. In microbursts, evaporative cooling helps to cool the air which leads to rapid acceleration. Wind speeds can exceed 100 mph. The winds expand out from the center of the burst.

6. Lightning and Thunder Lightning is the result of charge separation (part of the cloud develops an excess of negative charge, whereas another part acquires a positive charge). The rapid vertical motion, present in mature cumulonimbus clouds, is key to charge separation.

7. How do we separate all that charge? water + + + ice + - - - + - - - + The formation of ice in clouds is important. Charge separation within the freezing crystal is where it begins. • Interior of shell freezes • Expands & shatters • Leaves differently charged • fragments.

8. Discharge via cloud-to-ground lightning (10%)

9. Sprite! Sprite Blue Flash Cloud lightning

10. Thunder Lightning is a massive discharge of electricity. Electricity is made up of electrons. When lightning strikes, massive amounts of electrons shoot through the air at once. In the process, they bump into a lot of air molecules. When lightning strikes, intense heat (8,000°C - 33,000 °C) expands the air in and around it so rapidly that it results in a loud noise we know as thunder. THUNDER IS AN EXPLOSION!!

11. Tornadoes Tornadoes (twisters, cyclones) are violent windstorms that take the form of a rotating column of air Air flows from higher to lower pressure Wind speed is determined by the pressure gradient force The pressure at the center of a tornado is as much as 10 percent lower than just outside it How significant is a 10 percent drop?

12. Pressure gradients and winds 1032mb/996mb is ~ 3% difference… ...OVER 1500 miles! 1032mb/928mb is a 10% difference. The distance in a tornado is ~5 miles.

13. Strong tornadoes are made up of several suction vortices Suction vortices are 10–30 meters wide

14. Tornadoes Less than one percent of thunderstorms produce tornadoes. Tornadoes can form in any situation that produces severe weather including cold fronts, squall lines, and hurricanes. The most intense tornadoes are typically from supercell thunderstorms. For a tornado to occur, first amesocyclone (a vertical cylinder of rotating air 2-6 miles wide) must develop. Tornadoes often form about 30 minutes after mesocyclone formation.

15. Development of Tornadoes and Mesocyclones Wind speed shear and strong vertical updrafts lead to the rotating column of air.

16. Average annual tornado incidence per 10,000 square miles for a 27 year period. cP mT Interaction of cold, dry cP polar air masses with the warm, humid mT tropical air masses results in majority of tornadoes over the U.S.

17. Average number of tornadoes and tornado days each month in the U.S. Most tornadoes occur in the spring.

18. Profile of a Tornado • Average tornado has a • diameter of 500-2000 ft. • Tornadoes move at • about 45 mph • The typical path is approximately 16 miles long. Most tornadoes move to the northeast (recall winds in advance of a cold front are from the southwest).

19. Tornado Intensity: Fujita Scale • F0 <72 mph Light damage • F1 72-112 Moderate • F2 113-157 Considerable • F3 158-206 Severe • F4 207-260 Devastating • F5 >260 Incredible

20. Fujita Scale Tornadoes are classified according to the worst damage that they did anywhere along their path. College Park tornado was an F3: Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown

21. Doppler Radar Doppler radars and faster computers have greatly improved our ability to warn the public about tornadoes

22. We now have our own!Well, OK, so it belongs to Channel 9

23. Doppler Effect higher frequency Christian Johann Doppler in 1842 explained the process. lower frequency The wavelength of the radar echo changes is such a way to determine movement of the thunderstorm.

24. Doppler image—2 mesocyclones

25. The tornado itself! (F3)

26. Damage from it

27. College Park Tornado

50. 521 Reflectivity 516 Reflectivity 511 Reflectivity 526 Reflectivity 531 Reflectivity 526 Velocity