Chapter 10: Thunderstorms and Tornadoes

1. Chapter 10: Thunderstorms and Tornadoes • Thunderstorms • Tornadoes • Tornadic thunderstorms • Severe weather and doppler radar

2. Thunderstorms

3. Thunderstorms What do we already know about thunderstorms? Can produce intense rain, hail, wind, lightning Can make tornadoes Formed by rising air from: Uneven surface heating Uplift along a frontal boundary Topographical uplift

4. Thunderstorms Definitions: Supercell thunderstorms – large updraft storms that can produce flash floods, severe weather, tornadoes Severe Thunderstorm – has to have one of following: Hail greater than or equal to ¾ inch A tornado Wind gusts 50 mph

5. Ordinary Cell Thunderstorms Ordinary cell thunderstorm Form in region of low wind shear Can form due to surface air convergence Cumulus stage (growth stage) – due to rising warm, humid air condensing into cumulus cloud Latent heat release keeps cloud warm and unstable Grows quickly to towering cumulus Usually no precipitation because of updrafts No lightning or thunder

6. Ordinary Cell Thunderstorms

7. Ordinary Cell Thunderstorms Mature stage – marked by considerable downdrafts Cloud particles grow larger, and begin to fall Entrainment - Dry air is sucked into cloud, causing evaporation and cooling Heavier and cooler air descends…downdrafts Formation of updraft and downdraft cells Most intense time of thunderstorm Storm may grow as high as stratosphere (anvil) Heavy rain, lightning, small hail

8. Ordinary Cell Thunderstorms Mature stage Cloud top may overshoot into stratosphere

9. Ordinary Cell Thunderstorms Mature stage Cloud top may overshoot into stratosphere Downdraft reaches ground and spreads along surface as a gust front Sometimes rain may not reach ground, but cold air does

10. Ordinary Cell Thunderstorms

11. Ordinary Cell Thunderstorms Dissipating stage – thunderstorm weakens Updrafts begin to weaken after 15 to 30 min. Gust front moves too far from the storm, so updrafts have to weaken Light precipitation falls Only anvil remains All three stages in less than an hour

12. Ordinary Cell Thunderstorms

13. Stepped Art Fig. 10-1, p. 265

14. Severe Thunderstorms and the Supercell • So ordinary cell storms weaken because the updraft weakens • What if the updraft doesn’t weaken quickly? • If moderate wind shear pushes downdraft downwind, updraft is not cut off • If downdraft then undercuts updraft, a multicell storm forms

15. Severe Thunderstorms and the Supercell

16. Severe Thunderstorms and the Supercell Multicell storms Cells of varying age co-mingling Top of cloud well into stratosphere Updrafts allow hail to grow large

17. Severe Thunderstorms and the Supercell Supercells If wind shear is strong and changes direction with height, downdraft may not undercut updraft Updraft remains strong for long time (> 1 hour) One rotating vertical column Can create tornadoes Hail size of grapefruit

18. Severe Thunderstorms and the Supercell • At surface, open wave with warm, humid air ahead of cold front • Overrunning warm air just above surface • Dry air at 700 mb level • Trough of low pressure at 500 mb • Divergence at 300 mb

19. Severe Thunderstorms and the Supercell • Severe thunderstorms form in light green area because: • Warm air below cold air is conditionally unstable • Strong wind shear creates severe thunderstorms • In morning, atmos is stable, no thunderstorms • Surface heating creates thunderstorms

20. Squall Lines and Mesoscale Convective Complexes • Squall line – line of multicell t-storms extending many kilometers • Mesoscale convective complex- cluster of storms in one spot

21. Squall Lines and Mesoscale Convective Complexes Squall lines often form ahead of cold fronts (pre-frontal squall-line thunderstorms) MCCs are long-lasting and provide a lot of rain, and a ton of severe weather Form during summer

22. Dryline Thunderstorms • Dryline – narrow zone with a sharp moisture contrast. Where do the thunderstorms form?

23. Gust Fronts, Microbursts and Derechoes • Gust front – leading edge of the thunderstorm downdraft • Passage can feel like a cold front passage • Shelf cloud – warm air rising above gust front creates cloud • Roll cloud – cloud just behind the gust front that spins horizontally • Outflow boundary – a huge gust front formed by numerous thunderstorm gust front • Downbursts – localized downdraft like water from a tap • Microbursts – downbursts that extend 4 km or less

24. Gust Fronts, Microbursts and Derechoes

25. Gust Fronts, Microbursts and Derechoes

26. Fig. 10-11, p. 271

29. Microburst • Microbursts can be dangerous to aircrafts • Plane encounters uplift at (a), and pilot puts nose down • Unexpected downdraft crashes plane • Has caused several airplane crashes • Some airports have microburst detection

30. Bow Echo and Derecho • Bow echo – a line of thunderstorms often form a bow echo on radar • Derecho – winds associated with downdrafts that exceed 104 mph and can cause a lot of damage…20 derechoes each year in U.S.

31. Floods and Flash Floods • Flash floods – floods that rise rapidly with little to no advance warning • Thunderstorms cause flash floods in two ways: • Hovering above one area for a long time • Training – thunderstorms keep passing over same area

32. Distribution of Thunderstorms • 18 millions thunderstorms worldwide annually • Need to have a combination of moisture and warmth • So do thunderstorms form in the Arctic? ITCZ? Gulf?

33. Lightning and Thunder • Lightning – simply a discharge of electricity, usually in mature thunderstorms • Heats air up to 54,000F (5x hotter that surface of sun • Thunder – the explosive expansion of the hot air creates a sound wave that travels in all direction • According to the Guinness Book of World Records,Roy Sullivan holds the world’s record for being struckby lightning seven times between 1942 and his deathin 1983.

34. Lightning and Thunder Light travels faster than sound, so we see the lightning before we hear the thunder Sound takes 5 seconds to travel 1 mile So if we hear thunder 5 seconds after we see the lightning, the stroke was 1 mile away

35. Lightning and Thunder

36. Electrification of Clouds • Must have charging of the cloud to have lightning • Electrification of clouds not fully understood • Theory 1: Supercooled droplets fall through cloud and collide with warm hailstone. Latent heat is released (warm or cold?)

37. Electrification of Clouds Net transfer of positive ions from warm to cold object Falling hailstone is negatively charged, falls to bottom of cloud Light positively charge particle is lifted to top of cloud Small area of positively charge particles near melting level

38. Electrification of Clouds Theory 2: When precip forms, it has a neg in the upper portion and pos charge in the lower As droplets collide, the large droplets become neg and fall The small droplets are lifted by updrafts and rise

39. The Lightning Stroke • Basics of lightning • Opposite charges attract • Positive charges on ground follow negative charges at base of cloud • Electrical current will not flow because air is good insulator • Charge must be large (< 1million volts per meter) to create a lightning bolt

40. The Lightning Stroke Cloud-to-ground lightning Discharge of electrons from cloud to ground as a stepped leader (many times) Positive charges race back up from elevated object as a return stroke

41. The Lightning Stroke Cloud-to-ground lightning Many electrons flow to ground and a stronger return stoke follows (this is what you see) 1/10,000 of a second so it looks like one continuous flash

42. The Lightning Stroke Dart leader – subsequent initial stroke that follows same path as initial stepped leader Causes the multiple flash of the lightning

43. Types of Lightning • Forked lightning – dart leader takes different path than stepped leader • Ribbon lightning – wind blows charges into ribbon-like lightning • Dry lightning – lightning that occurs in a dry thunderstorm

44. Types of Lightning Heat lightning – lightning that is seen by not heard (can be orange) St. Elmo’s Fire – luminous green or blue halo around the top of pointed objects (antennas, masts of ships). Lightning may occur after this is seen.

45. Lightning Detection and Suppression • Lightning direction-finder

46. Tornado Life Cycles • Tornado – rotating column of air blowing around a small low pressure that reaches the ground

47. Tornado Life Cycles Funnel cloud Tornado that hasn’t reached the ground Dust-whirl stage Swirling dust at the ground marks the tornadoes circulation Mature stage Funnel at greatest width, most intense damage. Often vertical Decay stage Funnel shrinks, damage becomes less, tornado becomes stretched

48. Tornado Outbreaks • Tornado families – tornadoes spawned by the same thunderstorms • Tornado outbreaks – many tornadoes that form over same region • Much ground-breaking research on tornadoes wasconducted by Professor Ted Fujita of the Universityof Chicago. The “F-scale” of tornado intensity wasnamed after him.

49. Tornado Occurrence • Tornado alley – part of the Central Plains from Texas through Nebraska • Time of day – most often in the afternoon • Times of year – most often in Spring, lease often in Winter

50. Tornado Winds • Multi-vortex tornadoes • A single tornado with multiple rotating columns within it • Suction vortices • The small rotating columns within multi-vortex tornadoes