Convective Storms and By-Products: Thunderstorms and Tornadoes

1. Convective Storms and By-Products: Thunderstorms and Tornadoes

2. Thunder Storms • Cluster of clouds producing heavy rain, lightning, thunder, hail or tornados • Require large amounts of energy • Moist air, strong convection • Vary in length, precipitation and windiness

3. Thunderstorm Requirements • Warm moist air • Lifting – mountains or frontal cyclones • Thunderstorms often follow midlatitude storm tracks

4. Satellite View

5. Satellite View II

6. Growth and Development • Affected by • Unstable atmosphere • Environmental Temperature • Humidity • Wind speed and direction (surface to tropopause) • Vertical Wind Shear – adds spin • Nocturnal Jet – moisture and energy • Capping inversion – the lid on a boiling pot

7. Lifting Index • A measure of convective potential • Compares Tparcel to Tenvironment • When Tp >Te, convection is possible • Te-Tp • -3 to -6 marginal instability • -6 to -9 moderate instability • < -9 very unstable air

8. Types of Thunderstorms • Composed of cells • Ordinary- short lived and small • Super- large, last for hours • Single Cell • Multi Cell

9. Ordinary Single Cell • Short-lived, last for ~1 hour, localized • Stages • Cumulus • Mature • Dissapating

10. Cumulus stage • Moist surface air rises and cools at DALR until Lifting Condensation Level (LCL) is reached • Entrainment from dry environmental air • Evaporation of droplets, helps cool air • Variability in droplet size • If cloud is higher than freezing point mixed precipitation can form

11. Mature Stage • Precipitation begins to fall • Lightning, hail and rain maximized • Updrafts strongly organized • Falling precipitation occurs when air is unsaturated, promotes downdrafts of cool dense air

12. Dissipating stage • Updraft Collapses • Downdraft dominates, creates drag, snuffs updraft • Moisture source lost, convection slows • Dry environmental air entrains • Cloud dissipates

14. Ordinary Single Cell

15. Multi Cell Systems • Number of seperate individual cells at differing stages • Last several hours • 2 basic types • Squall lines • mesoscale

17. Shelf cloud at gust front

18. Squall line • Line, following frontal pattern • Boundaries of unstable air • 6 to 12 hours long • Span several states • Wind shear separates updraft, downdraft • Shelf cloud

19. Conditions for Squall line • Divergence aloft • Most low level inflow • Squall lines appear ahead of cold fronts

20. Squall Line

21. Squall line

22. Mesoscale Convective Complex • Complex arrangement of individual storms • 100 K Km2 (Iowa) • High pressure in upper levels • Do not require high wind shear • Long lived • Afternoon maturation • Die in early morning (dawn)

23. MMC requirements • Low level moisture source • Low level jet • Jet rises over downdrafts • Jet weakens during sunrise, MMC breaks up • Important source of water for US Great Plains

24. Super Cell • Rotating Single Cell system • Development depends on instability and wind shear (low level southerly, upper level westerly) • Updrafts and downdrafts are separate • Produces dangerous weather • Rain, hail, lightning, Tornadoes

25. Super Cell Structure

26. Structure of Supercell • Updraft goes in at rain free base, moves ahead and downwind • Anvil and overshooting tops indicate strong updrafts • Upper level winds help maintain movement • Downdraft in precipitation core

27. There’s no place like home…

28. Tornadoes • Rapidly Rotating columns of high wind around a low beneath a thunderstorm • Visible Funnel due to condensation, dust and debris in rapidly rising air • Funnel cloud is not a tornado

29. Funnel Cloud

30. Tornado

31. Just the facts • Less than one mile wide • Short lived <30 minutes • Hard to understand due to violent nature • Related to slowly rotating super cell thunderstorms • Movement with storm track, NE in US

32. Rotation • Begins in interplay between updrafts and downdrafts • Air spins around horizontal axis near front • Meso cyclone (5 to 20km wide) • Updrafts lift column and 2 columns form • Vertical axis • Left and Right movers • Vertical stretching increases spin

33. Spinning air lifted

34. Not a nice day for fishing

35. A twister is born • Cloud under spinning updraft lowers in a rotating cloud wall • Small compared to meso cyclone • Funnel Cloud • Water vapor makes circulation visible • Touchdown - start of tornado

36. Touchdown!!

37. Life Cycle • Organizing • Mature • Shrinking • Rope

38. Tornado Winds • 300 mph (480km/hr) • Force of wind proportional to v2 • 4 times more powerful than category 5 Hurricane • Ted Fujita • 1970 • Category F1 to F5 • 1% category 4,5

39. Source and Distribution • Source of winds unknown, strongest in direction of background flow • Strong tornadoes show multiple vortex • Distribution • Possible in any state • Areas of instability, wind shear, frontal movement

40. Tornado Alley

41. Tornado Season • Follows Jet stream (source of wind shear) • Minnesota- June • Mississippi- Spring and Fall • Could happen day or night • Attraction to trailer parks?

42. Severe Weather • Lightning • Hail • Floods • Severe winds

43. Lightning • Electrical discharge • Rising and sinking air motions • 85 deaths, 300 injured per year • 1 in 600,000 vs 1 in 5 billion • Can travel • Cloud to cloud • Cloud to ground • Inside individual clouds

44. Charge Separation • Charges distributed throughout cloud • Ice particle- graupel collisions • When T<-5oC • Graupel-negative • Ice Crystals-positive • Updrafts move and separate charges • Ice up • Graupel down • Cloud induces surface charge

45. Ground Charge • Attraction to cloud • High pointy metal structures • Large charge separation • Air acts to insulate, allows potential buildup • 3000 volts/ft • 9000 volts/m

46. Lightning Formation • Large charge buildup and separation • Pilot leader • Stepped leaders- branches act as conductive channels • Spark when channel is completed to ground • Electrons flow in series of flashes

47. Lightning crashes • Return stroke • Current flow upward • Dart leaders • Negative electrons, cloud to ground • Series of flashes

48. Lightning Stroke

49. Flash Floods • Input of water faster than removal, absorption or storage • Local • High volume • Short duration • Breaking dam

50. Influences • Rainfall intensity • Topography • Soil conditions • Ground cover • Steep terrain funnels flow • Extremes in soil moisture