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Lecture 19

Lecture 19. Chapter 11 Thunderstorms and Tornadoes. Thunder Storms. Cluster of clouds producing heavy rain, lightning, thunder, hail or tornados enormous energy Moist air, strong convection Vary in length, precipitation and windiness. Thunderstorm Requirements. Warm moist air

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Lecture 19

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  1. Lecture 19 Chapter 11 Thunderstorms and Tornadoes

  2. Thunder Storms • Cluster of clouds producing heavy rain, lightning, thunder, hail or tornados • enormous 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 • Squall line • Mesoscale convective complex

  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 dry adiabatic lapse rate 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 phase and 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

  13. Ordinary Single Cell

  14. Multi Cell Systems • Number of seperate individual cells at differing stages • Last several hours • 2 basic types • Squall line • Mesoscale convective complex (MCC)

  15. Note how the downdrafts assist the updrafts –provide lifting

  16. Shelf cloud above gust front

  17. Squall line • Line of storms often following or ahead of a front • Boundaries of unstable air • 6 to 12 hours long • Long (span several states) • Wind shear separates updraft, downdraft • Shelf cloud above gust front

  18. Conditions for Squall line • Divergence aloft • Most low level inflow • Squall lines often appear ahead of cold fronts in plains and midwest

  19. Squall Line

  20. Squall line

  21. 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 • Mature in late afternoon • Die in early morning (dawn)

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

  23. 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

  24. Super Cell Structure

  25. 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

  26. Auntie Em, it’s a twister

  27. 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 until it touches ground

  28. Funnel Cloud

  29. Tornado

  30. Just the facts • ~1.6 km wide • Short lived <30 minutes • Hard to understand due to violent nature • Related to rotating super cell thunderstorms • Movement with storm track, NE in US

  31. 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

  32. Spinning air lifted

  33. Not a nice day for fishing

  34. 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

  35. Touchdown!! Extra point is no good!

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

  37. 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

  38. Source and Distribution • strongest winds in direction of background flow • Strong tornadoes show multiple vortices • Geographical distribution • Possible in any state • Areas of instability, wind shear, frontal movement

  39. Tornado Alley

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

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

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

  43. Charge Separation • Charges distributed throughout cloud • Ice particle- graupel collisions • When T<-15oC • Graupel-negative • Ice Crystals-positive • Updrafts move and separate charges • Ice up • Graupel down • Cloud induces surface charge

  44. 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

  45. 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

  46. Lightning Stroke

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

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

  49. Kodak moment

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