2. Types of Thunderstorms 1.Airmass or Ordinary Cell Thunderstorms 2.Supercell / Severe Thunderstorms Limited wind shear Often form along shallow boundaries of converging surface winds Precipitation does not fall into the updraft Cluster of cells at various developmental stages due to cold outflow undercutting updraft

3. ORDINARY CELL THUNDERSTORMS 1.CUMULUS STAGE Sun heats the land Warm, humid air rises Condensation point is reached, producing a cumulus cloud Grows quickly (minutes) because of the release of latent heat Updrafts suspend droplets ‘Towering cumulus’ or cumulus congestus

4. 2. MATURE STAGE Droplets large enough to overcome resistance of updrafts (rain/hail) “Entrainment” Drier air is drawn in Air descends in downdraft, due to evaporative cooling and falling rain/hail Anvil head when stable layer reached (cloud follows horizontal wind) Strongest stage, with lightning and thunder

5. Mature, ordinary cell thunderstorm with anvil head

8. Microbursts create aviation hazards

9. 3. DISSIPATING STAGE Updrafts weaken as gust front moves away from the storm Downdrafts cut off the storm’s “fuel supply” Anvil head sometimes remains afterward Ordinary cell thunderstorms may pass through all three stages in only 60 minutes

10. Review of Stages: Developing (cumulus), mature and dissipating

11. Thunderstorms Typical conditions: 1. Conditional instability 2. Trigger Mechanism (eg. front, sea-breeze front, mountains, localized zones of excess surface heating, shallow boundaries of converging surface winds)

12. Conditional Instability

13. 1. Heating within boundary layer Air trapped here due to stable layer aloft increasing heat/moisture within boundary layer (BL). 2.External trigger mechanism forces air parcels to rise to the lifted condensation level (LCL) Clouds form and temperature follows MALR 3.Parcel may reach level of free convection (LFC). Parcel accelerates under own buoyancy. Warmer than surroundings - explosive updrafts 4.Saturated parcel continues to rise until stable layer is reached

14. CAPE Convective available potential energy (J/kg)

15. CAPE (J/kg) 0 Stable <1000 Marginally Unstable 1000-2500 Moderately Unstable 2500-3000 Very Unstable >3500 Extremely Unstable

16. The Severe Storm Environment 1.High surface dew point 2.Cold air aloft (increases conditional instability) 3.Shallow, statically-stable layer capping the boundary layer 4. Strong winds aloft (aids tornado development) 5. Wind shear in low levels (allows for long-lasting storms) 6.Dry air at mid-levels (increases downdraft velocities)

20. A squall line (MCS)

21. Radar image of squall line

22. Wind shear and vertical motions in a squall line thunderstorm

23. Mesoscale convective complex (MCC)

24. Outflow Boundaries

25. Thunderstorm movement in an MCC

26. See: http://rsd.gsfc.nasa.gov/rsd/movies/preview.html

27. Tornado Development 1.Pre-storm conditions: Horizontal shaft of rotating air at altitude of wind shift (generally S winds near surface and W winds aloft) 2. If capping is breached and violent convection occurs, the rotating column is tilted toward the vertical

29. Supercell Thunderstorms Defined by mid-level rotation (mesocyclone) Highest vorticity near updraft core Supercells form under the following conditions: High CAPE, capping layer, cold air aloft, large wind shear

34. Tornadogenesis 1.Mesocyclone 5-20 km wide develops 2.Vortex stretching: Lower portion of mesocyclone narrows in strong updrafts 3.Wind speed increases here due to conservation of angular momentum 4.Narrow funnel develops: visible due to adiabatic cooling associated with pressure droppage

37. 2 hours after the Lethbridge tornado

39. Tornado producing supercell [insert fig 11-29]

40. Global tornado frequency [insert fig 11-32]

41. [insert table 11- 2]

42. Waterspouts –Similar to tornadoes –Develop over warm waters –Smaller and weaker than tornadoes

43. Distribution of lightning strikes [insert fig 11-23]

44. Lightning Source of lightning: the cumulonimbus cloud Collisions between supercooled cloud particles and graupel (or hail) cause clouds to become charged Most of the base of the cumulonimbus cloud becomes negatively charged – the rest becomes positively charged (positive electric dipole) Net transfer of positive ions from warmer object to colder object (hailstone gets negatively charged & fall toward bottom - ice crystals get + charge) Many theories exist: open area of research

46. Development of lightning

47. Flashes per square kilometre per year

51. Four types of cloud- ground lightning Most common

53. Intracloud Discharges Cloud to Ground Discharges - death and destruction of property - disruption of power and communication - ignition of forest fires - Lightning is an excellent source of soil nitrogen!

54. Cloud-ground lightning 90% induced by negatively charged leaders 10% induced by positively charged leaders Sometimes, there are ground to cloud leaders Negative cloud-ground lightning Leaders branch toward the ground at about 200 km/s, with a current of 100-1000 Amperes The return stroke produces the bright flash

55. Potential difference between lower portion of negatively-charged leader and ground ~10,000,000 + V As the leader nears the ground, the electric potential breaks the threshold breakdown strength of air An upward-moving discharge is emitted from the Earth to meet with the leader

56. The return stroke lasts about 100 microseconds, and carries a charge of 30 kiloAmperes, producing the main flash The temperature along the channel heats to 30,000 + K, creating an expanding high pressure channel, producing shockwaves

57. Blue jet

58. Multiple suction vortices greatly increase damage [insert fig 11-37]