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THUNDERSTORMS

THUNDERSTORMS. Types of Thunderstorms Airmass or Ordinary Cell Thunderstorms 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

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THUNDERSTORMS

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  1. THUNDERSTORMS

  2. Types of Thunderstorms • Airmass or Ordinary Cell Thunderstorms • 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 • 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. 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

  6. Downdrafts and Gust Fronts

  7. Aviation Risk even in Ordinary Cell (Airmass) Thunderstorms

  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: • Conditional instability • Trigger Mechanism • (eg. front, sea-breeze front, mountains, • localized zones of excess surface heating, • shallow boundaries of converging surface • winds)

  12. Conditional Instability

  13. Thunderstorm Development • 1. Heating within boundary layer • Air trapped here due to stable layer aloft • increasing heat/moisture within boundary layer • (BL). • 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 • High surface dew point • Cold air aloft (increases conditional instability) • 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) • Dry air at mid-levels (increases downdraft • velocities)

  17. A squall line (MCS)

  18. Radar image of squall line

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

  20. Mesoscale convective complex (MCC)

  21. Thunderstorm movement in a MCC

  22. Outflow Boundaries

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

  24. 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 • Wind shear separates updraft from downdraft • so it can keep developing

  25. Tornado Development • 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

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

  27. Wall Cloud

  28. 2 hours after the Lethbridge tornado

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

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

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

  32. [insert table 11-2]

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

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

  35. Lightning • Source of lightning: the cumulonimbus cloud • Collisions between ice crystals and graupel/hail surrounded • by supercooled water droplets 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) • Result: positive charges well aloft, negative charges near the • cloud base

  36. Development of cloud to ground Lightning (20% of cases) Charge separation Stepped leader approaches ground Positive charge surges upward from ground Spark surges up from ground

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