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Chapter 11

Chapter 11. Thunderstorms and Tornadoes. Figure CO: Chapter 11, Thunderstorms and Tornadoes. © Dobresum/ShutterStock, Inc. What is a thunderstorm?. A cloud or a cluster of clouds that produces thunder, lightning, heavy rain, and sometimes hail and tornadoes Tall cumulonimbus clouds

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Chapter 11

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  1. Chapter 11 Thunderstorms and Tornadoes

  2. Figure CO: Chapter 11, Thunderstorms and Tornadoes © Dobresum/ShutterStock, Inc.

  3. What is a thunderstorm? • A cloud or a cluster of clouds that produces thunder, lightning, heavy rain, and sometimes hail and tornadoes • Tall cumulonimbus clouds • Form when air rises or is lifted from the surface or near the surface • Energy is released when saturated air condenses moisture • Air rises rapidly and to great heights—the anvil cloud • Overshooting tops briefly overshoot the tropopause • Mammatus cloud may form beneath the anvil

  4. Figure 01: Photo of thunderstorm Courtesy of Dr. John R. Mecikalski

  5. Figure 02A: Visible satellite image of thunderstorms. Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

  6. Figure 02B: Infrared image of thunderstorms. Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

  7. Figure 03: World thunderstorm climatology. Adapted from WMO (World Meteorological Organization), 1956: World Distribution of Thunderstorm Days. WMO Publ. No. 21, TP. 21.

  8. Figure 04: U.S. thunderstorm climatology Courtesy of Oklahoma Climatological Survey

  9. Conditions for Thunderstorm Formation • A warm moist air mass at or just above the surface • High dew-point temperature • Maritime tropical air mass • A deep layer of conditional instability • Saturated air parcels rise freely • In the tropics this is the average condition • Stability of an air mass can change

  10. Conditions for thunderstorm formation (continued) • How stability changes (becomes less stable or more unstable) • Warm air advection at low levels • Cold air advection at upper levels • Lifting of a stable layer that is humid at its base and dry at its top (convective or potential instability) • Also described a blowing a capping inversion • Surface heating

  11. Conditions for thunderstorm formation (continued) • Lifting mechanisms (more than one may be active) to get air parcels saturated • Free convection from buoyancy due to surface heating • Common in warm season • Thermals, then cumulus clouds • Forced lifting from topography • Many thunderstorms occur on the upwind side of mountains

  12. Conditions for thunderstorm formation (continued) • Frontal lifting • Especially cold fronts • Also drylines • Convergence • Sea breezes converging in central Florida • Low pressure centers in the tropics • Hurricane is the largest collection of thunderstorms • Nocturnal low-level jet (just above the surface) • An important ingredient in severe weather • Supplies moisture and energy at low levels

  13. Lifted Index • Is a way to describe stability with one number • Is not a perfect measure of stability • Is useful for forecasting, but not the only criterion forecasters use • Is a difference in temperature at 500mb • The parcel temperature is determined by raising a surface parcel to saturation at the DALR, and to 500mb at the SALR • LI = T(environment) – T(air parcel) • Negative values are unstable

  14. Figure T01: Lifted Index, Stability, and Possible Weather

  15. Figure 05: Satellite image of lifted index and severe weather reports Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

  16. Conditions for thunderstorm formation (continued) • For the more severe thunderstorms, vertical shear of the horizontal wind • The westerly (west to east) part of the wind increasing as height increases • Clockwise turning of the direction from which the wind blows as height increases • For example, southeasterlies at the surface, southerlies at 850mb, southwesterlies at 700mb, and westerlies at 500mb

  17. Figure 06: Severe weather schematic Adapted from Athrens, C.D. Meteorology Today, Ninth edition. Brooks Cole, 2009

  18. Figure 07: LANDSAT image of tornado destruction Courtesy of USGS EROS Data Center with processing by Environmental Remote/Landsat-7

  19. Thunderstorm Cells • A cell is a compact region of a cloud that has a strong vertical updraft • Ordinary cells are a few km in diameter, last less than an hour • Supercells are larger and can last several hours • Account for the vast majority of severe thunderstorm weather • Multicell thunderstorms are composed of lines or clusters of thunderstorm cells, ordinary, supercell, or both

  20. Figure T02: The three thunderstorm types

  21. Ordinary Single-Cell Thunderstorm • Life cycle has three distinct stages • Cumulus—parcels ascend in the updraft and get saturated at the lifting condensation level or LCL, which marks cloud base • Mixing with environment air is called entrainment • Mature—begins when precipitation starts to fall • Time of most lightning, rain, small hail • A downdraft develops with cooling due to evaporating precipitation • Dissipating—updraft weakens, downdraft dominates • Also known as the air mass thunderstorm

  22. Figure 08: Thunderstorm life cycle

  23. Figure 09: Photo of ordinary thunderstorm Courtesy of Tim Webster

  24. Multicell Thunderstorms • Composed of several individual single-cell storms, each one at a different stage of development • Can last several hours • A moderate amount of vertical wind shear • Updraft and downdraft can coexist • Updraft and downdraft meet at the gust front • Groups of multicell thunderstorms are called mesoscale convective systems

  25. Figure 10: Schematic of a multicell thunderstorm

  26. Squall Lines • A squall line is composted of individual intense thunderstorm cells arranged in a line or band • Occur along a boundary of unstable air • Have life spans of 6 to 12 hours or more • Extend over several states simultaneously • A shelf cloud is often observed above the gust front • Often observed ahead of a cold front • Divergence aloft and a broad, low-level inflow of moist air favor development of squall lines

  27. Figure 11A: Radar image of squall line Courtesy of NWS/NOAA

  28. Figure 12: Photo of shelf cloud © Peter Wollinga/Dreamstime.com

  29. Mesoscale Convective Complex (MCC) • An MCC is a complex of individual storms that covers a large area in an infrared satellite image and lives more than 6 hours • Often form in late afternoon and evening • In satellite images give the appearance of a large circular storm with cold cloud-top temperatures • Often form underneath a ridge of high pressure • Because upper-level divergence can occur in a ridge • Do not require as much vertical shear as squall lines • Can be maintained by the low-level jet

  30. Figure 13: Satellite image of MCC Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

  31. Figure 14: Time-lapse MCC schematic

  32. Supercell Thunderstorms • The supercell thunderstorm is a large single-cell storm, sometimes 32 km or more across, that almost always produces dangerous weather • Strong wind gusts, large hail, dangerous lightning and tornadoes • Require a very unstable atmosphere • Require both directional and speed shear • Vertical wind shear causes supercell thunderstorms to rotate about a vertical axis

  33. Figure 15: Hand and pencil explanation of supercell thunderstorm rotation

  34. Figure 16: Supercell thunderstorm, from the side Courtesy of NOAA

  35. Cyclonic Right-Moving Supercell • The spinning updraft is called a mesocyclone • Resembles an extratropical cyclone • 5 to 20 km across • Narrows and rotates more quickly when it stretches • Is too large and too slow in rotation to be a tornado • Has an overshooting cloud top • Has two downdrafts ahead of (forward flank) and behind (rear flank) the center of the storm • Downdrafts caused by precipitation • Gust fronts ahead of the downdrafts

  36. Figure 17: The surface conditions associated with a typical supercell thunderstorm Courtesy of Oklahoma Climatological Survey

  37. Microbursts • Microbursts develop when rain falling from a thunderstorm evaporates underneath the cloud, cooling the air beneath • Cold heavy air plunges to the surface and splashes against the ground • Air then rushes sideways and swirls upward as a result of the pressure gradient between the cold air and the warm surroundings • Microbursts can do as much damage as a small tornado

  38. Figure 18ab: Microburst Courtesy of Mike Smith, www.mikesmithenterprises.com

  39. Figure 18cde: Microburst Courtesy of Mike Smith, www.mikesmithenterprises.com

  40. Figure 19: Flightpath of an airplane through a microburst

  41. Figure 20: Ronald Reagan landing before a microburst Adapted from Fujita, T. The Downburst. University of Chicago Press, 1985.

  42. Tornadoes • Tornadoes are rapidly rotating columns or funnels of high wind that spiral around very narrow regions of low pressure beneath a thunderstorm • Visible because of condensation, dust, and debris • Nearly always rotate cyclonically, often move to the northeast • If the circulation does not reach the ground, called a funnel cloud • Usually < 1.6 km across

  43. Figure 21: Girl in front of tornado Courtesy of Marilee Thomas

  44. Figure B01: Don Lloyd photo of tornado © Cailyn Lloyd

  45. Tornado Formation • Most form underneath supercell thunderstorms • The cloud base underneath the updraft on the rear side of the thunderstorm may lower • Forming a rotating wall cloud • A rapidly rotating column of air much smaller than the mesocyclone may protrude beneath the wall cloud • As water vapor condenses in the air rushing up into this column, a funnel cloud may form and reach the ground, becoming a tornado

  46. Figure 22: The ominous approach of a rotating wall cloud is a sign that a tornado may develop at any moment. Courtesy of Nolan T. Atkins

  47. Tornado Life Cycle • Four stages in the life cycle • Organizing stage, funnel picks up debris, as it reaches the surface and widens • Mature stage, tornado at peak intensity and width • Shrinking stage, the funnel narrows • Decaying or rope stage, when the funnel thins out to a very narrow ropelike column after which it eventually dissipates

  48. Figure 23: The life cycle of the Union City, Oklahoma, tornado on May 24, 1973 Modified from Golden, J. H., and D. Purcell, Mon. Wea. Rev., 106 [1978]: 3–11.

  49. Tornado Signatures on Radar • Hook echo • The pattern of heavy rain inside a supercell forms a kind of hook around the region most likely to produce a tornado • Tornado vortex signature • A couplet of red and green colors on Doppler radar images • Mesocyclone signature • A couplet of red and green colors on Doppler radar images larger than the tornado vortex signature

  50. Figure 24A: Radar reflectivity image of Enterprise, AL tornado Courtesy of NCDC/NOAA

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