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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 – 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 Figure 01: Photo of thunderstorm Courtesy of Dr. John R. Mecikalski Figure 02A: Visible satellite image of thunderstorms. Courtesy of CIMSS/SSEC/University of Wisconsin-Madison Figure 02B: Infrared image of thunderstorms. Courtesy of CIMSS/SSEC/University of Wisconsin-Madison Figure 03: World thunderstorm climatology. Adapted from WMO (World Meteorological Organization), 1956: World Distribution of Thunderstorm Days. WMO Publ. No. 21, TP. 21. Figure 04: U.S. thunderstorm climatology Courtesy of Oklahoma Climatological Survey 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 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 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 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 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 Figure T01: Lifted Index, Stability, and Possible Weather Figure 05: Satellite image of lifted index and severe weather reports Courtesy of CIMSS/SSEC/University of Wisconsin-Madison 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 Figure 06: Severe weather schematic Adapted from Athrens, C.D. Meteorology Today, Ninth edition. Brooks Cole, 2009 Figure 07: LANDSAT image of tornado destruction Courtesy of USGS EROS Data Center with processing by Environmental Remote/Landsat-7 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 Figure T02: The three thunderstorm types 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 Figure 08: Thunderstorm life cycle Figure 09: Photo of ordinary thunderstorm Courtesy of Tim Webster 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 Figure 10: Schematic of a multicell thunderstorm 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 Figure 11A: Radar image of squall line Courtesy of NWS/NOAA Figure 12: Photo of shelf cloud © Peter Wollinga/Dreamstime.com 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 Figure 13: Satellite image of MCC Courtesy of SSEC and CIMSS, University of Wisconsin-Madison Figure 14: Time-lapse MCC schematic Supercell Thunderstorms • The supercell thunderstorm is a large singlecell 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 Figure 15: Hand and pencil explanation of supercell thunderstorm rotation Figure 16: Supercell thunderstorm, from the side Courtesy of NOAA 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 Figure 17: The surface conditions associated with a typical supercell thunderstorm Courtesy of Oklahoma Climatological Survey 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 Figure 18ab: Microburst Courtesy of Mike Smith, www.mikesmithenterprises.com Figure 18cde: Microburst Courtesy of Mike Smith, www.mikesmithenterprises.com Figure 19: Flightpath of an airplane through a microburst Figure 20: Ronald Reagan landing before a microburst Adapted from Fujita, T. The Downburst. University of Chicago Press, 1985. 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 Figure 21: Girl in front of tornado Courtesy of Marilee Thomas Figure B01: Don Lloyd photo of tornado © Cailyn Lloyd 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 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 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 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. 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 Figure 24A: Radar reflectivity image of Enterprise, AL tornado Courtesy of NCDC/NOAA Figure 24B: Radar velocity image of Enterprise, AL tornado Courtesy of NCDC/NOAA Tornado Winds and the EF Scale • The Enhanced Fujita (or EF) scale ranges from 0 to 5, with 5 the most damage • The scale uses 28 damage indicators, like schools, barns and vegetation and the damage to each helps place the tornado on the scale • The higher the scale number, the more severe are the tornado’s wind and damage • Some of the most severe tornadoes are multiple vortex tornadoes Figure T03: The Enhanced Fujita Scale for Tornadoes Figure 25: Fujita scale pie charts Courtesy of Tom Grazulis, tornadoproject.com Figure 26: Multiple-vortex tornado Modified from Tom Grazulis, The Tornado: Nature’s Ultimate Windstorm, Oklahoma University Press, 2000, p. 111. Figure 27: U.S. tornado climatology Courtesy of Oklahoma Climatological Survey Figure 28: A climatology of the relative frequency of killer tornado events from 1950 to 2004. Courtesy of Dr. Walker Ashley, Meteorology Program, Department of Geography, Northern Illinois University Figure 29: Time of year of maximum tornado risk Adapted from H. E. Brooks et al., Wea. Forecasting, 4 (2003). Figure 30: Line graph of tornado deaths per million Americans Courtesy of Dr. Charles A. Doswell III, www.flame.org/~cdoswell/Tornado_essay.html Figure B03: Photo of Parsons Company damage Courtesy of NOAA/Matt Dayhoff, Peoria Journal Star Figure 31: Tornado paths from Superoutbreak Source: NOAA Figure 32: Greensburg, KS devastation Courtesy of Greg Henshall/FEMA Figure 33: Path of Atlanta tornado in EF scale Courtesy of National Weather Service Forecast Office, Peachtree City, GA/NOAA Figure 34: Atlanta skyscraper post-tornado Courtesy of Bruce Bracey www.flickr.com/photos/broo2/2358025514 The Waterspout • Waterspouts are narrow spinning funnels of rising air that form underneath clouds – Usually shorter cumulus clouds that are not rotating – Low pressure at the center sets up a pressure gradient that drives air inward – Air rising and cooling condenses, making the funnel visible – Strongest waterspouts only as strong as the weakest tornadoes, < 160 km/hr Figure 35: Waterspout Courtesy of Dr. Joseph Golden/NOAA Lightning • Lightning is a huge electrical discharge • Lightning caused by rising and sinking air motions that occur in mature thunderstorms – Can travel from cloud to cloud, within the same cloud, or from cloud to ground – In-cloud discharges by far most common • A lightning bolt is actually a series of flashes Figure 36: Lightning © Harald Edens, www.weatherscapes.com Lightning Strikes a Tree • First, charge separates in the cloud – Collisions of ice crystal with graupel • Second, the ground becomes positively charged – The base of the cloud is negatively charged, and like charges repel – The voltage between cloud and ground builds up • Third, lightning formation begins – A pilot leader, then dart leaders of negative charge move down from the cloud • Fourth, a brilliant flash is observed – Current flows upward in the return stroke Figure 37A: Charges collect in the base of the cloud Figure 37B: Negative charges build up near the base of the cloud, the ground repels negative charges and changes from its usual negative to a positive charge. Figure 37C: The stepped leader connects the cloud to the ground Figure 37D: The bright return stroke surges upward Figure 38: Color graphic of lightning climatology Adapted from Orville, R., and Huffines, G., Monthly Weather Review, May 2001. Figure 39: Schematic of upper-atmosphere lightning: elves, sprites, etc. Courtesy of National Severe Storms Laboratory/NOAA Flash Floods and Flooding • A flood is a substantial rise in water that covers areas not usually submerged – Water flows into a region faster than it can be absorbed, stored, or removed into a drainage basin – Caused by high-intensity rainfall, prolonged rainfall, or both – Great threats to human life • A flash flood is a sudden local flood that has a great volume of water and a short duration – Key elements: rainfall intensity and duration Figure 40: Six Flags under water © Erik S. Lesser/Landov Hail • Hail is precipitation in the form of large balls or lumps of ice • Hailstones begin as small ice particle • Hailstones grow by accretion of supercooled water droplets • Dry growth occurs when the drops freeze on contact—little liquid water on the surface • Wet growth occurs when the droplet don’t freeze quickly and spread across the surface of the hailstone—a film of liquid water on the surface Figure 41: Largest hailstone ever Courtesy of NOAA Producing Hailstones • Production of large hailstones requires a strong updraft that is tilted and an abundant supply of supercooled water – Hail occurs in regions near the strong updraft – Supercell thunderstorms often produce the largest hail – The curtain of hailstones that falls below cloud base is called the hailshaft – The hailswath is the section of the ground covered with hail Figure 42: Graphic of hail occurrence across U.S. Hailstorms Across the Nation by S. Channgon, D. Channgnon, and S. Hilbert, Image courtesy of the Midwestern Regional Climate Center, Illinois State Water Survey