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AMS Weather Studies Introduction to Atmospheric Science, 4th Edition
Chapter 11Thunderstorms and Tornadoes
Case-In-PointMajor severe weather outbreak of 3 May 1999 More than 70 tornadoes were reported in Oklahoma, northern Texas, and south central Kansas, and 26 of these occurred in or around Oklahoma CityAn F5 tornado took 38 lives in Oklahoma City suburbsAn F4 tornado claimed 6 more lives in Haysville, KansasEssential ingredients for this outbreak:Warm, humid surface air layer was initially capped (capping inversion) with much drier air aloftTemperature and humidity contrast between low-level and upper-level layers grew throughout the day, increasing the potential for deep convection and severe weatherSounding indicated strong vertical wind shearAfternoon arrival of a jet streak lifted the air column and eliminated the capping inversionMassive supercell thunderstorms developed explosively and spawned violent tornadoes
Driving QuestionWhat conditions in the atmospheric favor development of severe convective weather systems?Tornadoes are the most intense of weather systems, but less than 1% of all thunderstorms spawn tornadoesThis chapter covers thunderstorms and tornadoes, their characteristics, life cycles, geographical and seasonal distributions and associated hazards
Thunderstorm Life CycleA thunderstorm is a meso-scale weather system that is accompanied by lightning and thunder, affects a relatively small area, and is short-lived. It is the product of vigorous convection extending high into the troposphere.
Thunderstorm Life CycleTowering Cumulus StageCumulus clouds build vertically and laterally, and surge upward to altitudes of 8,000-10,000 m (26,000-33,000 ft) over a period of 10-15 minutesProduced by convection within the atmosphereFree convection triggered by intense solar heating of Earths surfaceGenerally not powerful enough to produce thunderstormsForced convection orographic uplift or converging winds strengthen convectionThis is generally the cause of thunderstormsLatent heat released during condensation adds to buoyancyDuring the cumulus stage, the updraft is strong enough to keep water droplets and ice crystals suspendedAs a result, precipitation does not occur in the cumulus stage
Thunderstorm Life CycleMature Stage maximum intensityStage typically lasts about 10-20 minutesBegins when precipitation reaches Earths surfaceFeatures heaviest rain, frequent lightning, strong surface winds, and possible tornadoesWeight of droplets and ice crystals overcome the updraftDowndraft created when precipitation descending through the cloud drags the adjacent air downwardEntrained dry air at the edge of the cloud leads to evaporative cooling, which weakens the buoyant uplift and strengthens the downdraftAt the surface, the leading edge of downdraft air resembles a miniature cold front and is called a gust frontOminous-appearing low clouds associated with a gust front include a roll cloud and a shelf cloud
Thunderstorm Life CycleRoll cloudShelf CloudThunderstorms can develop along gust fronts ahead of the main storm
Thunderstorm Life CycleWhen the upward billowing cumulonimbus cloud reaches the tropopause, it spreads out forming a flat anvil top. Cloud tops during the mature stage can build to altitudes in excess of 18,000 m (about 60,000 ft).Thunderstorm cells appear bright white in visible satellite images
Thunderstorm Life CycleDissipating StagePrecipitation and the downdraft spread throughout the thunderstorm cell, heralding the cells demiseSubsiding air replaces the updraft and cuts off the supply of moistureAdiabatic compression warms the subsiding air and the clouds gradually vaporize
Thunderstorm ClassificationNOAA classification of thunderstorms, and the likelihood of severe weather.
Thunderstorm ClassificationThunderstorms are meso-scale convective systems (MCS) and are classified based on the number, organization, and intensity of their constituent cellsSingle-cell thunderstormsUsually a relatively a weak system forming along a boundary within an air mass (i.e., gust front)Typically completes its life cycle in 30 minutes or lessMulticellular thunderstormsCharacterizes most thunderstorms. Each cell may be at a different stage in its life cycle, and a succession of cells is responsible for a prolonged period of thunderstorm weather.Two types:Squall lineMesoscale convective complexEither can produce severe weather
Thunderstorm ClassificationA thunderstorm may track at some angle to the path of its constituent cells, complicating the weather system motion. In the above idealized situation, the component cells of a multicellular thunderstorm travel at about 20 degrees to the eastward moving thunderstorm. As they travel toward the northeast, the individual cells progress through their life cycle.
Thunderstorm ClassificationMulticellular thunderstorm typesSquall line elongated cluster of thunderstorm cells that is accompanied by a continuous gust front at the lines leading edgeMost likely to develop in the warm southeast sector of a mature extra-tropical cyclone, ahead of and parallel to the cold frontMesocyclone convective complex (MCC)A nearly circular cluster of many interacting thunderstorm cells with a lifetime of at least 6 hrs, and often 12-24 hrsThousands of times larger than a single cellPrimarily warm season phenomena (March September)Usually develop at night over the eastern 2/3 of the U.S.Is not associated with a frontUsually develops during weak synoptic-scale flow, often develops near an upper-level ridge of high pressure, and on the cool side of a stationary frontA low level jet feeds warm humid air into the systemSupercell thunderstorms are long-lived single cell stormsExceptionally strong updraft, with rotational circulation that may evolve into a tornado
Thunderstorm ClassificationRadar image of a squall lineInfrared satellite image showing a MCC over the south-central U.S.
Where and WhenConditions necessary for thunderstorms to develop include:Humid air in the low- to mid-troposphereOften mT air when that air mass is destabilizedAtmospheric instabilitymT air becomes unstable when lifted to the convective condensation levelA source of upliftAlong fronts, up mountain slopes, or via horizontal convergence of surface windsThe more humid the air, the less uplift needed to destabilize it
Where and WhenSolar heating drives atmospheric convectionThunderstorms are most frequent when and where solar radiation is most intenseAlso storms are most frequent during the warmest part of the dayThere are many exceptionsExample - the low-level jet stream up the Missouri/Mississippi River Valleys at night contributes to nocturnal thunderstorm maximumThunderstorm frequency is often expressed in thunderstorm days per yearThis is merely a count of the number of days in which thunder is heardThis does not account for days with multiple lines of thunderstorms passing over a weather station
Where and WhenIn the tropics and subtropics, intense solar heating may combine with converging surface winds to trigger thunderstorm developmentThis combination characterizes the ITCZIn North America, thunderstorm frequency increases from north to southHighest frequency over central Florida due to convergence of sea breezesSecond highest frequency over portions of the Rocky Mountain Front Range due to topographically related differences in heatingThunderstorms are unusual over coastal areas downwind from relatively cold ocean waters (i.e., coastal California)Infrequent in Hawaii due to trade wind inversion
Where and WhenAverage annual number of thunderstorm-days in the United States
Where and WhenConverging sea breezes over Florida
Severe ThunderstormsA severe thunderstorm is accompanied by locally damaging surface winds, frequent lightning, or large hailSurface winds stronger than 50 kts (58 mph) and/or hailstones 0.75 in. (1.9 cm) or larger in diameterMay also produce flash floods or tornadoesWhat causes some thunderstorms to be severe?Key is vertical wind shear, the change in horizontal wind speed and direction with increasing altitudeWeak vertical wind shear favors short-lived updrafts, low cloud tops, and weak thunderstormsStrong vertical wind shear favors vigorous updrafts, great vertical cloud development, and severe thunderstormsWith increasing vertical wind shear, the inflow of warm humid air is sustained for a longer period because the gust front cannot advance as far from the cell. Also, most precipitation falls alongside the titled updraft, sustaining the updraft.
Severe ThunderstormsA synoptic weather pattern that favors development of severe thunderstorms. A dryline is the western boundary of the mT air mass and brings about uplift in a manner similar to a cold front.
Severe ThunderstormsThe polar front jet stream produces strong vertical wind shearThis maintains a vigorous updraftThis supports great vertical development of thunderstormsThe jet contributes to stratification of air that increases the potential instability of the troposphereA jet streak induces both horizontal divergence and convergence of air in the upper troposphereConvergence occurs in the right front quadrant of a jet streak, causing weak subsidence of airSinking air is compressionally warmed and forms an inversion (capping inversion) over the mT air massThe underlying air mass becomes more humidContrast between air layers mountsAll that is needed is a lifting mechanism for severe weather to occur
Severe ThunderstormsA temperature sounding that favors the development of severe thunderstorm cells. A capping inversion separates subsiding dry air aloft from warm, humid air near the surface.