© ams 1 chapter 11 thunderstorms and tornadoes ams weather studies introduction to atmospheric...

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Chapter 11Chapter 11

Thunderstorms and Thunderstorms and TornadoesTornadoes

AMS Weather StudiesAMS Weather Studies Introduction to Atmospheric Science, 4Introduction to Atmospheric Science, 4 thth Edition Edition

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Case-In-PointCase-In-Point Major severe weather outbreak of 3 May 1999 Major severe weather outbreak of 3 May 1999

– More than 70 tornadoes were reported in Oklahoma, More than 70 tornadoes were reported in Oklahoma, northern Texas, and south central Kansas, and 26 of northern Texas, and south central Kansas, and 26 of these occurred in or around Oklahoma Citythese occurred in or around Oklahoma City

– An F5 tornado took 38 lives in Oklahoma City suburbsAn F5 tornado took 38 lives in Oklahoma City suburbs– An F4 tornado claimed 6 more lives in Haysville, KansasAn F4 tornado claimed 6 more lives in Haysville, Kansas– Essential ingredients for this outbreak:Essential ingredients for this outbreak:

Warm, humid surface air layer was initially capped (capping Warm, humid surface air layer was initially capped (capping inversion) with much drier air aloftinversion) with much drier air aloft

Temperature and humidity contrast between low-level and Temperature and humidity contrast between low-level and upper-level layers grew throughout the day, increasing the upper-level layers grew throughout the day, increasing the potential for deep convection and severe weatherpotential for deep convection and severe weather

Sounding indicated strong vertical wind shearSounding indicated strong vertical wind shear Afternoon arrival of a jet streak lifted the air column and Afternoon arrival of a jet streak lifted the air column and

eliminated the capping inversioneliminated the capping inversion Massive supercell thunderstorms developed explosively and Massive supercell thunderstorms developed explosively and

spawned violent tornadoesspawned violent tornadoes

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What conditions in the atmospheric favor What conditions in the atmospheric favor development of severe convective weather development of severe convective weather systems?systems?– Tornadoes are the most intense of weather Tornadoes are the most intense of weather

systems, but less than 1% of all thunderstorms systems, but less than 1% of all thunderstorms spawn tornadoesspawn tornadoes

– This chapter covers thunderstorms and This chapter covers thunderstorms and tornadoes, their characteristics, life cycles, tornadoes, their characteristics, life cycles, geographical and seasonal distributions and geographical and seasonal distributions and associated hazardsassociated hazards

Driving QuestionDriving Question

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Thunderstorm Life CycleThunderstorm Life Cycle A thunderstorm is a meso-scale weather system that is accompanied A thunderstorm is a meso-scale weather system that is accompanied

by lightning and thunder, affects a relatively small area, and is short-by lightning and thunder, affects a relatively small area, and is short-lived. It is the product of vigorous convection extending high into the lived. It is the product of vigorous convection extending high into the troposphere.troposphere.

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Thunderstorm Life CycleThunderstorm Life Cycle

Towering Cumulus StageTowering Cumulus Stage– Cumulus clouds build vertically and laterally, and surge upward to Cumulus 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-altitudes of 8,000-10,000 m (26,000-33,000 ft) over a period of 10-15 minutes15 minutes

– Produced by convection within the atmosphereProduced by convection within the atmosphere Free convection – triggered by intense solar heating of Earth’s Free convection – triggered by intense solar heating of Earth’s

surfacesurface– Generally not powerful enough to produce thunderstormsGenerally not powerful enough to produce thunderstorms

Forced convection – orographic uplift or converging winds Forced convection – orographic uplift or converging winds strengthen convectionstrengthen convection

– This is generally the cause of thunderstormsThis is generally the cause of thunderstorms– Latent heat released during condensation adds to buoyancyLatent heat released during condensation adds to buoyancy– During the cumulus stage, the updraft is strong enough to keep During the cumulus stage, the updraft is strong enough to keep

water droplets and ice crystals suspendedwater droplets and ice crystals suspended As a result, precipitation does not occur in the cumulus stageAs a result, precipitation does not occur in the cumulus stage

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Thunderstorm Life CycleThunderstorm Life Cycle Mature Stage – maximum intensityMature Stage – maximum intensity

– Stage typically lasts about 10-20 minutesStage typically lasts about 10-20 minutes– Begins when precipitation reaches Earth’s surfaceBegins when precipitation reaches Earth’s surface– Features heaviest rain, frequent lightning, strong surface Features heaviest rain, frequent lightning, strong surface

winds, and possible tornadoeswinds, and possible tornadoes– Weight of droplets and ice crystals overcome the updraftWeight of droplets and ice crystals overcome the updraft– Downdraft created when precipitation descending Downdraft created when precipitation descending

through the cloud drags the adjacent air downwardthrough the cloud drags the adjacent air downward Entrained dry air at the edge of the cloud leads to evaporative Entrained dry air at the edge of the cloud leads to evaporative

cooling, which weakens the buoyant uplift and strengthens the cooling, which weakens the buoyant uplift and strengthens the downdraftdowndraft

At the surface, the leading edge of downdraft air resembles a At the surface, the leading edge of downdraft air resembles a miniature cold front and is called a gust frontminiature cold front and is called a gust front

Ominous-appearing low clouds associated with a gust front Ominous-appearing low clouds associated with a gust front include a roll cloud and a shelf cloudinclude a roll cloud and a shelf cloud

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Roll cloudRoll cloud

Shelf CloudShelf Cloud

Thunderstorms can develop along Thunderstorms can develop along gust fronts ahead of the main stormgust fronts ahead of the main storm

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Dissipating StageDissipating Stage– Precipitation and the downdraft spread Precipitation and the downdraft spread

throughout the thunderstorm cell, heralding the throughout the thunderstorm cell, heralding the cell’s demisecell’s demise

– Subsiding air replaces the updraft and cuts off Subsiding air replaces the updraft and cuts off the supply of moisturethe supply of moisture

– Adiabatic compression warms the subsiding air Adiabatic compression warms the subsiding air and the clouds gradually vaporizeand the clouds gradually vaporize


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Thunderstorm ClassificationThunderstorm Classification

NOAA classification of thunderstorms, and the likelihood of severe weather.NOAA classification of thunderstorms, and the likelihood of severe weather.

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Thunderstorms are meso-scale convective systems (MCS) Thunderstorms are meso-scale convective systems (MCS) and are classified based on the number, organization, and and are classified based on the number, organization, and intensity of their constituent cellsintensity of their constituent cells

Single-cell thunderstormsSingle-cell thunderstorms– Usually a relatively a weak system forming along a boundary within Usually a relatively a weak system forming along a boundary within

an air mass (i.e., gust front)an air mass (i.e., gust front)– Typically completes its life cycle in 30 minutes or lessTypically completes its life cycle in 30 minutes or less

Multicellular thunderstormsMulticellular thunderstorms– Characterizes most thunderstorms. Each cell may be at a different Characterizes most thunderstorms. Each cell may be at a different

stage in its life cycle, and a succession of cells is responsible for a stage in its life cycle, and a succession of cells is responsible for a prolonged period of thunderstorm weather.prolonged period of thunderstorm weather.

– Two types:Two types: Squall lineSquall line Mesoscale convective complexMesoscale convective complex Either can produce severe weatherEither can produce severe weather

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A thunderstorm may track at some angle to the path of its constituent cells, A thunderstorm may track at some angle to the path of its constituent cells, complicating the weather system motion. In the above idealized situation, complicating the weather system motion. In the above idealized situation, the component cells of a multicellular thunderstorm travel at about 20 the component cells of a multicellular thunderstorm travel at about 20 degrees to the eastward moving thunderstorm. As they travel toward the degrees to the eastward moving thunderstorm. As they travel toward the northeast, the individual cells progress through their life cycle.northeast, the individual cells progress through their life cycle.

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Thunderstorm ClassificationThunderstorm Classification Multicellular thunderstorm typesMulticellular thunderstorm types

– Squall line – elongated cluster of thunderstorm cells that is Squall line – elongated cluster of thunderstorm cells that is accompanied by a continuous gust front at the line’s leading edgeaccompanied by a continuous gust front at the line’s leading edge Most likely to develop in the warm southeast sector of a mature Most likely to develop in the warm southeast sector of a mature

extra-tropical cyclone, ahead of and parallel to the cold frontextra-tropical cyclone, ahead of and parallel to the cold front– Mesocyclone convective complex (MCC)Mesocyclone convective complex (MCC)

A nearly circular cluster of many interacting thunderstorm cells A nearly circular cluster of many interacting thunderstorm cells with a lifetime of at least 6 hrs, and often 12-24 hrswith a lifetime of at least 6 hrs, and often 12-24 hrs

Thousands of times larger than a single cellThousands of times larger than a single cell Primarily warm season phenomena (March – September)Primarily warm season phenomena (March – September) Usually develop at night over the eastern 2/3 of the U.S.Usually develop at night over the eastern 2/3 of the U.S. Is not associated with a frontIs not associated with a front Usually develops during weak synoptic-scale flow, often Usually develops during weak synoptic-scale flow, often

develops near an upper-level ridge of high pressure, and on the develops near an upper-level ridge of high pressure, and on the cool side of a stationary frontcool side of a stationary front

A low level jet feeds warm humid air into the systemA low level jet feeds warm humid air into the system– Supercell thunderstorms are long-lived single cell stormsSupercell thunderstorms are long-lived single cell storms

Exceptionally strong updraft, with rotational circulation that may Exceptionally strong updraft, with rotational circulation that may evolve into a tornadoevolve into a tornado

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Radar image of a squall lineRadar image of a squall line Infrared satellite image showing a MCC Infrared satellite image showing a MCC over the south-central U.S.over the south-central U.S.

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Where and WhenWhere and When

Conditions necessary for thunderstorms to Conditions necessary for thunderstorms to develop include:develop include:– Humid air in the low- to mid-troposphereHumid air in the low- to mid-troposphere

Often mT air when that air mass is destabilizedOften mT air when that air mass is destabilized

– Atmospheric instabilityAtmospheric instability mT air becomes unstable when lifted to the convective mT air becomes unstable when lifted to the convective

condensation levelcondensation level

– A source of upliftA source of uplift Along fronts, up mountain slopes, or via horizontal Along fronts, up mountain slopes, or via horizontal

convergence of surface windsconvergence of surface winds The more humid the air, the less uplift needed to The more humid the air, the less uplift needed to

destabilize itdestabilize it

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Solar heating drives atmospheric convectionSolar heating drives atmospheric convection– Thunderstorms are most frequent when and where solar Thunderstorms are most frequent when and where solar

radiation is most intenseradiation is most intense– Also storms are most frequent during the warmest part of Also storms are most frequent during the warmest part of

the daythe day There are many exceptionsThere are many exceptions

– Example - the low-level jet stream up the Example - the low-level jet stream up the Missouri/Mississippi River Valleys at night contributes to Missouri/Mississippi River Valleys at night contributes to nocturnal thunderstorm maximumnocturnal thunderstorm maximum

Thunderstorm frequency is often expressed in Thunderstorm frequency is often expressed in thunderstorm days per yearthunderstorm days per year– This is merely a count of the number of days in which This is merely a count of the number of days in which

thunder is heardthunder is heard– This does not account for days with multiple lines of This does not account for days with multiple lines of

thunderstorms passing over a weather stationthunderstorms passing over a weather station

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In the tropics and subtropics, intense solar heating may In the tropics and subtropics, intense solar heating may combine with converging surface winds to trigger combine with converging surface winds to trigger thunderstorm developmentthunderstorm development– This combination characterizes the ITCZThis combination characterizes the ITCZ

In North America, thunderstorm frequency increases from In North America, thunderstorm frequency increases from north to southnorth to south– Highest frequency over central Florida due to convergence of sea Highest frequency over central Florida due to convergence of sea

breezesbreezes– Second highest frequency over portions of the Rocky Mountain Front Second highest frequency over portions of the Rocky Mountain Front

Range due to topographically related differences in heatingRange due to topographically related differences in heating Thunderstorms are unusual over coastal areas downwind Thunderstorms are unusual over coastal areas downwind

from relatively cold ocean waters (i.e., coastal California)from relatively cold ocean waters (i.e., coastal California) Infrequent in Hawaii due to trade wind inversionInfrequent in Hawaii due to trade wind inversion

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Severe ThunderstormsSevere Thunderstorms

A severe thunderstorm is accompanied by locally A severe thunderstorm is accompanied by locally damaging surface winds, frequent lightning, or damaging surface winds, frequent lightning, or large haillarge hail– Surface winds stronger than 50 kts (58 mph) and/or Surface winds stronger than 50 kts (58 mph) and/or

hailstones 0.75 in. (1.9 cm) or larger in diameterhailstones 0.75 in. (1.9 cm) or larger in diameter– May also produce flash floods or tornadoesMay also produce flash floods or tornadoes

What causes some thunderstorms to be severe?What causes some thunderstorms to be severe?– Key is vertical wind shear, the change in horizontal wind Key is vertical wind shear, the change in horizontal wind

speed and direction with increasing altitudespeed and direction with increasing altitude Weak vertical wind shear favors short-lived updrafts, low cloud Weak vertical wind shear favors short-lived updrafts, low cloud

tops, and weak thunderstormstops, and weak thunderstorms Strong vertical wind shear favors vigorous updrafts, great Strong vertical wind shear favors vigorous updrafts, great

vertical cloud development, and severe thunderstormsvertical cloud development, and severe thunderstorms With increasing vertical wind shear, the inflow of warm humid With increasing vertical wind shear, the inflow of warm humid

air is sustained for a longer period because the gust front air is sustained for a longer period because the gust front cannot advance as far from the cell. Also, most precipitation cannot advance as far from the cell. Also, most precipitation falls alongside the titled updraft, sustaining the updraft.falls alongside the titled updraft, sustaining the updraft.

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A synoptic weather pattern that favors development of A synoptic weather pattern that favors development of severe thunderstorms. A dryline is the western boundary of severe thunderstorms. A dryline is the western boundary of the mT air mass and brings about uplift in a manner similar the mT air mass and brings about uplift in a manner similar to a cold front.to a cold front.

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Severe ThunderstormsSevere Thunderstorms The polar front jet stream produces strong vertical The polar front jet stream produces strong vertical

wind shearwind shear– This maintains a vigorous updraftThis maintains a vigorous updraft– This supports great vertical development of This supports great vertical development of

thunderstormsthunderstorms– The jet contributes to stratification of air that increases The jet contributes to stratification of air that increases

the potential instability of the tropospherethe potential instability of the troposphere A jet streak induces both horizontal divergence and A jet streak induces both horizontal divergence and

convergence of air in the upper troposphereconvergence of air in the upper troposphere Convergence occurs in the right front quadrant of a jet streak, Convergence occurs in the right front quadrant of a jet streak,

causing weak subsidence of aircausing weak subsidence of air Sinking air is compressionally warmed and forms an inversion Sinking air is compressionally warmed and forms an inversion

(capping inversion) over the mT air mass(capping inversion) over the mT air mass The underlying air mass becomes more humidThe underlying air mass becomes more humid Contrast between air layers mountsContrast between air layers mounts All that is needed is a lifting mechanism for severe weather to All that is needed is a lifting mechanism for severe weather to

occuroccur

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A temperature sounding that favors the development of severe A temperature sounding that favors the development of severe thunderstorm cells. A capping inversion separates subsiding thunderstorm cells. A capping inversion separates subsiding dry air aloft from warm, humid air near the surface.dry air aloft from warm, humid air near the surface.

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Mammatus clouds occur on the underside of a thunderstorm anvil and Mammatus clouds occur on the underside of a thunderstorm anvil and sometimes indicate a severe storm system. Their appearance is caused by blobs sometimes indicate a severe storm system. Their appearance is caused by blobs of cold, cloudy air that descend from the anvil into the clear air beneath the anvil.of cold, cloudy air that descend from the anvil into the clear air beneath the anvil.

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Thunderstorm HazardsThunderstorm Hazards

LightningLightning– A brilliant flash of light A brilliant flash of light

caused by an electrical caused by an electrical discharge within a discharge within a cumulonimbus cloud or cumulonimbus cloud or between the cloud and between the cloud and Earth’s surfaceEarth’s surface

– Direct hazard to human lifeDirect hazard to human life– Ignites forest and brush firesIgnites forest and brush fires– Very costly to electrical Very costly to electrical

utilitiesutilities– Lightning detection network Lightning detection network

provides real-time provides real-time informationinformation

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Thunderstorm HazardsThunderstorm Hazards LightningLightning

– What causes lightning?What causes lightning? Large differences in electrical charge develop within a cloud, Large differences in electrical charge develop within a cloud,

between clouds, or between a cloud and the groundbetween clouds, or between a cloud and the ground– Upper portion and much smaller region of the cumulonimbus cloud Upper portion and much smaller region of the cumulonimbus cloud

become positively charged, with a disk-shaped zone of negative become positively charged, with a disk-shaped zone of negative charge in between. A positive charge is induced on the ground charge in between. A positive charge is induced on the ground directly under the clouddirectly under the cloud

Lightning may forge a path between oppositely charged regionsLightning may forge a path between oppositely charged regions Charge separation within a cloud may be due to collisions Charge separation within a cloud may be due to collisions

between descending graupel striking smaller ice crystals in their between descending graupel striking smaller ice crystals in their path. At temperatures < -15 path. At temperatures < -15 °°C (5 C (5 °°F) graupel become F) graupel become negatively charged while ice crystals become positively negatively charged while ice crystals become positively charged. Vigorous updrafts carry ice crystals to upper portions charged. Vigorous updrafts carry ice crystals to upper portions of the cloud.of the cloud.

Positive charge near cloud base also due to graupel-ice crystal Positive charge near cloud base also due to graupel-ice crystal collision, but temps > -15 collision, but temps > -15 °°C (5 C (5 °°F) induce positive charge to F) induce positive charge to graupel and negative charge to ice crystalsgraupel and negative charge to ice crystals

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Thunderstorm HazardsThunderstorm Hazards LightningLightning

– A cloud-to-ground lightning flash involves a regular sequence of A cloud-to-ground lightning flash involves a regular sequence of eventsevents Stepped ladders: streams of electrons surge from the cloud base to the Stepped ladders: streams of electrons surge from the cloud base to the

ground in discrete stepsground in discrete steps Return stroke: forms as an ascending electric current when the positive Return stroke: forms as an ascending electric current when the positive

and negative charges recombine; often emanates from tall, pointed and negative charges recombine; often emanates from tall, pointed structuresstructures

Dart leaders, subsequent surges of electrons from the cloud, follow the Dart leaders, subsequent surges of electrons from the cloud, follow the same conducting pathsame conducting path

Sequence takes place in < two-tenths of a secondSequence takes place in < two-tenths of a second

– Lightning causes intense heating of air so rapidly that air density Lightning causes intense heating of air so rapidly that air density cannot initially respondcannot initially respond Shock wave is generated and propagates outward, producing sound Shock wave is generated and propagates outward, producing sound

waves heard as thunderwaves heard as thunder

– Flash-to-bang method: Thunder takes about 3 seconds to travel 1 Flash-to-bang method: Thunder takes about 3 seconds to travel 1 km (or 5 seconds to travel 1 mi)km (or 5 seconds to travel 1 mi) If you must wait 9 seconds between lightning flash and thunderclap, the If you must wait 9 seconds between lightning flash and thunderclap, the

lightning is about 3 km (1.8 mi) awaylightning is about 3 km (1.8 mi) away

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Thunderstorm Hazards - LightningThunderstorm Hazards - Lightning

Steps in a cloud-to-ground lightning dischargeSteps in a cloud-to-ground lightning discharge

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Thunderstorm Hazards - LightningThunderstorm Hazards - Lightning

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Thunderstorm HazardsThunderstorm Hazards DownburstsDownbursts

– Exceptionally strong downdrafts that occur with or without rainExceptionally strong downdrafts that occur with or without rain– Starburst pattern causes ground destructionStarburst pattern causes ground destruction– Also very dangerous to aircraft because they trigger wind shearAlso very dangerous to aircraft because they trigger wind shear

Aircraft have warning systems that use the same principle as Doppler radarAircraft have warning systems that use the same principle as Doppler radar

– A macroburst cuts a swath of destruction > 4 km (2.5 mi) wide with surface A macroburst cuts a swath of destruction > 4 km (2.5 mi) wide with surface winds that may top 210 km per hr (130 mph)winds that may top 210 km per hr (130 mph)

– A microburst is smaller and shorter livedA microburst is smaller and shorter lived– Derecho: a family of straight-line downburst winds that may be hundreds of Derecho: a family of straight-line downburst winds that may be hundreds of

kilometers long; sustained winds in excess of 94 km per hr (58 mph)kilometers long; sustained winds in excess of 94 km per hr (58 mph)

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Flash FloodsFlash Floods– Short-term, localized, often unexpected rise in stream Short-term, localized, often unexpected rise in stream

level usually in response to torrential rain falling over a level usually in response to torrential rain falling over a relatively small geographical arearelatively small geographical area

– Caused by excessive rainfall in slow moving or Caused by excessive rainfall in slow moving or stationary thunderstorm cellsstationary thunderstorm cells

– Atmospheric conditions that favor flash floods:Atmospheric conditions that favor flash floods: More common at night and form in an atmosphere with weak More common at night and form in an atmosphere with weak

vertical wind shear and abundant moisture through great depthsvertical wind shear and abundant moisture through great depths Precipitation efficient atmosphere has high values of Precipitation efficient atmosphere has high values of

precipitable water and relative humidity and a thunderstorm precipitable water and relative humidity and a thunderstorm cloud base with temperatures above freezingcloud base with temperatures above freezing

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A hydrograph A hydrograph showing showing changes in changes in gauge level and gauge level and discharge in discharge in response to a response to a heavy rain heavy rain event. On the event. On the top graph, top graph, precipitation and precipitation and runoffrunoffare shown in 6-are shown in 6-hr intervals. hr intervals.

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Flash FloodsFlash Floods– Especially hazardous in mountainous Especially hazardous in mountainous

terrainterrain Big Thompson Canyon, CO flood on 31 Big Thompson Canyon, CO flood on 31

July 1976July 1976 Fort Collins, CO flood on 28 July 1997Fort Collins, CO flood on 28 July 1997

– Urban areas are prone to flash floods Urban areas are prone to flash floods during intense downpoursduring intense downpours Concrete and asphalt city surfaces Concrete and asphalt city surfaces

impervious to water and elaborate impervious to water and elaborate storm sewer systems may be unable to storm sewer systems may be unable to handle excess runoffhandle excess runoff

– Flash floods can also be caused by Flash floods can also be caused by breaching of a dam or levee, or by the breaching of a dam or levee, or by the sudden release of water during sudden release of water during breakup of a river ice jambreakup of a river ice jam

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Thunderstorm Thunderstorm Hazards-Hazards-Flash FloodsFlash Floods

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HailHail– Frozen precipitation in the form of balls or lumps of ice > Frozen precipitation in the form of balls or lumps of ice >

5 mm (0.2 in.) in diameter, called hailstones5 mm (0.2 in.) in diameter, called hailstones– Almost always falls from cumulonimbus clouds that are Almost always falls from cumulonimbus clouds that are

characterized by a strong updraft, great vertical characterized by a strong updraft, great vertical development, and an abundance of supercooled waterdevelopment, and an abundance of supercooled water

– Develops when an ice pellet is transported vertically Develops when an ice pellet is transported vertically through portions of the cloud containing varying through portions of the cloud containing varying concentrations of supercooled water dropletsconcentrations of supercooled water droplets Composed of alternating layers of glaze and rimeComposed of alternating layers of glaze and rime

– Grows by accretion (addition) of freezing water droplets Grows by accretion (addition) of freezing water droplets and falls out of cloud base when it becomes to large and and falls out of cloud base when it becomes to large and heavy to be supported by updraftsheavy to be supported by updrafts

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Thunderstorm Hazards - HailThunderstorm Hazards - Hail

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Thunderstorm HazardsThunderstorm Hazards HailHail

– May accumulate on the ground in a long, narrow strip May accumulate on the ground in a long, narrow strip known as a hailstreak; typically 2 km (1.2 mi) wide and known as a hailstreak; typically 2 km (1.2 mi) wide and 10 km (6.2 mi) long10 km (6.2 mi) long

– The figure below is a model of hailstreak developmentThe figure below is a model of hailstreak development

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Thunderstorm HazardsThunderstorm Hazards HailHail

– In the U.S. each year, hail causes an average $1 billion in In the U.S. each year, hail causes an average $1 billion in damage, mostly to crops, livestock, and roofsdamage, mostly to crops, livestock, and roofs Farmers cope with hazard by purchasing insuranceFarmers cope with hazard by purchasing insurance

– The figure below shows that the annual number of severe hail The figure below shows that the annual number of severe hail reports has increased exponentially due to greater public reports has increased exponentially due to greater public awareness, easier report filing, and other factorsawareness, easier report filing, and other factors

– In the U.S., severe hail is most likely in In the U.S., severe hail is most likely in tornado alleytornado alley

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TornadoesTornadoes About 10% of the annual About 10% of the annual

10,000 U.S. severe 10,000 U.S. severe thunderstorms produce thunderstorms produce tornadoestornadoes

A tornado is a violently A tornado is a violently rotating column of air in rotating column of air in contact with the groundcontact with the ground

Most are small and short-Most are small and short-lived and often strike lived and often strike sparsely-populated regionssparsely-populated regions

The most prolific tornado The most prolific tornado outbreak on record occurred outbreak on record occurred over the Great Plains and over the Great Plains and Midwest on 29-30 May 2004Midwest on 29-30 May 2004– 170 tornadoes were 170 tornadoes were

reportedreported

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Tornado CharacteristicsTornado Characteristics The most striking characteristic is the funnel-shaped cloud The most striking characteristic is the funnel-shaped cloud

composed of tiny water dropletscomposed of tiny water droplets– If the vortex remains aloft, it is called a funnel cloudIf the vortex remains aloft, it is called a funnel cloud– If the cloud extends to the ground, it is called a tornadoIf the cloud extends to the ground, it is called a tornado– The funnel cloud forms in response to the steep pressure gradient The funnel cloud forms in response to the steep pressure gradient

directed from the tornado’s outer edge towards its centerdirected from the tornado’s outer edge towards its center A weak tornado’s path is typically < 1.6 km (1 mi) long and A weak tornado’s path is typically < 1.6 km (1 mi) long and

100 m (330 ft) wide with a lifetime of a few minutes100 m (330 ft) wide with a lifetime of a few minutes A violent tornado can have a path > 160 km (100 mi) long A violent tornado can have a path > 160 km (100 mi) long

and 1.0 km (3000 ft) wide with a lifetime of 10 min to > 2 hrsand 1.0 km (3000 ft) wide with a lifetime of 10 min to > 2 hrs– Wind speeds may be up to 500 km per hr (300 mph)Wind speeds may be up to 500 km per hr (300 mph)

Most are spawned by and travel with severe thunderstormsMost are spawned by and travel with severe thunderstorms Usually track from SW to NE, but may go any directionUsually track from SW to NE, but may go any direction

– Average forward speed is 48 km per hr (30 mph)Average forward speed is 48 km per hr (30 mph) An exceptionally great horizontal air pressure gradient is An exceptionally great horizontal air pressure gradient is

responsible for a tornado’s vigorous circulationresponsible for a tornado’s vigorous circulation

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Tornadoes have occurred in all 50 states, but most occur in Tornadoes have occurred in all 50 states, but most occur in tornado alley, a N-S corridor stretching from eastern Texas tornado alley, a N-S corridor stretching from eastern Texas and the Texas Panhandle northward to southeastern South and the Texas Panhandle northward to southeastern South DakotaDakota

Weak tornadoes are more likely over flat than rough terrain, Weak tornadoes are more likely over flat than rough terrain, but strong to violent tornadoes are largely unaffected by but strong to violent tornadoes are largely unaffected by terrainterrain

U.S. typically has 1300 tornadoes/yrU.S. typically has 1300 tornadoes/yr Peak activity in May and June due to in part to:Peak activity in May and June due to in part to:

– Relative instability of the lower atmosphereRelative instability of the lower atmosphere– Favorable synoptic weather conditions: well-defined polar front and Favorable synoptic weather conditions: well-defined polar front and

intense cyclonesintense cyclones Center of maximum tornado frequency follows the sunCenter of maximum tornado frequency follows the sun

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Average number of tornadoes per 10,000 square Average number of tornadoes per 10,000 square miles by state, 1999-2008miles by state, 1999-2008

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Tornado Hazards and the EF-ScaleTornado Hazards and the EF-Scale Tornadoes are a threat to people and property Tornadoes are a threat to people and property

because ofbecause of– Extremely high windsExtremely high winds

Blow down structuresBlow down structures Flying debris main cause of death and injuryFlying debris main cause of death and injury Multi-vortex tornadoes are most destructiveMulti-vortex tornadoes are most destructive It is no longer recommended that windows be opened; most It is no longer recommended that windows be opened; most

buildings have sufficient air leaks so that a potentially explosive buildings have sufficient air leaks so that a potentially explosive pressure differential never developspressure differential never develops

– A strong updraft A strong updraft – Subsidiary vorticesSubsidiary vortices– An abrupt drop in air pressureAn abrupt drop in air pressure

Some tornadoes consist of two or more subsidiary Some tornadoes consist of two or more subsidiary vortices that orbit each other or a common centervortices that orbit each other or a common center– These multi-vortex tornadoes are the most destructiveThese multi-vortex tornadoes are the most destructive

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Tornado Hazards and the EF-ScaleTornado Hazards and the EF-Scale The F-scale was revised in 2007 as The F-scale was revised in 2007 as

the EF-scale (Enhanced F-Scale)the EF-scale (Enhanced F-Scale)– The EF-scale is based on The EF-scale is based on

rotational wind speeds rotational wind speeds

estimated from property estimated from property

damagedamage– Ranges from EF0 to EF5Ranges from EF0 to EF5

EF5 tornadoes are rareEF5 tornadoes are rare About 77% of tornadoes in the U.S. are About 77% of tornadoes in the U.S. are

considered weak (EF0 to EF1) and 95% are considered weak (EF0 to EF1) and 95% are below EF3below EF3

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Tornado Hazards and the EF-ScaleTornado Hazards and the EF-Scale

Path of the Wichita-Andover, Kansas tornado of 26 Path of the Wichita-Andover, Kansas tornado of 26 April 1991. Numbers along the path indicate rating on April 1991. Numbers along the path indicate rating on the F-scale.the F-scale.

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The Tornado-Thunderstorm ConnectionThe Tornado-Thunderstorm Connection

Most violent tornadoes come from supercellsMost violent tornadoes come from supercells– Very energetic with updraft speeds sometimes in excess Very energetic with updraft speeds sometimes in excess

of 240 km per hr (150 mph); they can last for several of 240 km per hr (150 mph); they can last for several hours and produce more than one tornadohours and produce more than one tornado

Supercell formation and characteristicsSupercell formation and characteristics– Horizontal wind exhibits strong speed and directional Horizontal wind exhibits strong speed and directional

shear, which causes air to rotate about a horizontal axis. shear, which causes air to rotate about a horizontal axis. The updraft tilts tube of rotating air to vertical and a The updraft tilts tube of rotating air to vertical and a mesocyclone is formed.mesocyclone is formed.

– A roughly circular lowered portion of the rain-free base A roughly circular lowered portion of the rain-free base of a thunderstorm, called a wall cloud, often of a thunderstorm, called a wall cloud, often accompanies a mesocycloneaccompanies a mesocyclone Most wall clouds do not produce tornadoesMost wall clouds do not produce tornadoes Tornadic wall clouds have strong and persistent rotation before Tornadic wall clouds have strong and persistent rotation before

the appearance of a tornadothe appearance of a tornado

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Supercell formation and characteristics, cont.Supercell formation and characteristics, cont.– A mesocyclone circulation is most intense at 6100 m A mesocyclone circulation is most intense at 6100 m

(20,000 ft); in a tornadic supercell it narrows and builds (20,000 ft); in a tornadic supercell it narrows and builds downward towards the grounddownward towards the ground As the spinning column of air narrows its circulation increases, As the spinning column of air narrows its circulation increases,

similar to an ice skater spinning faster as she pulls in her armssimilar to an ice skater spinning faster as she pulls in her arms A tornado typically appears near the updraft and toward the rear A tornado typically appears near the updraft and toward the rear

of a supercellof a supercell– As the tornadic circulation descends to the surface, a As the tornadic circulation descends to the surface, a

downdraft develops near the rear edge of the supercell. downdraft develops near the rear edge of the supercell. Eventually the downdraft surrounds the tornado and the Eventually the downdraft surrounds the tornado and the tornado dissipatestornado dissipates

Potentially destructive tornadoes can also develop Potentially destructive tornadoes can also develop in multi-cellular thunderstorm clusters and in multi-cellular thunderstorm clusters and hurricaneshurricanes

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Schematic view of a tornadic supercell Schematic view of a tornadic supercell thunderstormthunderstorm

Plan view of a tornadic supercell Plan view of a tornadic supercell thunderstormthunderstorm

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A A thunderstorm wall cloud may accompany a mesocyclone, thunderstorm wall cloud may accompany a mesocyclone, but most do not produce a tornadobut most do not produce a tornado

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Monitoring Tornadic Monitoring Tornadic ThunderstormsThunderstorms

Direct monitoring is generally not feasibleDirect monitoring is generally not feasible Instead, storm chasers rely on photography, balloon-borne Instead, storm chasers rely on photography, balloon-borne

instruments that monitor surrounding atmospheric instruments that monitor surrounding atmospheric conditions, and portable Doppler radar to detect circulation conditions, and portable Doppler radar to detect circulation within supercellswithin supercells

Doppler radar Doppler radar – In the reflectivity mode, can show a hook echo when the parent In the reflectivity mode, can show a hook echo when the parent

mesocyclone is presentmesocyclone is present– In the velocity mode it monitors circulation; a tornado circulation In the velocity mode it monitors circulation; a tornado circulation

may show up as a tornado vortex signature (TVS), a small region of may show up as a tornado vortex signature (TVS), a small region of rapidly changing winds within a mesocyclonerapidly changing winds within a mesocyclone

Storm spotters and visual surveillance of thunderstorms Storm spotters and visual surveillance of thunderstorms are still essentialare still essential

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Monitoring Tornadic ThunderstormsMonitoring Tornadic Thunderstorms

On 3 May 1999, an F-5 tornado devastated Moore, OK. (A) is the On 3 May 1999, an F-5 tornado devastated Moore, OK. (A) is the weather radar image showing the hook echo associated with this weather radar image showing the hook echo associated with this tornado. In (B), a tornado vortex signature (TVS) is visible near tornado. In (B), a tornado vortex signature (TVS) is visible near Moore. [NOAA/NWS/Storm Prediction Center]Moore. [NOAA/NWS/Storm Prediction Center]

AA BB

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Monitoring Tornadic ThunderstormsMonitoring Tornadic Thunderstorms

The annual number of reports of tornadoes in the U.S.The annual number of reports of tornadoes in the U.S. 1950-20081950-2008

© ams 1 chapter 11 thunderstorms and tornadoes ams weather studies introduction to atmospheric...

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