3 Thunderstorms What do we already know about thunderstorms? Can produce intense rain, hail, wind, lightningCan make tornadoesFormed by rising air from:Uneven surface heatingUplift along a frontal boundaryTopographical uplift3
4 Thunderstorms Definitions: Supercell thunderstorms – large updraft storms that can produce flash floods, severe weather, tornadoesSevere Thunderstorm – has to have one of following:Hail greater than or equal to ¾ inchA tornadoWind gusts 50 mph4
5 Ordinary Cell Thunderstorms Form in region of low wind shearCan form due to surface air convergenceCumulus stage (growth stage) – due to rising warm, humid air condensing into cumulus cloudLatent heat release keeps cloud warm and unstableGrows quickly to towering cumulusUsually no precipitation because of updraftsNo lightning or thunder5
7 Ordinary Cell Thunderstorms Mature stage – marked by considerable downdraftsCloud particles grow larger, and begin to fallEntrainment - Dry air is sucked into cloud, causing evaporation and coolingHeavier and cooler air descends…downdraftsFormation of updraft and downdraft cellsMost intense time of thunderstormStorm may grow as high as stratosphere (anvil)Heavy rain, lightning, small hail7
8 Ordinary Cell Thunderstorms Mature stageCloud top may overshoot into stratosphere8
9 Ordinary Cell Thunderstorms Mature stageCloud top may overshoot into stratosphereDowndraft reaches ground and spreads along surface as a gust frontSometimes rain may not reach ground, but cold air does9
11 Ordinary Cell Thunderstorms Dissipating stage – thunderstorm weakensUpdrafts begin to weaken after 15 to 30 min.Gust front moves too far from the storm, so updrafts have to weakenLight precipitation fallsOnly anvil remainsAll three stages in less than an hour11
13 Figure 10.1: Simplified model depicting the life cycle of an ordinary thunderstorm that is nearly stationary. (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.)Watch this Active Figure on ThomsonNow website atStepped ArtFig. 10-1, p. 265
14 Severe Thunderstorms and the Supercell So ordinary cell storms weaken because the updraft weakensWhat if the updraft doesn’t weaken quickly?If moderate wind shear pushes downdraft downwind, updraft is not cut offIf downdraft then undercuts updraft, a multicell storm forms
16 Severe Thunderstorms and the Supercell Multicell stormsCells of varying age co-minglingTop of cloud well into stratosphereUpdrafts allow hail to grow large16
17 Severe Thunderstorms and the Supercell SupercellsIf wind shear is strong and changes direction with height, downdraft may not undercut updraftUpdraft remains strong for long time (> 1 hour)One rotating vertical columnCan create tornadoesHail size of grapefruit17
18 Severe Thunderstorms and the Supercell At surface, open wave with warm, humid air ahead of cold frontOverrunning warm air just above surfaceDry air at 700 mb levelTrough of low pressure at 500 mbDivergence at 300 mb18
19 Severe Thunderstorms and the Supercell Severe thunderstorms form in light green area because:Warm air below cold air is conditionally unstableStrong wind shear creates severe thunderstormsIn morning, atmos is stable, no thunderstormsSurface heating creates thunderstorms19
20 Squall Lines and Mesoscale Convective Complexes Squall line – line of multicell t-storms extending many kilometersMesoscale convective complex- cluster of storms in one spot
21 Squall Lines and Mesoscale Convective Complexes Squall lines often form ahead of cold fronts (pre-frontal squall-line thunderstorms)MCCs are long-lasting and provide a lot of rain, and a ton of severe weatherForm during summer21
22 Dryline Thunderstorms Dryline – narrow zone with a sharp moisture contrast. Where do the thunderstorms form?
23 Gust Fronts, Microbursts and Derechoes Gust front – leading edge of the thunderstorm downdraftPassage can feel like a cold front passageShelf cloud – warm air rising above gust front creates cloudRoll cloud – cloud just behind the gust front that spins horizontallyOutflow boundary – a huge gust front formed by numerous thunderstorm gust frontDownbursts – localized downdraft like water from a tapMicrobursts – downbursts that extend 4 km or less
26 Figure 10.11: A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest.Fig , p. 271
27 Figure 10.11: A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest.27
28 Figure 10.11: A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest.28
29 Microburst Microbursts can be dangerous to aircrafts Plane encounters uplift at (a), and pilot puts nose downUnexpected downdraft crashes planeHas caused several airplane crashesSome airports have microburst detection29
30 Bow Echo and DerechoBow echo – a line of thunderstorms often form a bow echo on radarDerecho – winds associated with downdrafts that exceed 104 mph and can cause a lot of damage…20 derechoes each year in U.S.30
31 Floods and Flash Floods Flash floods – floods that rise rapidly with little to no advance warningThunderstorms cause flash floods in two ways:Hovering above one area for a long timeTraining – thunderstorms keep passing over same area
32 Distribution of Thunderstorms 18 millions thunderstorms worldwide annuallyNeed to have a combination of moisture and warmthSo do thunderstorms form in the Arctic? ITCZ? Gulf?
33 Lightning and ThunderLightning – simply a discharge of electricity, usually in mature thunderstormsHeats air up to 54,000F (5x hotter that surface of sunThunder – the explosive expansion of the hot air creates a sound wave that travels in all directionAccording to the Guinness Book of World Records, Roy Sullivan holds the world’s record for being struck by lightning seven times between 1942 and his death in 1983.
34 Lightning and ThunderLight travels faster than sound, so we see the lightning before we hear the thunderSound takes 5 seconds to travel 1 mileSo if we hear thunder 5 seconds after we see the lightning, the stroke was 1 mile away
36 Electrification of Clouds Must have charging of the cloud to have lightningElectrification of clouds not fully understoodTheory 1: Supercooled droplets fall through cloud and collide with warm hailstone. Latent heat is released (warm or cold?)
37 Electrification of Clouds Net transfer of positive ions from warm to cold objectFalling hailstone is negatively charged, falls to bottom of cloudLight positively charge particle is lifted to top of cloudSmall area of positively charge particles near melting level37
38 Electrification of Clouds Theory 2: When precip forms, it has a neg in the upper portion and pos charge in the lowerAs droplets collide, the large droplets become neg and fallThe small droplets are lifted by updrafts and rise38
39 The Lightning Stroke Basics of lightning Opposite charges attract Positive charges on ground follow negative charges at base of cloudElectrical current will not flow because air is good insulatorCharge must be large (< 1million volts per meter) to create a lightning bolt
40 The Lightning Stroke Cloud-to-ground lightning Discharge of electrons from cloud to ground as a stepped leader (many times)Positive charges race back up from elevated object as a return stroke40
41 The Lightning Stroke Cloud-to-ground lightning Many electrons flow to ground and a stronger return stoke follows (this is what you see)1/10,000 of a second so it looks like one continuous flash41
42 The Lightning StrokeDart leader – subsequent initial stroke that follows same path as initial stepped leaderCauses the multiple flash of the lightning42
43 Types of LightningForked lightning – dart leader takes different path than stepped leaderRibbon lightning – wind blows charges into ribbon-like lightningDry lightning – lightning that occurs in a dry thunderstorm
44 Types of LightningHeat lightning – lightning that is seen by not heard (can be orange)St. Elmo’s Fire – luminous green or blue halo around the top of pointed objects (antennas, masts of ships). Lightning may occur after this is seen.44
45 Lightning Detection and Suppression Lightning direction-finder
46 Tornado Life CyclesTornado – rotating column of air blowing around a small low pressure that reaches the ground
47 Tornado Life Cycles Funnel cloud Tornado that hasn’t reached the groundDust-whirl stageSwirling dust at the ground marks the tornadoes circulationMature stageFunnel at greatest width, most intense damage. Often verticalDecay stageFunnel shrinks, damage becomes less, tornado becomes stretched47
48 Tornado OutbreaksTornado families – tornadoes spawned by the same thunderstormsTornado outbreaks – many tornadoes that form over same regionMuch ground-breaking research on tornadoes was conducted by Professor Ted Fujita of the University of Chicago. The “F-scale” of tornado intensity was named after him.
49 Tornado OccurrenceTornado alley – part of the Central Plains from Texas through NebraskaTime of day – most often in the afternoonTimes of year – most often in Spring, lease often in Winter
50 Tornado Winds Multi-vortex tornadoes Suction vortices A single tornado with multiple rotating columns within itSuction vorticesThe small rotating columns within multi-vortex tornadoes
51 Seeking ShelterTornado watch – tornadoes are likely to develop within the next few hoursTornado warning – issued once a tornado is spottedIt’s always a good idea to know what to do if a tornado watch or warning is issued for your area.
52 The Fujita ScaleTornado classification based on damage
53 Supercell TornadoesMesocyclones – Rising, rotating column on the south side of a supercell. Acts to increase the updraftBounded weak echo region – area inside t-storm where radar does not pick up precipHook echo – Radar representation of a tornado (looks like a hook)Rotating clouds – First sign tornado is about to formWall cloud – rotating clouds that have lowered below the base of a supercellA rotating wall cloud is an unforgettable sight - just ask a successful storm chaser.
54 Figure 10.35: Some of the features associated with a tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.Fig , p. 290
55 Figure 10.35: Some of the features associated with a tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.55
56 Nonsupercell Tornadoes Gustnadoes – tornadoes that flow along a gust frontLandspouts – similar to water spouts and form due to developing cumulus congestus
57 Severe Weather and Doppler Radar Doppler shift – Change in frequency in sound waves for moving objectsTornado vortex signature – rapidly changing wind direction in mesocycloneNEXRAD – Network of 150 radar stations in U.S.Waterspout – Tornado that forms over water