Chapter 10: Thunderstorms and Tornadoes Tornadic thunderstorms Severe weather and doppler radar

Thunderstorms

Thunderstorms What do we already know about thunderstorms? Can produce intense rain, hail, wind, lightning Can make tornadoes Formed by rising air from: Uneven surface heating Uplift along a frontal boundary Topographical uplift 3

Thunderstorms Definitions: Supercell thunderstorms – large updraft storms that can produce flash floods, severe weather, tornadoes Severe Thunderstorm – has to have one of following: Hail greater than or equal to ¾ inch A tornado Wind gusts 50 mph 4

Ordinary Cell Thunderstorms Form in region of low wind shear Can form due to surface air convergence Cumulus stage (growth stage) – due to rising warm, humid air condensing into cumulus cloud Latent heat release keeps cloud warm and unstable Grows quickly to towering cumulus Usually no precipitation because of updrafts No lightning or thunder 5

Ordinary Cell Thunderstorms 6

Ordinary Cell Thunderstorms Mature stage – marked by considerable downdrafts Cloud particles grow larger, and begin to fall Entrainment - Dry air is sucked into cloud, causing evaporation and cooling Heavier and cooler air descends…downdrafts Formation of updraft and downdraft cells Most intense time of thunderstorm Storm may grow as high as stratosphere (anvil) Heavy rain, lightning, small hail 7

Ordinary Cell Thunderstorms Mature stage Cloud top may overshoot into stratosphere 8

Ordinary Cell Thunderstorms Mature stage Cloud top may overshoot into stratosphere Downdraft reaches ground and spreads along surface as a gust front Sometimes rain may not reach ground, but cold air does 9

Ordinary Cell Thunderstorms 10

Ordinary Cell Thunderstorms Dissipating stage – thunderstorm weakens Updrafts begin to weaken after 15 to 30 min. Gust front moves too far from the storm, so updrafts have to weaken Light precipitation falls Only anvil remains All three stages in less than an hour 11

Ordinary Cell Thunderstorms 12

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 at www.thomsonedu.com/login. Stepped Art Fig. 10-1, p. 265

Severe Thunderstorms and the Supercell So ordinary cell storms weaken because the updraft weakens What if the updraft doesn’t weaken quickly? If moderate wind shear pushes downdraft downwind, updraft is not cut off If downdraft then undercuts updraft, a multicell storm forms

Severe Thunderstorms and the Supercell 15

Severe Thunderstorms and the Supercell Multicell storms Cells of varying age co-mingling Top of cloud well into stratosphere Updrafts allow hail to grow large 16

Severe Thunderstorms and the Supercell Supercells If wind shear is strong and changes direction with height, downdraft may not undercut updraft Updraft remains strong for long time (> 1 hour) One rotating vertical column Can create tornadoes Hail size of grapefruit 17

Severe Thunderstorms and the Supercell At surface, open wave with warm, humid air ahead of cold front Overrunning warm air just above surface Dry air at 700 mb level Trough of low pressure at 500 mb Divergence at 300 mb 18

Severe Thunderstorms and the Supercell Severe thunderstorms form in light green area because: Warm air below cold air is conditionally unstable Strong wind shear creates severe thunderstorms In morning, atmos is stable, no thunderstorms Surface heating creates thunderstorms 19

Squall Lines and Mesoscale Convective Complexes Squall line – line of multicell t-storms extending many kilometers Mesoscale convective complex- cluster of storms in one spot

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 weather Form during summer 21

Dryline Thunderstorms Dryline – narrow zone with a sharp moisture contrast. Where do the thunderstorms form?

Gust Fronts, Microbursts and Derechoes Gust front – leading edge of the thunderstorm downdraft Passage can feel like a cold front passage Shelf cloud – warm air rising above gust front creates cloud Roll cloud – cloud just behind the gust front that spins horizontally Outflow boundary – a huge gust front formed by numerous thunderstorm gust front Downbursts – localized downdraft like water from a tap Microbursts – downbursts that extend 4 km or less

Gust Fronts, Microbursts and Derechoes 24

Gust Fronts, Microbursts and Derechoes 25

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. 10-11, p. 271

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

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

Microburst Microbursts can be dangerous to aircrafts Plane encounters uplift at (a), and pilot puts nose down Unexpected downdraft crashes plane Has caused several airplane crashes Some airports have microburst detection 29

Bow Echo and Derecho Bow echo – a line of thunderstorms often form a bow echo on radar Derecho – winds associated with downdrafts that exceed 104 mph and can cause a lot of damage…20 derechoes each year in U.S. 30

Floods and Flash Floods Flash floods – floods that rise rapidly with little to no advance warning Thunderstorms cause flash floods in two ways: Hovering above one area for a long time Training – thunderstorms keep passing over same area

Distribution of Thunderstorms 18 millions thunderstorms worldwide annually Need to have a combination of moisture and warmth So do thunderstorms form in the Arctic? ITCZ? Gulf?

Lightning and Thunder Lightning – simply a discharge of electricity, usually in mature thunderstorms Heats air up to 54,000F (5x hotter that surface of sun Thunder – the explosive expansion of the hot air creates a sound wave that travels in all direction According 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.

Lightning and Thunder Light travels faster than sound, so we see the lightning before we hear the thunder Sound takes 5 seconds to travel 1 mile So if we hear thunder 5 seconds after we see the lightning, the stroke was 1 mile away

Lightning and Thunder

Electrification of Clouds Must have charging of the cloud to have lightning Electrification of clouds not fully understood Theory 1: Supercooled droplets fall through cloud and collide with warm hailstone. Latent heat is released (warm or cold?)

Electrification of Clouds Net transfer of positive ions from warm to cold object Falling hailstone is negatively charged, falls to bottom of cloud Light positively charge particle is lifted to top of cloud Small area of positively charge particles near melting level 37

Electrification of Clouds Theory 2: When precip forms, it has a neg in the upper portion and pos charge in the lower As droplets collide, the large droplets become neg and fall The small droplets are lifted by updrafts and rise 38

The Lightning Stroke Basics of lightning Opposite charges attract Positive charges on ground follow negative charges at base of cloud Electrical current will not flow because air is good insulator Charge must be large (< 1million volts per meter) to create a lightning bolt

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 stroke 40

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 flash 41

The Lightning Stroke Dart leader – subsequent initial stroke that follows same path as initial stepped leader Causes the multiple flash of the lightning 42

Types of Lightning Forked lightning – dart leader takes different path than stepped leader Ribbon lightning – wind blows charges into ribbon-like lightning Dry lightning – lightning that occurs in a dry thunderstorm

Types of Lightning Heat 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

Lightning Detection and Suppression Lightning direction-finder

Tornado Life Cycles Tornado – rotating column of air blowing around a small low pressure that reaches the ground

Tornado Life Cycles Funnel cloud Tornado that hasn’t reached the ground Dust-whirl stage Swirling dust at the ground marks the tornadoes circulation Mature stage Funnel at greatest width, most intense damage. Often vertical Decay stage Funnel shrinks, damage becomes less, tornado becomes stretched 47

Tornado Outbreaks Tornado families – tornadoes spawned by the same thunderstorms Tornado outbreaks – many tornadoes that form over same region Much 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.

Tornado Occurrence Tornado alley – part of the Central Plains from Texas through Nebraska Time of day – most often in the afternoon Times of year – most often in Spring, lease often in Winter

Tornado Winds Multi-vortex tornadoes Suction vortices A single tornado with multiple rotating columns within it Suction vortices The small rotating columns within multi-vortex tornadoes

Seeking Shelter Tornado watch – tornadoes are likely to develop within the next few hours Tornado warning – issued once a tornado is spotted It’s always a good idea to know what to do if a tornado watch or warning is issued for your area.

The Fujita Scale Tornado classification based on damage

Supercell Tornadoes Mesocyclones – Rising, rotating column on the south side of a supercell. Acts to increase the updraft Bounded weak echo region – area inside t-storm where radar does not pick up precip Hook echo – Radar representation of a tornado (looks like a hook) Rotating clouds – First sign tornado is about to form Wall cloud – rotating clouds that have lowered below the base of a supercell A rotating wall cloud is an unforgettable sight - just ask a successful storm chaser.

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. 10-35, p. 290

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

Nonsupercell Tornadoes Gustnadoes – tornadoes that flow along a gust front Landspouts – similar to water spouts and form due to developing cumulus congestus

Severe Weather and Doppler Radar Doppler shift – Change in frequency in sound waves for moving objects Tornado vortex signature – rapidly changing wind direction in mesocyclone NEXRAD – Network of 150 radar stations in U.S. Waterspout – Tornado that forms over water