Decoding Thunderstorms and Tornadoes

Chapter 10: Thunderstorms and Tornadoes • Thunderstorms • Tornadoes • Tornado formation • Observing tornadoes and severe weather • Waterspouts

Thunderstorms • Thunderstorm: storm with lightning and thunder; they are convective storms that form with rising air in a • conditionally unstable environment • The trigger needed to start air moving upward may be • surface heating; • topographic lift; • convergence zone (e.g., sea breeze leading edge); • frontal lift; • divergence aloft • Q: Conditionally unstable environment means that the environmental lapse rate is • a) > 10 K/km, b) < 6 K/km, c) between 6 – 10 K/km

Ordinary Cell Thunderstorms • Three stages: cumulus, mature, dissipating • Ordinary cell thunderstorms are sometimes called ‘air mass thunderstorms’ or `ordinary thunderstorm’, because they form in conditionally unstable air masses and are not necessarily associated with fronts or severe weather • Shortlived (<1 hr), less than 1 km wide, low wind shear, rarely produce strong wind or large hail Q: Is it possible to drop air temperature by 40F in an hour? A: Yes. On 7/13/1999 in California, a strong downdraft from a mature thunderstorm dropped the air temperature from 97F to a chilly 57F in one hour! Q: Why does low wind shear produce an ordinary rather than severe thunderstorm? A: because downdraft would cutoff the supply of moisture

Moisten environment latent heat warms downdraft cuts off the cloud layer humid updraft; Q: why is the thunderstorm downdraft usually cold? A: Entrained dry air causes raindrop evaporation, cooling the air which descends as a downdraft

Severe Thunderstorms and Supercell Severe thunderstorm is defined as a thunderstorm with at least one of the following: large hail with a diameter > ¾ inch, surface wind gusts >50 knots (58 mi/hr), and/or produces a tornado multicell storms moderate wind shear; lasting several hours; multiple ordinary or supercell thunderstorms A B C Q: which cloud in the figure is at the mature stage? a) A, b) B, c) C

Gust Front • gust front: leading edge of cold air originating inside a thunderstorm • shelf cloud and roll cloud • outflow boundary: merging several gust fronts

Shelf cloud roll cloud

Microbursts • downbursts (intense downdraft) and microbursts (< 4km; caused aircraft crash) Dust clouds due to mictoburst tail wind headwind Microbursts present a severe hazard to aircraft, especially during takeoff and landing. Several airports have installed microburst detection instruments.

Squall Lines • squall line: multicell storms as a line of thunderstorms extending for many kilometers (up to 1000 km) • Pre-frontal squall line may be initiated by gravity waves

Strong downdrafts of squall lines cause bow-shaped signal in a Doppler radar image, called bow echo. When the damage associated with the straight-line winds extends for a considerable distance along the squall line’s path, the wind storm is called a derecho (day-ray-sho).

mesoscale convective complex (MCC): multicell storms as a large circular cluster of storms; tend to form in summer in regions where the upper-level winds are weak; large size (100,000 square km); last for several hours Q: Who first defined MCC? a) a UA scientist; b) a non-UA scientist

Supercell: Strong wind shear in speed and direction; Shallow inversion above warm and humid layer acts as a lid; Long-lasting (hours); Larger than 1 km in diameter; Single violently rotating updraft; Produces tornado, large hail, strong gusts Q: A thunderstorm with weak wind shear is most probably a) ordinary cell, b) multicell, c) supercell

A model of classical supercell

Q: why is the weather pattern left is favorable for supercell? A: a) with wind shear, downdraft would not cut off the updraft; b) upper level divergence and lower level convergence strengthen the updraft Q: Why do wind shear (e.g., increasing wind speed with height) and upward motion would cause the updraft to rotate?

Dryline Thunderstorms • dryline • These storms occur frequently in the southernGreat Plains of the US.

Q: A circular cluster of storms is called • a) MCC, b) squall line, c) dryline • Q: Intense downdraft is called • a) Derecho, b) gust front, c) downburst • Q: What is most damaging for aviation? • a) Derecho, b) gust front, c) microburst • Q: Which has the largest cloud area as seen from • satellites? • dryline, b) MCC, c) squall line, d) supercell • Q: If the vertical wind shear is weak, the thunderstorm is • a) ordinary cell, b) multicell, c) supercell

Thunderstorms and Flooding • flash floods: floods that rise rapidly with little or no advance warning 1976 Big Thompson flash flood (12 inch rain in 4 hours) The Great Flood of 1993 over the upper midwest

Distribution of Thunderstorms • combination of warmth and moisture • geographical placement Thunderstorm days Hail days Q: why are the Great Plains more favorable for hails than, say, Florida (right panel above)? A: The Great Plains are more favorable for severe thunderstorms (including larger hails); the warm subcloud layer in Florida melts hail before reaching the ground.

Lightning and Thunder • Lightning: a discharge of electricity, a giant spark, which usually occurs in mature thunderstorms (and may also occur in snowstorms and dust storms); majority of lightning strokes within clouds with only 20% between clouds and surface; a lightning stroke can heat the air it travels to 30,000C, 5 times as hot as the Sun’s surface Q: why would a lightning cause a thunder? A: the extreme heating from a lightning causes the air to expand explosively, thus initiating a shock wave that becomes a booming sound wave (or a thunder) Q: Do you see the lightning or hear the thunder first? a) see the lightning first, b) hear the thunder first, c) at the same time Q: Assuming sound speed is 330 m/s and you hear the thunder 5 seconds after seeing the lightning, what is the distance of the lightning stroke? a) 330 m, b) 1 km, c) 1 mile

Electrification of Clouds For normal fair weather, the atmosphere is usually characterized by a negatively charged surface and a positively charged upper atmosphere • relationships of updrafts and downdrafts to electrical charges in clouds There is a net transfer of positive ions (charged molecules) from the warmer (and larger) hailstone to the colder (and smaller) ice crystal or supercooled droplets which are lifted to the upper layer of clouds Q: Why are hailstones warmer than smaller droplets? a) because latent heat release; b) because hailstones are bigger; c) because smaller droplets are supercool

The Lightning Stroke • cloud-to-ground lightning • stepped leader • return stroke: large number of electrons flow to the ground and a much larger, more luminous return stroke (current) to cloud • dart leader – subsequent leader

Types of Lightning • forked lightning • ribbon lightning: hanging from clouds due to winds • dry lightning: not producing rain; cause forest fire • heat lightning (in summer): seen but not heard because sound wave propagation is affected by air • St. Elmo’s fire: a corona discharge or sparks, can cause the top of a ship’s mast to glow; also seen over power lines and aircraft wings

Lightning Detection and Protection • lightning direction-finder: detecting the radio waves produced by lightning • Satellites can also monitor global lightning activities Q: who created the U.S. National Lightning Detection Network? a) a UA scientist; b) a non-UA scientist

Lightning Protection Q: who invented the lightning rod? a) Jefferson, b) Lincoln, c) Washington, d) Franklin

Q: Where do you stay under • thunderstorm? A: • Not under trees; • Avoid elevated places; • Keep your head as low as • possible but not touch ground • Inside a building; • Inside a car; • Not in a golf cart • Q: why don’t you want to lie down on the ground? • A: lightning channels usually emanate outward at a point of lightning strike, a surface current may travel through your body and injure or kill you

Tornadoes • tornado or twister: typically 100-600 m, maybe >1 mi, usually moves at 20-40 knots • funnel cloud: not reach the ground • dust-whirl stage • mature stage • decay stage Tornadoes ripped through the Dallas region on 4/3/2012 and a truck became airborne.

Tornado Occurrence • tornado alley: Great Plains • time of day: most frequent at 4-6pm LT • times of year: May and June Annual number per state; Annual number per 100 mi by 100 mi

Tornado Winds • multi-vortex tornadoes • suction vortices Q: what is the wind speed at A or C? a) 100 knots, b) 112 knots, c) 150 knots

Seeking Shelter • tornado watch: likely to form • tornado warning: spotted visually or by radar • It’s always a good idea to know what to do if a tornado watch or warning is issued for your area. • Take shelter in the basement or small room in the middle of the house at a lower level and cover your head • Lie flat on the ground in a ditch (but not lie down on a flat surface) • Don’t stay under a highway overpass • Don’t stay near window or wall • Don’t stay in a mobile home

The Fujita Scale • tornado classification based on damage • The “F-scale” was named after Prof. Ted Fujita. • Wind damage is proportional to the square of wind speed Q: How many times is the damage from F3 (160 knots) as that from F0 (40 knots)? a) 2, b) 4, c) 8, d) 16

Tornado outbreaks Tornado families: different tornados spawned by the same thunderstorm Tornado outbreaks: 6 or more tornados over a particular region Tri-state tornado outbreak on 3/18/1925: at least 7 tornadoes traveled a total of 437 miles across portions of Missouri, Illinois, and Indiana, causing 747 human casulties. Meteorologists are doing a better job now in protecting lives.

Tornado Formation Supercell Tornadoes • mesocyclones • bounded weak echo region: inside mesocyclone (no precip) • Radar hook echo: rotating precipitation around mesocyclone • wall cloud • A rotating wall cloud is an unforgettable sight - justask a successful storm chaser.

hook echo Q: What is the percentage of supercells producing tornadoes: a) 100%, b) 70%, c) 40%, d) 15% Q: Is mesocyclone caused by the vertical tilting of the horizontal vortex tube? a) yes, b) no

Nonsupercell Tornadoes • Gustnadoes: tornado along a gust front • Landspouts: weak and short-lived, from congestus • cold air funnels: cold air aloft; short-lived; weak • Waterspouts: similar to landspouts

Observing Tornadoes and Severe Weather • Doppler shift: similar to change of sound frequency as a train approaches the observer • tornado vortex signature: rapidly changing wind direction • Doppler lidar: use light beam (instead of microwave in radar), higher spatial resolution • NEXRAD: >150 Doppler radars over continental U.S. (www.weather.gov) Yellow and red: moving away Green and blue: moving toward the radar

Decoding Thunderstorms and Tornadoes