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Unformatted text preview: Chapter 10: Thunderstorms and Tornadoes Thunderstorms Tornadoes Tornadic thunderstorms Severe weather and doppler radar Thunderstorms 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 Ordinary Cell Thunderstorms cumulus stage mature stage dissipating stage Ordinary cell thunderstorms are sometimes called `air mass thunderstorms', 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 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! Moisten environment latent heat warms the cloud layer; entrained dry air causes evaporative cooling and downdraft and gust front downdraft cuts off humid updraft; gust front is too far away to enhance updraft Fig. 10-1, p. 265 Severe Thunderstorms and the Supercell
Severe thunderstorm is defined as a thunderstorm with at least one of the following: large hail with a diameter > inch, surface wind gusts >50 knots (58 mi/hr), and/or produces a tornado multicell storms moderate wind shear 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 Fig. 10-5, p. 268 squall line: multicell storms as a line of thunderstorms extending for many kilometers (up to 1000 km) Squall Lines and Mesoscale Convective Complexes prefrontal squall line circular cluster of storms; tend to form in summer in regions where the upperlevel winds are weak; large size (100,000 square km) mesoscale convective complex: multicell storms as a large Dryline Thunderstorms dryline These storms occur frequently in the southern Great Plains of the US. gust front: leading edge of cold air originating inside a thunderstorm shelf cloud and roll cloud outflow boundary: merging several gust fronts downbursts (intense downdraft) and microbursts (< 4km; caused aircraft crash) Gust Fronts, Microbursts and Derechoes with rotation) strong wind extending for 100's km produced by strong downdrafts; producing large hail and sometimes tornado Derecho (dayraysho): straightline (not associated Fig. 10-11, p. 271 Microbursts present a severe hazard to aircraft, especially during takeoff and landing. Several airports have installed microburst detection instruments.
Fig. 10-13, p. 272 A squall line thunderstorms appear in the shape of a bow (or bow echo) on a radar screen and produce a straight-line wind (derecho) Fig. 10-16, p. 273 Floods and Flash Floods flash floods: floods that rise rapidly with little or no advance warning The Great Flood of 1993 over the upper midwest 1976 Big Thompson flash flood (12 inch rain in 4 hours) Distribution of Thunderstorms combination of warmth and moisture geographical placement Great Plains are more favorable for hails partly because evaporation of hail surface water cools the dry air Lightning and Thunder 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 through it travels to 30,000C, 5 times hotter than the Sun's surface Lightning: a discharge of electricity, a giant spark, which usually Thunder: The extreme heating causes the air to expand explosively, thus initiating a shock wave that becomes a booming sound wave Do you see the lightning or hear the thunder first? For normal fair weather, the atmosphere is usually characterized by a negatively charged surface and a positively charged upper atmosphere Electrification of Clouds electrical charges in clouds relationships of updrafts and downdrafts to electrical charges 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 The Lightning Stroke subsequent leader cloudtoground lightning stepped leader return stroke dart leader Types of Lightning
forked lightning ribbon lightning: hanging from clouds due to winds cause forest fire heat lightning: seen but not heard 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 dry lightning: lightning directionfinder
Detecting the radio waves produced by lightning Lightning Detection and Suppression Where do you stay under thunderstorm? Not under trees; Avoid elevated places; Keep your head as low as possible but not touch ground Inside a building; Inside a car Figure 2, p. 282 Tornado Life Cycles usually at 2040 knots tornado or twister: typically 100600 m, may >1 mi, funnel cloud: not reach the ground different tornados spawned by the same thunderstorm dustwhirl stage mature stage decay stage tornado families: tornado outbreaks: 6 or more tornados over a particular region Tornado Occurrence tornado alley time of day: most frequent at 46pm LT times of year Annual number per state; Annual number per 100 mi by 100 mi Tornado Winds multivortex tornadoes suction vortices 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 Don't stay under a highway overpass Don't stay near window or wall tornado classification based on damage The Fujita Scale The "F-scale" was named after Prof. Ted Fujita. Wind damage is proportional to the square of wind speed Tornadic Thunderstorms
mesocyclones bounded weak echo region hook echo rotating clouds wall cloud A rotating wall cloud is an unforgettable sight - just ask a successful storm chaser. Fig. 10-35, p. 290 Nonsupercell Tornadoes Gustnadoes: along a gust front Landspouts: weak and short
lived, from congestus cold air funnels: cold air aloft Doppler shift tornado vortex signature rapidly changing wind direction Doppler lidar Severe Weather and Doppler Radar use light beam, higher resolution NEXRAD waterspout ...
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This note was uploaded on 04/06/2008 for the course NATS - 101 taught by Professor Zeng during the Fall '07 term at University of Arizona- Tucson.
- Fall '07