Class #9: Monday, July 19 Thunderstorms and tornadoes

Class #9, Monday, July 19, 2010 1

Class #9: Monday, July 19Thunderstorms and tornadoes

Chapter 14

Class #9, Monday, July 19, 2010 2Fig. 14-CO, p. 370

Class #9, Monday, July 19, 2010 3Fig. 14-1, p. 372


Thunderstorms and Tornadoes

Chapter 14


Thunderstorms

• A storm containing lightening and thunder; convective storms

• Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50kts, or tornado

• Ordinary Cell Thunderstorms– Air-mass thunderstorms: limited wind sheer– Stages: cumulus, mature, dissipating– Entrainment, downdraft, gust front








Thunderstorms

Multi-cell ThunderstormsThunderstorms that contain a number of convection

cells, each in a different stage of development, moderate to strong wind shear; tilt, over shooting top

Gust Front: leading edge of the cold air out-flowing air; shelf cloud, roll cloud, outflow boundary

Micro-bursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind; wind shear, virga







Thunderstorms

• Multi-cell Thunderstorms– Squall-line thunderstorms; line of multi-cell

thunderstorms, pre-frontal squall-line, derecho– Meso-scale Convective Complex: a number of

individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system; summer, 10,000km2



Class #9, Monday, July 19, 2010 21Stepped Art

Fig. 14-11, p. 378








Thunderstorms Supercell thunderstorms

Large, long-lasting thunderstorm with a single rotating updraft

Strong vertical wind shearOutflow never undercuts updraftClassic, high precipitation and low precipitation

supercellsCap and convective instabilityRain free base, low-level jetSurface, 850mb, 700mb, 500mb, 300mb conditions








Thunderstorms

Thunderstorms and the DrylineSharp, horizontal change in moistureThunderstorms form just east of drylinecP, mT, cT

Floods and Flash FloodsFlash floods rise rapidly with little or no advance

warning; many times caused by stalled or slow thunderstorm

Large floods can be created by training of storm systems, Great Flood of 1993

Class #9, Monday, July 19, 2010 36Fig. 1, p. 386



Thunderstorms

• Topic: Big Thompson Canyon– July 31, 1976, 12 inches of rain in 4 hours created

a flood associated with $35.5million in damage and 135 deaths

• Distribution of Thunderstorms– Most frequent Florida, Gulf Coast, Central Plains– Fewest Pacific coast and Interior valleys– Most frequent hail Central Plains





Thunderstorms

• Lightening and Thunder– Lightening: discharge of electricity in mature storms

(within cloud, cloud to cloud, cloud to ground)– Thunder: explosive expansion of air due to heat

from lightening– Electrification of Clouds: graupel and hailstones fall

through supercooled water, ice crystals become negatively charged

– Upper cloud positive, bottom cloud negative






Thunderstorms

• Observations: Elves– Blue jets, red sprite, ELVES

• The Lightening Stroke– Positive charge on ground, cloud to ground

lightening– Stepped leader, ground stroke, forked lightening,

ribbon lightening, bead lightening, corona discharge



Thunderstorms

• Observation: Apple tree– DO NOT seek shelter during a thunderstorm under

an isolated tree.• Lightening Detection and Suppression– Lightening direction finder detects radiowaves

produced by lightening, spherics– National Lightening Detection Network– Suppression: seed clouds with aluminum











Tornadoes

• Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground.

• Tornado life cycle– Organizing, mature, shrinking, decay stage

• Tornado outbreaks– Families, super outbreak


Tornadoes Tornado Occurrence

US experiences most tornadoesTornado Alley (warm, humid surface; cold dry air aloft)Highest spring, lowest winter

Tornado windsMeasurement based upon damage after storm or

Doppler radarFor southwest approaching storms, winds strongest in

the northeast of the storm, 220 kts maximumMulti-vortex tornados






Tornadoes

• Seeking shelter– Basement or small, interior room on ground floor– Indoor vs. outdoor pressure

• The Fujita Scale– Based upon the damage created by a storm– F0 weakest, F5 strongest– Enhanced Fujita Scale


Tornadic Formation

• Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear

• Supercell Tornadoes– Wind sheer causes spinning vortex tube that is

pulled into thunderstorm by the updraft– Mesocyclone, BWER, rear flank downdraft, vertical

stretching, funnel cloud, rotating cloud, wall cloud

Class #9, Monday, July 19, 2010 67Table 14-1, p. 399








Class #9, Monday, July 19, 2010 75Stepped Art

Fig. 14-46, p. 402


Tornadic Formation

• Nonsupercell Tornadoes– Gustnadoes– Land spout– Cold-air funnels




Severe Weather and Doppler Radar

• Doppler radar measures the speed of precipitation toward and away radar unit

• Two Doppler radars can provide a 3D view• TVS, Doppler lidar• NEXRAD





Waterspouts

• Rotating column of air that is connected to a cumuliform cloud over a large body of water

• Tornadic waterspout


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Class #9: Monday, July 19 Thunderstorms and tornadoes