A storm containing lightning and thunder; convective storms Chapter 14 Severe thunderstorms: At least one: large hail wind gusts greater than or equal to 50 kt Tornado 1 2 Ordinary Cell Ordinary Cell AKA Air-mass thunderstorms: form with limited wind sheer, vertically stacked Stages: cumulus, mature, dissipating Cumulus Stage (Growth Stage) Warm air rises. Cloud droplets evaporate at top and make air more humid, allowing higher growth. Condensation releases heat, which causes more rising. No precipitation (held aloft by updrafts), no lightning. Mature Stage Droplets grow large enough to fall. Drier air is drawn into cloud from sides (entrainment). This causes evaporates some drops, which cools air. Cooling air leads to downdrafts, also enhanced by falling precipitation. Anvil-Shaped cumulonimbus is formed, cloud top can be 40000 high. Updrafts collide with downdrafts- turbulence. Cool downdrafts spread sideways at the ground along a gust front. Turbulence along gust front. Warm air rises over gust front. 3 4 Ordinary Cell Dissipating Stage Storm generally dissipates after 15-30 minutes. Gust front moves away from the storm and no longer enhances updrafts. Downdrafts dominate, no more updrafts to fuel the storm. Whole process can last only one hour. bring summer rain and welcome temperature relief, though brief. 5 6 1
Multi-cell that contain a number of convection cells, each in a different stage of development Caused by moderate to strong wind shear, which produces tilt Sometimes there is an over shooting top Mammatus clouds can form below the anvil Gust Front: leading edge of the cold air out-flowing air Can form shelf clouds or roll clouds. Combined edge of gust fronts is called the outflow boundary. Downbursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind Microbusts: 4km spread or less. Cause wind shear, dangerous to planes. Plane crash in Dallas, 1985: 100 deaths. Virga may form. Warm downbursts are called heat bursts. 7 8 9 Fig. 14-4, p. 10 375 Fig. 14-4, p. 11 375 12 2
5/2/11 13 14 15 16 17 18 Multi-cell Squall-line thunderstorms: a line of multi-cell thunderstorms Pre-frontal squall-line Bow Echo: a bow-shaped squall line Derecho: High winds along several hundred kilometers of squall line The Rear Inflow Jet Meso-scale Convective Complex (MCC): a number of individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system Damaging straight-line winds Happens in summer Can last 12 hours, cover 10,000 km2 3
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Supercell Large, long-lasting thunderstorm with a single rotating updraft Strong vertical wind shear: horizontal rotation becomes vertical. Outflow never undercuts updraft The Mesocyclone and the Overshooting Top Wall clouds Three types of supercell: Classic High precipitation Low precipitation Rain free base, low-level jet Surface, 850mb, 700mb, 500mb, 300mb conditions 25 26 27 28 Inversion layer caps air, convective instability: breakthrough 29 30 5
Floods Flash Floods Flash 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 31 32 Distribution of Most frequent Florida, Gulf Coast, Central Plains Fewest Pacific coast and Interior valleys Most frequent hail Central Plains 33 34 Lightning and Thunder Lightning: 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 35 36 6
37 38 The Lightning Stroke Positive charge on ground, cloud to ground lightning Stepped leader, ground stroke, forked lightening, ribbon lightning, bead lightning, corona discharge 39 40 Observation: Apple tree DO NOT seek shelter during a thunderstorm under an isolated tree. Lightning Detection and Suppression Lightning direction finder detects radiowaves produced by lightning: Spherics National Lightning Detection Network Suppression: seed clouds with aluminum 41 42 7
Tornadoes Tornadoes Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground. Funnel cloud: A tornado that has not reached the ground Tornado life cycle Organizing, mature, shrinking, decay stage Tornado outbreaks Families, super outbreak Tornado Occurrence US experiences most tornadoes Tornado Alley (warm, humid surface; cold dry air aloft, wind shear helped by jet stream) Highest in spring, lowest in winter Tornado winds Measurement based upon damage after storm or Doppler radar For southwest approaching storms, winds strongest in the northeast of the storm, 220 kts maximum If a tornado approaches, on which side is the wind fastest? Multi-vortex tornados 43 44 45 46 47 48 8
Tornadoes Seeking shelter Basement or small, interior room on ground floor Indoor vs. outdoor pressure, p. 398 The Fujita Scale Based upon the damage created by a storm F0 weakest, F5 strongest Enhanced Fujita Scale 49 Table 14-1, p. 50 399 Table 14-2, p. 51 400 Table 14-3, p. 52 400 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 (bounded weak echo region), rear flank downdraft, vertical stretching, funnel cloud, rotating cloud, wall cloud 53 54 9
5/2/11 Concordia University Geog/ Sci-381 Chapter 14 55 56 Art Stepped Fig. 14-46, p. 402 Tornadic Formation Nonsupercell Tornadoes Gustnadoes Form along gust front, short-lived and weak Land spout Common over East-Central Colorado Cold-air funnels Formed by cold air aloft, common along US West Coast 57 58 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 59 60 10
Fig. 14-49, p. 61 405 Fig. 14-50, p. 62 405 Waterspouts Rotating column of air that is connected to a cumuliform cloud over a large body of water Tornadic waterspout 63 Fig. 14-51, p. 64 406 Homework for Chapter 14 Project for Chapter 14 Chapter 14 Questions for Review, p. 407 #2-6, 19, 25, 29 Chapter 14 Questions for Thought, p. 408 #7 Chapter 14 Problems and Exercises, p. 409 None #4 65 66 11